“2vowAsrra Proceedings of ‘Workshop on...
“2vowAsrra
Proceedings of ‘Workshop on Medical Malacology in tica’, Harare, Zimbabwe. September 22-26, 1997, p. 173-184
POPULATION DYNAMKS OF SCHISTOSOME INTEJXMEDIATE HOST SNAILS IN
A VILLAGE IN THE DELTA OF THE SENEGAL RIVER BASIN
BY O.T. DIAW
Service de Parasitologie L.N.E.R lr /LS.RA. B.P.2037 Dakar- Senegal
ABSTRAC-I-
Durin_o the last decades major changes in water management and other ecological factors bave
caused an increase in snail habitats in the Senegal river basin and these optimal condiuons havc
promored transmission of schistosomiasiso particularly in thc delta. Thr objectives of the present
studv were to investigare dexity fluctuations in populations of intermediate hosts and the number of
schistosome infected snails and to identifv the most important transmission period in Mbodiene. a
reccnt foci of schistosomiasis in the delta, over a period of 30 months from July 1991 to Decemher
1996.In Mbodiene. a small irrigation canal and the Lampsar river are the onlx reliable sources of
domestic lvater for the population. Biomphalario pfelfiri and Buiirzus globosus are the principal
snails in these two aquatic systems which are invoived in the transmission of urinary and intestinal
schistosomiasis. S hoematobium and S. mansoni. respectively. In the Lampsar river. \\thich is a
psrmanrnt water body. Biomphalaria was more abundant with highsst densities from \\lay to
.-\\tgst a period in which 1: giobosus was also at its peak. Transmission of S haematobium and S
nxzzso~t occurred throughout the year and was at its peak betxveen May and Augst. The overali
infection rate of S. mansoni was 75% and 47% for S. haematobium.
B. globosus was the most abundant in the irrigation canal with highest densities in Au_oust 1994
and 6om August to December 1996. while Biomphaluria peaked in October and August 1991. This
uater body is temporq. it dries out 5 to 6 months per year and the irrigation is no: regular During
the sune?. the transmission period of S. haematobwn was in .August 1991 and mainly from .k~gxr
to December 1996. while August and September 1995 \\vere the only transmission periods of S
mansoni. The overaii infection rate of .Y haematobium was 3 1% and for S. mmsom 75?,0 . the samc
a in thr Lampsar river. Other snails such as B. truncarus. B. jorskalii and B. seneguiensis. ivhich are
nnt important fer the transmission of schistosomiasis. \\sere colkcted in !ox numbers
Thz results of the present study provide a bztter understanding of the dbnamics of snail
intermediate hosts of schistosomes, their fluctuations and transmission periods and also make it
advantageous to plan a stratea for snail control
Key words: schistosome, B. truncatus, B. forskulii. B. senegalenxs. B. pj$ëri. Senegal river basin
INTRODUCTION
The delta of the river in Senegal extends over 120 km inland fiom the sea and is up to 50
km wide. In 1970 and early 1980 many surveys conducted along the Senegal river basin and
particularly in the delta, showed that urinary schistosomiasis was restricted to small foci
with a lou- prevalence. exept in Lampsar where prevalence was high (Chaine & Malek
-
--
174
Population ajmamics of schistosome intermediate hosts in Senegal
1983), whereas Schistosoma mansoni was unknown in n0rtkx-n Senegal. Biomph&&
pfelfleri, the in ermediate host of
t
5’. mansoni, was only seen occasionally (small unstable
population in four isolated sites ) but not firmly established in this area (Senegal river basin
and Lac de Guiers) (Diaw 1980)
Since 1986 several ecological events occurred in the Senegal river basin: i) the opening
of the Diama dam (1986) in the lower valley, preventing the flow of sea water into the
Senegal river during the dry season and ii) the opening of the Mammtali dam (1990) and
reservoir in the Upper reaches regulating the flow at high water levels. These hydrological
changes permit the increase of irrigation in the delta and around Lac de Guiers and
subsequently the increase of schistosomiasis (Malek & Chaine 1989).
In 1987-88 the fïrst cases of 5’. marzsoni were identifïed in the Senegal river basin at the
town of Richard-Toll and thereafter the number of cases increased rapidly (Talla et ul.
1990). In 1988-89 new cases of Schistosoma haemarobiunz were observed in the delta
village of Mbodiene (Verle et al. 1994). Malacological surveys carried out in this area from
1988 to 1990 revealed a rapid proliferation of pulmonate snails, particularly the
intermediate host for schistosomiasis, Biomphalaria pfeifSeri* which seemed to have
disappeared since 1977 and Bulinus spp. ( Diaw er al. 1990 j. These were the first signs of
change in ecological conditions whose major impact was the increase of these diseases
three years after the construction of the dams.
In 1992 a survey for urinary schistosomiasis was done in Mbodiene and the prevalence
vvas found to be 87% (Verle et al. 1994). A cross-sectional malacological survey was done
to identify the water bodies and the snails involved in the transmission, after which a
longitudinal survey was undertaken to investigate the density fluctuations in populations of
intermediate hosts, their infestation and their epidemiological role in the transmission of
schistosomiasis.
This paper reports the fïndings of monthly survey-s in the two water bodies of Mbodiene,
the Lampsar river and the irrigation canal, from July 1994 to December 1996. The objective
was to detect the dynamics of snail intermediate hosts for schistosomiasis. particularly
Mirzus globoszis and Biomphalaria pferfiri, and thereby identify the most important
transmission period in order to suggest optimal snail control operations for the village of
M bodiene.
MATERIAL AND METHODS
Stu& area
Mbodiene is a small village along the national road between the sea (40 km) and
Richard-TO11 (65 km). The two principal water bodies are the Lampsar river and the
irrigation scheme built in the 1980s for growing rice. The rice fields are separated from the
village by the road and by a small irrigation canal.
The Lampsar river is a branch of the Senegal river. lt is a permanent water body- but the
level is subject to significant fluctuations. It is used as a drinking water source and for
bathing and recreational activities. The irrigation canal is 1 m wide, the water level depends
on irrigation activities and it is usually dry for several months of the year. The canal is next
O.T. Diaw
175
to the habitation and is used for domestic pur-poses and sometimes for recreational activities
by the children. These two water bodies are the only reliable sources of domestic water, SO
water contact is very frequent and intensive.
1t4A URITANIA
FIG 1. The map of the Senegal River Delta. Senegal It show the area of the srudy.
Snail survey
Selection of snails sumpling sites and sampplingprocedures
A preiiminary survey along the Lampsar river at the foci of Mbodiene and the irrigation
canal \\Vas conducted to identifj. the different water contact sites where people \\Ver-e
invol\\.ed in domestic and recreational activities.
Fi\\e sites were selected on the irrigation canal and three on the Lampsar river. Snail
searches were performed every month from July 1994 to December 1996 (30 months).
Durins monthly samplin; snails were collected at each site by hvo technicians who scooped
for 10 minutes. Snails were kept in separate plastic containers with some water. Only the
potential intermediate host snails of schistosomes were collected, the surface water
temperature was recorded at each site and the vegetation and other site conditions u‘ere
noted. At the end of the search of the eight sites, the snails collected were taken to a field
laboratory at Richard-TO11 where they were washed, identifïed (Brown 1994) and counted,
after \\+.hich they were placed individually in glass tubes with about 10 ml of fïhered water
and esposed to direct sunlight or to electric light for 30 to 40 minutes to induce cercarial
shedding. The cercariae were then checked under a dissecting microscope and only, the
Schistosoma cercariae were recorded (Frandsen & Christensen 1984). Snails which did not
176
Population dynamics of schistosome intermediate hosts in Senegal
.
release cercariae were kept ovemight and a second examination for cercariae shedding was
done.
RESULTS
Snailjndings in Mbodienc
Biomphalaria pfe@eri, Bulinus globosus, B. truncatus, B. senegalensis and B. forskalii
are the snail species recorded in the hvo water bodies in Mbodiene (Table 1). The irrigation
canai had the most diverse snail fauna with a11 fïve species present. Bulinus globosus was
the dominant species. although the densities were very low. Al1 species except Bulinus
senegalensis were found in the Lampsar river and densities were higher than in the
irrigation canai. Biomphalaria was the dominant species.
T.GLE 1. Total number of snails collected in the Lampsar river and me irrigation canal during
the 30 months ofsune) (July 1994 - December 1996 ) in Mbodiene.
Biomphalaria
Bulinus
Bulinus
Bulinus
Bulitws
pfeifferi
globosus
truncatus
forskalii
senepalensis
Irrigation canal (5 sites)
No. collected
290
4s:
78
225
9
No. infected
218
152
4
25
6
Infection rate (%)
75.1
3 1.3
5.1
11.0
66.6
Lampsar river (3 sites)
No. collected
1557
-s-q
3 3 3
127
19
0
No. infected
1170
156
22
0
0
Infection rate (%)
75.1
45.8
17.3
0.0
0.0
Irrigation canal + Lampsar river
No. collected
1847
816
205
244
9
Ko. infected
1388
308
26
25
0
Infection rate (%)
75.1
37.7
12.6
10.2
66.6
Previous studies on the intermediate host snails of schistosomes in the area of
Mbodiene have indicated limited the presence of few species occurring at low densities
(Diaw 1980). Up to 1986, only Bulinus truncatus, B. forskalii and B. globosus. which
transmitted S. haematobium, were recorded in the Lampsar river at the focus of the village
Lampsar (20 km from Mbodiene). Since the opening of the Diama dam there has been an
evolution in the malacological fauna in this area of Mbodiene.
Two cross-sectional studies conducted in September 1991 (Diaw, Sarr & Dia110 1991)
and in July 1992 (Verle et al. 1994) and the results of this survey from 1994 to 1996
confirm this evolution and the spread of these intermediate hosts, particularly Biomphalaria
O.T. Diaw
177
and B. globosus. (Piquet et a!. 1996). In 1991, B. senegalensis and B. forskalii were the only
snails coilected in Mbodiene and they were found in the irrigation canal .
The fïrst Biomphalaria (collected only in the Lampsar river) and B. globosus (localized
in the irrigation canal with high density) were found in Mbodiene, which was the beginning
of the colonization of the Lampsar river by these snails.
At the same time in 1991 and 1992, BiomphdariapfeifSeri and B. globosus were found
in high densities in the Lampsar river at the focus of other villages near Mbodiene (Diaw et
al. 1991).
Dynamics ofsnails in the Lampsar river (Fig. 2)
In the Larnpsar river a total of 2036 snails were collected. i.e. 65% of the total number
of snails in the two water bodies. Biomphalaria pfe$eri constituted 76% of the total
number of snails collected in the Lampsar river, while B. globosus represented 16%. In
relation to schistosomiasis. only Biomphalaria and B. globosza participated in the
transmission.
600 --
v)
“r
5 0 0 -’
CO
I
0
4 0 0
!
0
3 0 0 -
.
z
-
,:
2 0 0 .-
,
100 ~-.
JASONDJFMAMJJASONDJFMAMJJASOND
1
1994
/
1 9 9 5
I
1 9 9 6
I
- Ekpfeifferi
-- Rglobosus 7 Rpfeifferi n ELglobosus
FIG. 2. Density fluctuation of Biomphalaria pfeiffri and Bulinus globosw from JuIy 199-I to
December 1996 in the Lampsar river. Snail counts are the total number collected (linesl in
three sites and the number of Schisrosoma infected snails (bars).
In this water body’, snail densities were very lovv, with the highest number of
Bionzphalaria found from July 1994 to August 1995 (1,393 snails representing 89% of the
total) with hvo peaks in May (291) and August (690).
Density of B. globosus was generally much lower than Biomphalaria, but the same
pattem of variation was followed with the highest densities fiom July 1994 to August 1995
178
Population a$namics of schistosome intermediate hosts in Senegal
(202 specimens representing 61% of the total) and with peaks in May (35) and August (47).
Conceming the other snails, the densities were very low. Bulinus truncatus and B. forskdii
represented 7% of the total number of snails. The maximum number of B. truncatus were
collected from August 1994 to May 1993 with peaks in Aqgust 1994 (55 specimens) and in
May 1995 (24 specimens).
Dynamics of snails in the irrigation canal (Fig. 3)
The irrigation canal is temporary and during the survey it dried out for fïve months f?om
December 1994 to April 1995, then again in October 1995 and also for fïve months from
February to June 1996. The drought is not regtrlar from one year ta an other. but the period
is the same with a difference of 1 or 2 months. Just after the dry period no snails were
collected, with the fïrst snails being observed after two months and at very low densities.
i
Dry
,A.
y
-
0"
,- _ - C L - - . -
J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D
1
1994
1
1 9 9 5
I
1 9 9 6
I
.---~ .--
+ B.pfei#eri
-- B.g/obosus 1 Rpfeifferi a B.g/obosus
_--_-..-
FG. 3. Density fluctuation of Biomphalaria I~feiffêri and b’rtirnus globoszcs h-on1 July 199-I to
December 1996 in the irrigation canal. Snail counts are the total number collected in five sites
(lines) and the number of Schistosoma infected snails (bars),
Fewer snails were collected in the irrigation canal (1,085 specimens) than in the river.
representing 35% of the total number of mails collected in the two water bodies. With 483
specimens, B. globosus was more abundant than Bion~pldaria and represented 44% of the
total number of snails in the irrigation canal. The highest densities were observed in August
1994 and from September to December 1996, with tvvo peaks in ,4ugust (205 specimens)
and in November 1996 (110 specimens).
Density of Biomphalaria pfeifferi was lower (290 specimens) representing 27% of the
snails in the canal. The highest densities were observed in October, 1994 and in August and
O.T. Diaw
179
in September 1995, with a maximum of 191 Biomphalaria collected in Aqgst.
Biomphalaria and B. globosus did not have not the same pattem in the canal.
For the other snails, B. forskalii was more abundant than B. truncatus and B.
senegalensis. The highest numbers of B. forskalii and B. truncatus were collected in the
periods between July and August 1994 and between September and October 1996. The
highest densities were observed in August 1994 (76 specimens) and September 1996 (60
specimens) for B. forshzlii, and for B. truncatus the peak was in July 1994 (38 specimens).
Transmission of Schistosoma haematobium and S. mansoni in the Lampsar river (Fig. 2)
In the Lampsar river, schistosome infected Biomphalaria and B. globosus were found
during almost the whole time and transmission took place throughout the survey with
fluctuations fiom month to month. S haematobium infected B. globosus were collected
during 36 out of 30 months. but there were transmission peaks which coincided Lvith the
peak densities, in July 1994 and from May to August 1995. The maximum number of
infected snails was 47 in August 1995.
S. nzansoni infected B. pfe@ri were found durinp a period of 22 out of 30 months, Lvith
peaks in August 1994 and fiom May to August 1995. The highest number of infected
Biomphalaria were found at the same periods with maGmums of 631 and 273 infected
specimens respectively, in August and May 1995. The infection rates were very high, from
75% to 94%.
The overall infection rate is 75% for S. mansoni and 47% for S. haenzatobium. In this
water bodl; Biomphalaria is more abundant than B. globoszrs and the transmission of S
mansoni is very high.
Transmission of Schistosoma haematobium and S. mansoni in the irrigation canal fFig. 3)
The irrigation canal is dB. for 11 months out of 30 during the survey, 4 to 5 months per
year accord@ to the agricultural cycle. Calendar and there is a seasonality in snail
populations and in transmission.
S. haenzarobium transmission was focal and occurred mainly in August 1994 and from
September to December 1996. during which periods infection rates were high (28% to
749’0).
Transmission of S. mansoni is very focal and occurred from August to September 1995:
and during this period infection rates were very high (95% in August with 183 infected
snails out of 19 1 collected and 90% in September with 28 infected out of 3 1). Overall, 76%
of the total infected Biomphalaria collected was concentrated in this period.
The global infection rate of S. haematobium is 3 1% and for S. mansoni it is 75%. There
are two different pattems conceming the transmission in this Lvaterbody, S. mansoni being
transmitted from August to September and 5’. haematobium in two periods, in August 1994
and mainly fiom September to December 1996.
The global infection rate for S. haematobium was higher in the Lampsar river than in the
irrigation canal, while the latter had the same number of infected B. globosus but with
transmission being focal in two periods.
180
Population dynamics of schistosome intermediate hosts in S’enegal
The global infection rate for S. munsoni is the same in the two systems, but the highest
densities of infected Biomphalar-ia were found in the Lampsar river where they are most
abundant and transmission occurred throughout the year.
DISCUSSION
This study has shown that transmission of Schistosoma mansoni and S. haematobium
occurs in Mbodiene, taking place bath in the river and irrigation canais. S haematobium is
anterior to S. mansoni. The results of this survey resemble other similar studies in West
Afi-ica (McCullough 1957; Onabamiro 1972: Sodema 1979; Saladin et al. 1980; Coulibaly
& Madsen 1990; Istifanus, Fbiyi & Ndifon 1996).
According to the findings of previous suneys in the area of Mbodiene in 1991 and 1992
(Diaw et al. 1991; Verle et al. 1994) and findings of the present stud,y. there is an evolution
in the colonization of these two water bodies in the number of species of potential
intermediate host snails of schistosomiasis and in the size of the populations of these snails.
Bulinus globosus and Biomphalaria are most abundant in the two systems and are
recognized as the principal intermediate host snails of schistosomiasis (B. giobosus for S.
haematobium and Biomphalaria pfez$ëri for .Y. mansoni). but other potential interrnediate
host snails of human and animal schistosomiasis were collected in Mbodiene and some
were infected with schistosome cercariae. In Senegal Bulinusforskal~i is known only for the
transmission of S. bovis (Vercruysse, Southgate & Rollinson 1985; Diaw & Vassiliades
1987); Bulinus truncarus is intermediate host of S. bwis and is very compatible with this
parasite. In Senegal and particularly in tht: Senegal ri\\.er basin. several studies and
observations have shokvn that Bulinus trzrrrarw is incompatible with the strain of S.
haematobium (Lemasson & Diaw 1977; Vercruysse et al. 1985; Piquet et ai. 1996) but B.
twncatus is an important intermediate host for S. haematobium elsewhere in Africa.(Brown
1994). The available data suggest that the infestations observed were not those of S.
haematobium but S. bovis (the cercariae were not characterized). More recently, Bulime
truncatus collected in the delta (villages around Mbodiene) were found naturally infected
Lvith S. haematobium (confirmed by isoenzyme analysis) (Emould 1996).
These first fmdings showing the compatibility of B. nwzcatus with S. haematobium in
the delta are important in the epidemiolo,,m. of urina?. schistosomiasis. :\\lthough the
infection rates were low, B. truncatus participated in the transmission of S. haematobium in
this area and we have to reconsider its epidemiological role (compatibility) (Combes 1995)
in the context of these major ecological changes and disease events in the Senegal river
basin.
Bulinus senegalensis was collected from a few sites only in the irrigation canal. This
snail is known to be involved in the transmission of S. huematobium in the middle valley
around Podor and Matam (Vercruysse et al. 3982; Piquet er al. 1996)and also in the
transmission of S. bovis in the Senegambia (Smithers 1956). In Senegal it is not involved in
the transmission of animal schistosomiasis (5’. bovis and 5’. curassoni) (Vercruysse et al.
1985; Diaw et al. 1987). In Mbodiene it was found to be infected with schistosome cercariae
but the species were not identified. According to these results and the fact that B.
senegalensis is compatible with S. haematobium, it is possible that this snail plays a role in
the transmission of S. haematobium in the delta as it does in the middle valley.
O.T. Diaw
1 8 1
With these major changes in the environment and the increase of schistosomiasis in the
Senegal river basin, we need regular surveys (malacological and epidemiological) to detect
any changes in the epidemiology of schistosomiasis and the behaviour of intennediate host
snails and parasites. A good characterization of snails and their parasites (cercariae) is
needed particularly in this area.
The transmission of 5’. haematobium has now evolved and that of S. mansoni is in
progress as judged by the high prevalence in 3. globosus and Biomphalaria pfeifferi
(overall infection rate of 38% for 5’. haematobium and 75% for S. mansoni), there is a need
to conduct a parasitological survey in Mbodiene to establish the real prevalence in the
human population, particularly for 5’. mansoni.
The Lampsar river has been colonized by Biomphalaria which is becoming widespread
in the delta, not only at the focus of Mbodiene but a11 along the river with new foci
(Savoigne, Boundoume & Diagambale) (Emould 1996). In the delta Bionzphalaria is
becoming the principal snail and the most abundant.
The irrigation canal is fed from the Lampsar river according to the agicultural calendar,
SO the irrigation is not regular and the canal dries up for five to six months per year. There
is a long delay in the recolonization of the irrigation canal afier the reappearance of lvater.
Snails were found only after more than two months. The same observations were noticed in
certain canais in the delta Lvhere the recolonization occurred afier a delay of six v-eeks to
hvo months (Emould 1996).
Bdinus globosus, B. truncatus, Biomphalaria pfeifseri and B. senegalensis are the snails
found in the irrigation canal. and are reported to have a certain ability to resist drought
(Lariviere, Hoquet & Ranque 1962; Cridland 1967; Diaw. Seye & Sarr 1988, 1989).
The question arises of ho\\\\, the snails re-emerge in th,p canal afier h%‘o months - h!.
aesti\\.ating snails or by drifiing? If repopulation was done by aestivating snails as in
temporary ponds. just after refillin g, the flrst snails (medium sized 6 to 8 mm) would be
collected in low density just afier some hours or some days: but not after this long delay of
two months (Diaw et al. 1989). In this particular situation of the delta, recolonization mi@
be done by the combination of the hvo stratesies, aestivating snails and drifiing from the
Lampsar river. Biomphalaria is not a good aestivator, and the high densiF in August 1995,
four rnonths after the retum of the water: may be the result of drifiing and intense
reproduction.
The establishment of Biompha/aria in the delta and Lac de Guiers is thz reason for the
de\\-elopment and multiplication of new foci of S. nzansoni. l’he changes that bave occurred
in the last decades in the Senegal river basin have enabled the pulmonate potential
inrermediate hosts of human and animal schistosomiasis to flourish, increasing in
population size and estending their distribution, pa.rticularl>. Biomphdaria and B. globosus.
The same observations have been made for Lymnaea natalensis and other snails involved in
the transmission of animal trematodosis (Diaw et al. 1990).
CONCLUSION AND RECOMMENDATIONS
Lrinary and intestinal schistosomiasis are becoming important health problems in the
delta and Senegal river basin. Bionzphalaria pfe@ri and B. gZobosus are colonizing new
habitats and increasing in population size (Piquet er al. 1996). The transmission of S.
182
Population dynamics of schistosome intermediate hosts in Senegal
munsoni and S. haematobium takes place in the two water bodies, throughout the year in the
l-
1’ -I ’ . .
1
184
Population dynamics of schistosome intermediate hosts in Senegal
Saladin, B., Saladin, K., Dennis, E. & Degrémont A. (1980). Prehmi.nary epidemiological survey of
schistosomiasis in central and southem Liberia Acta Tropica, 37: 53-62.
Smithers, S.R (1956). On the ecology of schistosome vectors in the Gambia, with evidence of their role in
transmission. Transactions of the Royal Society of Tropical Medicine and Hygkne, 50: 354-365.
Sodema, WA. Jr. (1979). A longitudinal study of schistosome vector snail populations in Liberia American
Journal of Tropical Medicine and Hygiene, 28: 53 l-538.
Talla I., Kongs, A, Verle, P., Belot, J., Sarr S. & Coll, A.M. (1990). Outbreak of intestinal schistosomiasis in the
Senegal river basin. Annales de la Societé Belge de Médecine Tropicale, 70: 173-180.
Vercruysse, J., Southgate, V.R. & Rolliin, D. (1985). The epidemiology of human and animal schistosomiasis
in the Senegal river basin. Acta Trop& 42: 249-259.
Verle, P., Stelma, F., Desrumaux, P., Dieng, A., Diaw, O,T., Kongs, A., Niang, M., Sow, S., Talla, L, Sturrock,
RF., Gryseels, B. & Capron, A. (1994). Preliminary study of urinary schistosomiasis in a village in the delta
of Senegal river basin. Transactiom of the Royal Society of Tropical Medicine and Hygikne, 88 : 401-405.
O.T. Diaw
181
With these major changes in the environment and the increase of schistosomiasis in the
Senegal river basin, we need re3ular surveys (malacological and epidemiological) to detect
any changes in the epidemiology of schistosomiasis and the behaviour of intermediate host
snails and parasites. A good characterization of snails and their parasites (cercariae) is
needed particularly in this area.
The transmission of S. haematobium bas now evolved and that of S. maxsoni is in
progress as judged by the high prevalence in B. globosus and Biomphalaria pfeifferi
(overall infection rate of 38% for S haematobium and 75% for S. mansoni), there is a need
to conduct a parasitological survey in Mbodiene to establish the real prevalence in the
human population, particularly for S. mansoni.
The Lampsar river has been colonized by Bionzphalaria which is becoming widespread
in the delta, not only at the focus of Mbodiene but a11 along the river with new foci
(Savoigne. Boundoume &r Diagambale) (Emould 1996). In the delta Biomphalaria is
becoming the principal snail and the most abundant.
The irrigation canal is fed from the Lampsar river according to the agricultural calendar,
SO the irrigation is not regular and the canai dries up for fïve to six months per year. There
is a long delay in the recolonization of the irrigation canal alter the reappearance of n’ater.
Snails were found only alter more than two months. The same observations Lvere noticed in
certain canais in the delta Lvhere the recolonization occurred after a delay of sis x\\,esl;s to
hvo months (Emould 1996).
Bzdinus globosza, B. bw2catu.s: Bionzphalaria pfeifferi and B. senegalensis are the snails
found in the irrigation canal. and are reported to have a certain ability to resist drought
(Lariviere: Hoquet & Ranque 1962; Cridland 1967; Diaw, Seye & Sarr 1988, 1989).
The question arises of hov. the snails re-emerge in the canal after tw.0 months - bu,
aesti\\.ating snails or by drifting? If repopulation was done by aestivating snails as in
temporq ponds. just alter refillin g, the ftrst snails (medium sized 6 to S mm) would be
collected in low density just afier some hours or some days, but not after this long delay of
hvo months (Diaw el al. 1989). In this particular situation of the delta, recolonization mi&t
be done by the combination of the hvo strategies, aestivating snails and drifting from the
Lampsar river. Bionzphalaria is not a good aestivator, and the high densit) in August 1995,
four months after the ri-mm of the water, may be the result of drifting and intense
reproduction.
The establishment of Biomphalaria in the delta and Las de Guiers is the reason for the
de\\.rlopment and multiplication of new foci of S. nzansoni. The changes that have occurred
in ths last decades in the Senegal river basin have enabled the pulmonate potential
intennediate hosts of human and animal schistosomiasis to flourish, increasing in
population size and extending their distribution, particularly Biomphalaria and B. globosus.
The same observations have been made for Lymnaea natalensis and other snails involved in
the transmission of animal trematodosis (Diaw et al. 1990).
CONCLUSION AND RECOMMENDATIONS
Urinary and intestinal schistosomiasis are becoming important health problems in the
delta and Senegal river basin. Bionzphalariapfeifferi and B. globosus are colonizing new
habitats and increasing in ‘population size (Piquet et al. 1996). The transmission of S.
1 8 2
Population &namics of schistosome intermediate hosts in Senegal
mansorzi and s. haematobium t&es place in the hvo water bodies, throughout the year in the
Lampsar river and seasonal in the irrigation canal.
Tbe fmdings of the present study provide a basis for plannmg a stmtegy to control the
snails. Preventing snaiis from entering the irrigation canal when pumping water from the
Lmpsar river by putting a screen at the mout.h of me pump Will rrot eliminate snails m the
canal but could be suffrcient to reduce the density of snails, mainly B. &$Os~ and
Biomphalaria pfeifferi.
Water management is recommended in the different human water contact sites in the
Lampsar river and particularly in the irrigation canal. Judicio,us use of molluscicides,
carefully timed with the expansion phase and with peak snail density, must be
recommended. A tria1 is suggested for the irrigation canal. i.e. one application during the
colonization of the canal (in the fïrst three months alter water retums) to reduce the build-
up of populations and another before the main period of transmission to keep snails
numbers lovv.
In the irrigation canal the molluscicide must be combined with water management, i.e.
the canai should be cleaned of aquatic vegetation regularly, the vegetation should be
removed regularly and the banks made steeper to discourage the snails. The soi1 bas to be
tumed over and the dead vegetation bumed during the period when the canal is dc.
In the Lampsar river focal application of the molluscicide must be tested at sites with
high transmission but with much attention to the environment. Al1 these activities should be
combined with health education, through which individuals cari avoid the main trans-
mission periods and thus minimize the risks of infestation. Snail control seems to be easier
in the irrigation canal than in the Lampsar river with adequate \\.vater management in this
irrigation scheme.
The environmental changes in the Senegal river basin faveur the increase of schisto-
somiasis and animal trematodosis (faacioliasis, schistosomiasis, paramphistosomiasis)
making this area a permanent risk area. These risks are becoming worse with the national
programme of “Revitalisation of Fossil Valley” and the “C<anal du Cayor” (an open canal
running from Lac de Guiers to the region of Dakar) which creates suitable habitats for
snails and mean the extension of a11 these diseases from the delta and Lac de Guiers to the
rest of the counrn; if nothing is monitored now.
\\Vith the initiation of a national schistosomiasis contrai programme, these fïndings
should pro\\-ide a better understanding of the fluctuations in intemlediate host populations
and should be of major importance for cuntrol.
ACKNOWLEDGEMENTS
This lvork L+‘as performed as part of the ESPOIR programme for research and control of
schistosomiasis in the Senegal River Basin. The fïeld technicians M. Johnson and 0. SOM
are acknowledged as well as a11 the people involved in schistosomiasis in ESPOIR.
O.T. Diaw
183
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Population dynamics of schistosome intermediate hosts in Senegal
--T
*-a
1
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schistosomiasis in central and southern Liberia Acta Tropica, 37: 53-62.
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Journal of Tropical Medicine andHygiene, 28: 531-538.
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Senegal river basin. Annales de IaSoc~téBelge de Médecine Tropicale, 70: 173-180.
Vercruysse, J., Southgate, V.R. & Rolliin, D. (1985). The epidemiology of human and animal schistosomiasis
l
in the Senegal river basin. Acta Tropica, 42: 249-259.
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RF., Gryseels, B. & Capron, A. (1994). Preliminaty smdy of urinai-y schistosomiasis in a village in the delta
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1
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!
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j
i
t
1
/
1iI
Proceedings of ‘Workshop on Medical Malacology in tica’, Harare, Zimbabwe. September 22-26, 1997, p. 173-184
POPULATION DYNAMKS OF SCHISTOSOME INTEJXMEDIATE HOST SNAILS IN
A VILLAGE IN THE DELTA OF THE SENEGAL RIVER BASIN
BY O.T. DIAW
Service de Parasitologie L.N.E.R lr /LS.RA. B.P.2037 Dakar- Senegal
ABSTRAC-I-
Durin_o the last decades major changes in water management and other ecological factors bave
caused an increase in snail habitats in the Senegal river basin and these optimal condiuons havc
promored transmission of schistosomiasiso particularly in thc delta. Thr objectives of the present
studv were to investigare dexity fluctuations in populations of intermediate hosts and the number of
schistosome infected snails and to identifv the most important transmission period in Mbodiene. a
reccnt foci of schistosomiasis in the delta, over a period of 30 months from July 1991 to Decemher
1996.In Mbodiene. a small irrigation canal and the Lampsar river are the onlx reliable sources of
domestic lvater for the population. Biomphalario pfelfiri and Buiirzus globosus are the principal
snails in these two aquatic systems which are invoived in the transmission of urinary and intestinal
schistosomiasis. S hoematobium and S. mansoni. respectively. In the Lampsar river. \\thich is a
psrmanrnt water body. Biomphalaria was more abundant with highsst densities from \\lay to
.-\\tgst a period in which 1: giobosus was also at its peak. Transmission of S haematobium and S
nxzzso~t occurred throughout the year and was at its peak betxveen May and Augst. The overali
infection rate of S. mansoni was 75% and 47% for S. haematobium.
B. globosus was the most abundant in the irrigation canal with highest densities in Au_oust 1994
and 6om August to December 1996. while Biomphaluria peaked in October and August 1991. This
uater body is temporq. it dries out 5 to 6 months per year and the irrigation is no: regular During
the sune?. the transmission period of S. haematobwn was in .August 1991 and mainly from .k~gxr
to December 1996. while August and September 1995 \\vere the only transmission periods of S
mansoni. The overaii infection rate of .Y haematobium was 3 1% and for S. mmsom 75?,0 . the samc
a in thr Lampsar river. Other snails such as B. truncarus. B. jorskalii and B. seneguiensis. ivhich are
nnt important fer the transmission of schistosomiasis. \\sere colkcted in !ox numbers
Thz results of the present study provide a bztter understanding of the dbnamics of snail
intermediate hosts of schistosomes, their fluctuations and transmission periods and also make it
advantageous to plan a stratea for snail control
Key words: schistosome, B. truncatus, B. forskulii. B. senegalenxs. B. pj$ëri. Senegal river basin
INTRODUCTION
The delta of the river in Senegal extends over 120 km inland fiom the sea and is up to 50
km wide. In 1970 and early 1980 many surveys conducted along the Senegal river basin and
particularly in the delta, showed that urinary schistosomiasis was restricted to small foci
with a lou- prevalence. exept in Lampsar where prevalence was high (Chaine & Malek
-
--
174
Population ajmamics of schistosome intermediate hosts in Senegal
1983), whereas Schistosoma mansoni was unknown in n0rtkx-n Senegal. Biomph&&
pfelfleri, the in ermediate host of
t
5’. mansoni, was only seen occasionally (small unstable
population in four isolated sites ) but not firmly established in this area (Senegal river basin
and Lac de Guiers) (Diaw 1980)
Since 1986 several ecological events occurred in the Senegal river basin: i) the opening
of the Diama dam (1986) in the lower valley, preventing the flow of sea water into the
Senegal river during the dry season and ii) the opening of the Mammtali dam (1990) and
reservoir in the Upper reaches regulating the flow at high water levels. These hydrological
changes permit the increase of irrigation in the delta and around Lac de Guiers and
subsequently the increase of schistosomiasis (Malek & Chaine 1989).
In 1987-88 the fïrst cases of 5’. marzsoni were identifïed in the Senegal river basin at the
town of Richard-Toll and thereafter the number of cases increased rapidly (Talla et ul.
1990). In 1988-89 new cases of Schistosoma haemarobiunz were observed in the delta
village of Mbodiene (Verle et al. 1994). Malacological surveys carried out in this area from
1988 to 1990 revealed a rapid proliferation of pulmonate snails, particularly the
intermediate host for schistosomiasis, Biomphalaria pfeifSeri* which seemed to have
disappeared since 1977 and Bulinus spp. ( Diaw er al. 1990 j. These were the first signs of
change in ecological conditions whose major impact was the increase of these diseases
three years after the construction of the dams.
In 1992 a survey for urinary schistosomiasis was done in Mbodiene and the prevalence
vvas found to be 87% (Verle et al. 1994). A cross-sectional malacological survey was done
to identify the water bodies and the snails involved in the transmission, after which a
longitudinal survey was undertaken to investigate the density fluctuations in populations of
intermediate hosts, their infestation and their epidemiological role in the transmission of
schistosomiasis.
This paper reports the fïndings of monthly survey-s in the two water bodies of Mbodiene,
the Lampsar river and the irrigation canal, from July 1994 to December 1996. The objective
was to detect the dynamics of snail intermediate hosts for schistosomiasis. particularly
Mirzus globoszis and Biomphalaria pferfiri, and thereby identify the most important
transmission period in order to suggest optimal snail control operations for the village of
M bodiene.
MATERIAL AND METHODS
Stu& area
Mbodiene is a small village along the national road between the sea (40 km) and
Richard-TO11 (65 km). The two principal water bodies are the Lampsar river and the
irrigation scheme built in the 1980s for growing rice. The rice fields are separated from the
village by the road and by a small irrigation canal.
The Lampsar river is a branch of the Senegal river. lt is a permanent water body- but the
level is subject to significant fluctuations. It is used as a drinking water source and for
bathing and recreational activities. The irrigation canal is 1 m wide, the water level depends
on irrigation activities and it is usually dry for several months of the year. The canal is next
O.T. Diaw
175
to the habitation and is used for domestic pur-poses and sometimes for recreational activities
by the children. These two water bodies are the only reliable sources of domestic water, SO
water contact is very frequent and intensive.
1t4A URITANIA
FIG 1. The map of the Senegal River Delta. Senegal It show the area of the srudy.
Snail survey
Selection of snails sumpling sites and sampplingprocedures
A preiiminary survey along the Lampsar river at the foci of Mbodiene and the irrigation
canal \\Vas conducted to identifj. the different water contact sites where people \\Ver-e
invol\\.ed in domestic and recreational activities.
Fi\\e sites were selected on the irrigation canal and three on the Lampsar river. Snail
searches were performed every month from July 1994 to December 1996 (30 months).
Durins monthly samplin; snails were collected at each site by hvo technicians who scooped
for 10 minutes. Snails were kept in separate plastic containers with some water. Only the
potential intermediate host snails of schistosomes were collected, the surface water
temperature was recorded at each site and the vegetation and other site conditions u‘ere
noted. At the end of the search of the eight sites, the snails collected were taken to a field
laboratory at Richard-TO11 where they were washed, identifïed (Brown 1994) and counted,
after \\+.hich they were placed individually in glass tubes with about 10 ml of fïhered water
and esposed to direct sunlight or to electric light for 30 to 40 minutes to induce cercarial
shedding. The cercariae were then checked under a dissecting microscope and only, the
Schistosoma cercariae were recorded (Frandsen & Christensen 1984). Snails which did not
176
Population dynamics of schistosome intermediate hosts in Senegal
.
release cercariae were kept ovemight and a second examination for cercariae shedding was
done.
RESULTS
Snailjndings in Mbodienc
Biomphalaria pfe@eri, Bulinus globosus, B. truncatus, B. senegalensis and B. forskalii
are the snail species recorded in the hvo water bodies in Mbodiene (Table 1). The irrigation
canai had the most diverse snail fauna with a11 fïve species present. Bulinus globosus was
the dominant species. although the densities were very low. Al1 species except Bulinus
senegalensis were found in the Lampsar river and densities were higher than in the
irrigation canai. Biomphalaria was the dominant species.
T.GLE 1. Total number of snails collected in the Lampsar river and me irrigation canal during
the 30 months ofsune) (July 1994 - December 1996 ) in Mbodiene.
Biomphalaria
Bulinus
Bulinus
Bulinus
Bulitws
pfeifferi
globosus
truncatus
forskalii
senepalensis
Irrigation canal (5 sites)
No. collected
290
4s:
78
225
9
No. infected
218
152
4
25
6
Infection rate (%)
75.1
3 1.3
5.1
11.0
66.6
Lampsar river (3 sites)
No. collected
1557
-s-q
3 3 3
127
19
0
No. infected
1170
156
22
0
0
Infection rate (%)
75.1
45.8
17.3
0.0
0.0
Irrigation canal + Lampsar river
No. collected
1847
816
205
244
9
Ko. infected
1388
308
26
25
0
Infection rate (%)
75.1
37.7
12.6
10.2
66.6
Previous studies on the intermediate host snails of schistosomes in the area of
Mbodiene have indicated limited the presence of few species occurring at low densities
(Diaw 1980). Up to 1986, only Bulinus truncatus, B. forskalii and B. globosus. which
transmitted S. haematobium, were recorded in the Lampsar river at the focus of the village
Lampsar (20 km from Mbodiene). Since the opening of the Diama dam there has been an
evolution in the malacological fauna in this area of Mbodiene.
Two cross-sectional studies conducted in September 1991 (Diaw, Sarr & Dia110 1991)
and in July 1992 (Verle et al. 1994) and the results of this survey from 1994 to 1996
confirm this evolution and the spread of these intermediate hosts, particularly Biomphalaria
O.T. Diaw
177
and B. globosus. (Piquet et a!. 1996). In 1991, B. senegalensis and B. forskalii were the only
snails coilected in Mbodiene and they were found in the irrigation canal .
The fïrst Biomphalaria (collected only in the Lampsar river) and B. globosus (localized
in the irrigation canal with high density) were found in Mbodiene, which was the beginning
of the colonization of the Lampsar river by these snails.
At the same time in 1991 and 1992, BiomphdariapfeifSeri and B. globosus were found
in high densities in the Lampsar river at the focus of other villages near Mbodiene (Diaw et
al. 1991).
Dynamics ofsnails in the Lampsar river (Fig. 2)
In the Larnpsar river a total of 2036 snails were collected. i.e. 65% of the total number
of snails in the two water bodies. Biomphalaria pfe$eri constituted 76% of the total
number of snails collected in the Lampsar river, while B. globosus represented 16%. In
relation to schistosomiasis. only Biomphalaria and B. globosza participated in the
transmission.
600 --
v)
“r
5 0 0 -’
CO
I
0
4 0 0
!
0
3 0 0 -
.
z
-
,:
2 0 0 .-
,
100 ~-.
JASONDJFMAMJJASONDJFMAMJJASOND
1
1994
/
1 9 9 5
I
1 9 9 6
I
- Ekpfeifferi
-- Rglobosus 7 Rpfeifferi n ELglobosus
FIG. 2. Density fluctuation of Biomphalaria pfeiffri and Bulinus globosw from JuIy 199-I to
December 1996 in the Lampsar river. Snail counts are the total number collected (linesl in
three sites and the number of Schisrosoma infected snails (bars).
In this water body’, snail densities were very lovv, with the highest number of
Bionzphalaria found from July 1994 to August 1995 (1,393 snails representing 89% of the
total) with hvo peaks in May (291) and August (690).
Density of B. globosus was generally much lower than Biomphalaria, but the same
pattem of variation was followed with the highest densities fiom July 1994 to August 1995
178
Population a$namics of schistosome intermediate hosts in Senegal
(202 specimens representing 61% of the total) and with peaks in May (35) and August (47).
Conceming the other snails, the densities were very low. Bulinus truncatus and B. forskdii
represented 7% of the total number of snails. The maximum number of B. truncatus were
collected from August 1994 to May 1993 with peaks in Aqgust 1994 (55 specimens) and in
May 1995 (24 specimens).
Dynamics of snails in the irrigation canal (Fig. 3)
The irrigation canal is temporary and during the survey it dried out for fïve months f?om
December 1994 to April 1995, then again in October 1995 and also for fïve months from
February to June 1996. The drought is not regtrlar from one year ta an other. but the period
is the same with a difference of 1 or 2 months. Just after the dry period no snails were
collected, with the fïrst snails being observed after two months and at very low densities.
i
Dry
,A.
y
-
0"
,- _ - C L - - . -
J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D
1
1994
1
1 9 9 5
I
1 9 9 6
I
.---~ .--
+ B.pfei#eri
-- B.g/obosus 1 Rpfeifferi a B.g/obosus
_--_-..-
FG. 3. Density fluctuation of Biomphalaria I~feiffêri and b’rtirnus globoszcs h-on1 July 199-I to
December 1996 in the irrigation canal. Snail counts are the total number collected in five sites
(lines) and the number of Schistosoma infected snails (bars),
Fewer snails were collected in the irrigation canal (1,085 specimens) than in the river.
representing 35% of the total number of mails collected in the two water bodies. With 483
specimens, B. globosus was more abundant than Bion~pldaria and represented 44% of the
total number of snails in the irrigation canal. The highest densities were observed in August
1994 and from September to December 1996, with tvvo peaks in ,4ugust (205 specimens)
and in November 1996 (110 specimens).
Density of Biomphalaria pfeifferi was lower (290 specimens) representing 27% of the
snails in the canal. The highest densities were observed in October, 1994 and in August and
O.T. Diaw
179
in September 1995, with a maximum of 191 Biomphalaria collected in Aqgst.
Biomphalaria and B. globosus did not have not the same pattem in the canal.
For the other snails, B. forskalii was more abundant than B. truncatus and B.
senegalensis. The highest numbers of B. forskalii and B. truncatus were collected in the
periods between July and August 1994 and between September and October 1996. The
highest densities were observed in August 1994 (76 specimens) and September 1996 (60
specimens) for B. forshzlii, and for B. truncatus the peak was in July 1994 (38 specimens).
Transmission of Schistosoma haematobium and S. mansoni in the Lampsar river (Fig. 2)
In the Lampsar river, schistosome infected Biomphalaria and B. globosus were found
during almost the whole time and transmission took place throughout the survey with
fluctuations fiom month to month. S haematobium infected B. globosus were collected
during 36 out of 30 months. but there were transmission peaks which coincided Lvith the
peak densities, in July 1994 and from May to August 1995. The maximum number of
infected snails was 47 in August 1995.
S. nzansoni infected B. pfe@ri were found durinp a period of 22 out of 30 months, Lvith
peaks in August 1994 and fiom May to August 1995. The highest number of infected
Biomphalaria were found at the same periods with maGmums of 631 and 273 infected
specimens respectively, in August and May 1995. The infection rates were very high, from
75% to 94%.
The overall infection rate is 75% for S. mansoni and 47% for S. haenzatobium. In this
water bodl; Biomphalaria is more abundant than B. globoszrs and the transmission of S
mansoni is very high.
Transmission of Schistosoma haematobium and S. mansoni in the irrigation canal fFig. 3)
The irrigation canal is dB. for 11 months out of 30 during the survey, 4 to 5 months per
year accord@ to the agricultural cycle. Calendar and there is a seasonality in snail
populations and in transmission.
S. haenzarobium transmission was focal and occurred mainly in August 1994 and from
September to December 1996. during which periods infection rates were high (28% to
749’0).
Transmission of S. mansoni is very focal and occurred from August to September 1995:
and during this period infection rates were very high (95% in August with 183 infected
snails out of 19 1 collected and 90% in September with 28 infected out of 3 1). Overall, 76%
of the total infected Biomphalaria collected was concentrated in this period.
The global infection rate of S. haematobium is 3 1% and for S. mansoni it is 75%. There
are two different pattems conceming the transmission in this Lvaterbody, S. mansoni being
transmitted from August to September and 5’. haematobium in two periods, in August 1994
and mainly fiom September to December 1996.
The global infection rate for S. haematobium was higher in the Lampsar river than in the
irrigation canal, while the latter had the same number of infected B. globosus but with
transmission being focal in two periods.
180
Population dynamics of schistosome intermediate hosts in S’enegal
The global infection rate for S. munsoni is the same in the two systems, but the highest
densities of infected Biomphalar-ia were found in the Lampsar river where they are most
abundant and transmission occurred throughout the year.
DISCUSSION
This study has shown that transmission of Schistosoma mansoni and S. haematobium
occurs in Mbodiene, taking place bath in the river and irrigation canais. S haematobium is
anterior to S. mansoni. The results of this survey resemble other similar studies in West
Afi-ica (McCullough 1957; Onabamiro 1972: Sodema 1979; Saladin et al. 1980; Coulibaly
& Madsen 1990; Istifanus, Fbiyi & Ndifon 1996).
According to the findings of previous suneys in the area of Mbodiene in 1991 and 1992
(Diaw et al. 1991; Verle et al. 1994) and findings of the present stud,y. there is an evolution
in the colonization of these two water bodies in the number of species of potential
intermediate host snails of schistosomiasis and in the size of the populations of these snails.
Bulinus globosus and Biomphalaria are most abundant in the two systems and are
recognized as the principal intermediate host snails of schistosomiasis (B. giobosus for S.
haematobium and Biomphalaria pfez$ëri for .Y. mansoni). but other potential interrnediate
host snails of human and animal schistosomiasis were collected in Mbodiene and some
were infected with schistosome cercariae. In Senegal Bulinusforskal~i is known only for the
transmission of S. bovis (Vercruysse, Southgate & Rollinson 1985; Diaw & Vassiliades
1987); Bulinus truncarus is intermediate host of S. bwis and is very compatible with this
parasite. In Senegal and particularly in tht: Senegal ri\\.er basin. several studies and
observations have shokvn that Bulinus trzrrrarw is incompatible with the strain of S.
haematobium (Lemasson & Diaw 1977; Vercruysse et al. 1985; Piquet et ai. 1996) but B.
twncatus is an important intermediate host for S. haematobium elsewhere in Africa.(Brown
1994). The available data suggest that the infestations observed were not those of S.
haematobium but S. bovis (the cercariae were not characterized). More recently, Bulime
truncatus collected in the delta (villages around Mbodiene) were found naturally infected
Lvith S. haematobium (confirmed by isoenzyme analysis) (Emould 1996).
These first fmdings showing the compatibility of B. nwzcatus with S. haematobium in
the delta are important in the epidemiolo,,m. of urina?. schistosomiasis. :\\lthough the
infection rates were low, B. truncatus participated in the transmission of S. haematobium in
this area and we have to reconsider its epidemiological role (compatibility) (Combes 1995)
in the context of these major ecological changes and disease events in the Senegal river
basin.
Bulinus senegalensis was collected from a few sites only in the irrigation canal. This
snail is known to be involved in the transmission of S. huematobium in the middle valley
around Podor and Matam (Vercruysse et al. 3982; Piquet er al. 1996)and also in the
transmission of S. bovis in the Senegambia (Smithers 1956). In Senegal it is not involved in
the transmission of animal schistosomiasis (5’. bovis and 5’. curassoni) (Vercruysse et al.
1985; Diaw et al. 1987). In Mbodiene it was found to be infected with schistosome cercariae
but the species were not identified. According to these results and the fact that B.
senegalensis is compatible with S. haematobium, it is possible that this snail plays a role in
the transmission of S. haematobium in the delta as it does in the middle valley.
O.T. Diaw
1 8 1
With these major changes in the environment and the increase of schistosomiasis in the
Senegal river basin, we need regular surveys (malacological and epidemiological) to detect
any changes in the epidemiology of schistosomiasis and the behaviour of intennediate host
snails and parasites. A good characterization of snails and their parasites (cercariae) is
needed particularly in this area.
The transmission of 5’. haematobium has now evolved and that of S. mansoni is in
progress as judged by the high prevalence in 3. globosus and Biomphalaria pfeifferi
(overall infection rate of 38% for 5’. haematobium and 75% for S. mansoni), there is a need
to conduct a parasitological survey in Mbodiene to establish the real prevalence in the
human population, particularly for 5’. mansoni.
The Lampsar river has been colonized by Biomphalaria which is becoming widespread
in the delta, not only at the focus of Mbodiene but a11 along the river with new foci
(Savoigne, Boundoume & Diagambale) (Emould 1996). In the delta Bionzphalaria is
becoming the principal snail and the most abundant.
The irrigation canal is fed from the Lampsar river according to the agicultural calendar,
SO the irrigation is not regular and the canal dries up for five to six months per year. There
is a long delay in the recolonization of the irrigation canal afier the reappearance of lvater.
Snails were found only after more than two months. The same observations were noticed in
certain canais in the delta Lvhere the recolonization occurred afier a delay of six v-eeks to
hvo months (Emould 1996).
Bdinus globosus, B. truncatus, Biomphalaria pfeifseri and B. senegalensis are the snails
found in the irrigation canal. and are reported to have a certain ability to resist drought
(Lariviere, Hoquet & Ranque 1962; Cridland 1967; Diaw. Seye & Sarr 1988, 1989).
The question arises of ho\\\\, the snails re-emerge in th,p canal afier h%‘o months - h!.
aesti\\.ating snails or by drifiing? If repopulation was done by aestivating snails as in
temporary ponds. just after refillin g, the flrst snails (medium sized 6 to 8 mm) would be
collected in low density just afier some hours or some days: but not after this long delay of
two months (Diaw et al. 1989). In this particular situation of the delta, recolonization mi@
be done by the combination of the hvo stratesies, aestivating snails and drifiing from the
Lampsar river. Biomphalaria is not a good aestivator, and the high densiF in August 1995,
four rnonths after the retum of the water: may be the result of drifiing and intense
reproduction.
The establishment of Biompha/aria in the delta and Lac de Guiers is thz reason for the
de\\-elopment and multiplication of new foci of S. nzansoni. l’he changes that bave occurred
in the last decades in the Senegal river basin have enabled the pulmonate potential
inrermediate hosts of human and animal schistosomiasis to flourish, increasing in
population size and estending their distribution, pa.rticularl>. Biomphdaria and B. globosus.
The same observations have been made for Lymnaea natalensis and other snails involved in
the transmission of animal trematodosis (Diaw et al. 1990).
CONCLUSION AND RECOMMENDATIONS
Lrinary and intestinal schistosomiasis are becoming important health problems in the
delta and Senegal river basin. Bionzphalaria pfe@ri and B. gZobosus are colonizing new
habitats and increasing in population size (Piquet er al. 1996). The transmission of S.
182
Population dynamics of schistosome intermediate hosts in Senegal
munsoni and S. haematobium takes place in the two water bodies, throughout the year in the
l-
1’ -I ’ . .
1
184
Population dynamics of schistosome intermediate hosts in Senegal
Saladin, B., Saladin, K., Dennis, E. & Degrémont A. (1980). Prehmi.nary epidemiological survey of
schistosomiasis in central and southem Liberia Acta Tropica, 37: 53-62.
Smithers, S.R (1956). On the ecology of schistosome vectors in the Gambia, with evidence of their role in
transmission. Transactions of the Royal Society of Tropical Medicine and Hygkne, 50: 354-365.
Sodema, WA. Jr. (1979). A longitudinal study of schistosome vector snail populations in Liberia American
Journal of Tropical Medicine and Hygiene, 28: 53 l-538.
Talla I., Kongs, A, Verle, P., Belot, J., Sarr S. & Coll, A.M. (1990). Outbreak of intestinal schistosomiasis in the
Senegal river basin. Annales de la Societé Belge de Médecine Tropicale, 70: 173-180.
Vercruysse, J., Southgate, V.R. & Rolliin, D. (1985). The epidemiology of human and animal schistosomiasis
in the Senegal river basin. Acta Trop& 42: 249-259.
Verle, P., Stelma, F., Desrumaux, P., Dieng, A., Diaw, O,T., Kongs, A., Niang, M., Sow, S., Talla, L, Sturrock,
RF., Gryseels, B. & Capron, A. (1994). Preliminary study of urinary schistosomiasis in a village in the delta
of Senegal river basin. Transactiom of the Royal Society of Tropical Medicine and Hygikne, 88 : 401-405.
O.T. Diaw
181
With these major changes in the environment and the increase of schistosomiasis in the
Senegal river basin, we need re3ular surveys (malacological and epidemiological) to detect
any changes in the epidemiology of schistosomiasis and the behaviour of intermediate host
snails and parasites. A good characterization of snails and their parasites (cercariae) is
needed particularly in this area.
The transmission of S. haematobium bas now evolved and that of S. maxsoni is in
progress as judged by the high prevalence in B. globosus and Biomphalaria pfeifferi
(overall infection rate of 38% for S haematobium and 75% for S. mansoni), there is a need
to conduct a parasitological survey in Mbodiene to establish the real prevalence in the
human population, particularly for S. mansoni.
The Lampsar river has been colonized by Bionzphalaria which is becoming widespread
in the delta, not only at the focus of Mbodiene but a11 along the river with new foci
(Savoigne. Boundoume &r Diagambale) (Emould 1996). In the delta Biomphalaria is
becoming the principal snail and the most abundant.
The irrigation canal is fed from the Lampsar river according to the agricultural calendar,
SO the irrigation is not regular and the canai dries up for fïve to six months per year. There
is a long delay in the recolonization of the irrigation canal alter the reappearance of n’ater.
Snails were found only alter more than two months. The same observations Lvere noticed in
certain canais in the delta Lvhere the recolonization occurred after a delay of sis x\\,esl;s to
hvo months (Emould 1996).
Bzdinus globosza, B. bw2catu.s: Bionzphalaria pfeifferi and B. senegalensis are the snails
found in the irrigation canal. and are reported to have a certain ability to resist drought
(Lariviere: Hoquet & Ranque 1962; Cridland 1967; Diaw, Seye & Sarr 1988, 1989).
The question arises of hov. the snails re-emerge in the canal after tw.0 months - bu,
aesti\\.ating snails or by drifting? If repopulation was done by aestivating snails as in
temporq ponds. just alter refillin g, the ftrst snails (medium sized 6 to S mm) would be
collected in low density just afier some hours or some days, but not after this long delay of
hvo months (Diaw el al. 1989). In this particular situation of the delta, recolonization mi&t
be done by the combination of the hvo strategies, aestivating snails and drifting from the
Lampsar river. Bionzphalaria is not a good aestivator, and the high densit) in August 1995,
four months after the ri-mm of the water, may be the result of drifting and intense
reproduction.
The establishment of Biomphalaria in the delta and Las de Guiers is the reason for the
de\\.rlopment and multiplication of new foci of S. nzansoni. The changes that have occurred
in ths last decades in the Senegal river basin have enabled the pulmonate potential
intennediate hosts of human and animal schistosomiasis to flourish, increasing in
population size and extending their distribution, particularly Biomphalaria and B. globosus.
The same observations have been made for Lymnaea natalensis and other snails involved in
the transmission of animal trematodosis (Diaw et al. 1990).
CONCLUSION AND RECOMMENDATIONS
Urinary and intestinal schistosomiasis are becoming important health problems in the
delta and Senegal river basin. Bionzphalariapfeifferi and B. globosus are colonizing new
habitats and increasing in ‘population size (Piquet et al. 1996). The transmission of S.
1 8 2
Population &namics of schistosome intermediate hosts in Senegal
mansorzi and s. haematobium t&es place in the hvo water bodies, throughout the year in the
Lampsar river and seasonal in the irrigation canal.
Tbe fmdings of the present study provide a basis for plannmg a stmtegy to control the
snails. Preventing snaiis from entering the irrigation canal when pumping water from the
Lmpsar river by putting a screen at the mout.h of me pump Will rrot eliminate snails m the
canal but could be suffrcient to reduce the density of snails, mainly B. &$Os~ and
Biomphalaria pfeifferi.
Water management is recommended in the different human water contact sites in the
Lampsar river and particularly in the irrigation canal. Judicio,us use of molluscicides,
carefully timed with the expansion phase and with peak snail density, must be
recommended. A tria1 is suggested for the irrigation canal. i.e. one application during the
colonization of the canal (in the fïrst three months alter water retums) to reduce the build-
up of populations and another before the main period of transmission to keep snails
numbers lovv.
In the irrigation canal the molluscicide must be combined with water management, i.e.
the canai should be cleaned of aquatic vegetation regularly, the vegetation should be
removed regularly and the banks made steeper to discourage the snails. The soi1 bas to be
tumed over and the dead vegetation bumed during the period when the canal is dc.
In the Lampsar river focal application of the molluscicide must be tested at sites with
high transmission but with much attention to the environment. Al1 these activities should be
combined with health education, through which individuals cari avoid the main trans-
mission periods and thus minimize the risks of infestation. Snail control seems to be easier
in the irrigation canal than in the Lampsar river with adequate \\.vater management in this
irrigation scheme.
The environmental changes in the Senegal river basin faveur the increase of schisto-
somiasis and animal trematodosis (faacioliasis, schistosomiasis, paramphistosomiasis)
making this area a permanent risk area. These risks are becoming worse with the national
programme of “Revitalisation of Fossil Valley” and the “C<anal du Cayor” (an open canal
running from Lac de Guiers to the region of Dakar) which creates suitable habitats for
snails and mean the extension of a11 these diseases from the delta and Lac de Guiers to the
rest of the counrn; if nothing is monitored now.
\\Vith the initiation of a national schistosomiasis contrai programme, these fïndings
should pro\\-ide a better understanding of the fluctuations in intemlediate host populations
and should be of major importance for cuntrol.
ACKNOWLEDGEMENTS
This lvork L+‘as performed as part of the ESPOIR programme for research and control of
schistosomiasis in the Senegal River Basin. The fïeld technicians M. Johnson and 0. SOM
are acknowledged as well as a11 the people involved in schistosomiasis in ESPOIR.
O.T. Diaw
183
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McCullough. FS. (1957). Seasonal densih of populations of Bzknus (Physopsis) globosus and B. forsi,& in
natural habitats in Ghana Annals qf Tropical Medicine and Parasitology, j 1: 235-248.
Onabamiro. S.D. (1972). Studies on schistosomiasis in Sierra Leone, II. Seasond fluctuation in population
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(1996). The epidemiology of human schistosomiasis in the Senegal river basin. Transactionr o,fthe &y~/
Society of Tropical Medicine and Hygiene, 90: 340-346.
-
184
Population dynamics of schistosome intermediate hosts in Senegal
--T
*-a
1
Saladin, B., Saladin, K., Dennis, E. & Degrémont A (1980). Preliminary epidemiological survey of
I
schistosomiasis in central and southern Liberia Acta Tropica, 37: 53-62.
Smithers, S.R (1956). On the ecology of schistosome vectors in the Gambia, with evidence of their role in
transmission. Transactions of the Royal Socîety of Tropical Medicine ami Hygiene, 50: 354-365.
Sodema, WA. Jr. (1979). A longitudii study of schistosome vector snail populations in Liberia American
Journal of Tropical Medicine andHygiene, 28: 531-538.
Talla I., Kongs, A., Verle, P., Belof J., Sarr S. & Coll, A.M. (1990). Outbreak of intestinal schistosomiasis in the
Senegal river basin. Annales de IaSoc~téBelge de Médecine Tropicale, 70: 173-180.
Vercruysse, J., Southgate, V.R. & Rolliin, D. (1985). The epidemiology of human and animal schistosomiasis
l
in the Senegal river basin. Acta Tropica, 42: 249-259.
Verle, P., Stelma, F., Desrumaux, P., Dieng A., Diaw, O,T., Kongs, A., Niang, h4., Sow, S., Talla, I., Sturroc~
RF., Gryseels, B. & Capron, A. (1994). Preliminaty smdy of urinai-y schistosomiasis in a village in the delta
of Senegal river basin. Transactions of the Royal Smie& of Tropical Medicine and Hygiene, 88: 401-405.
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