Proceedings of ‘Worlshop on Medical and Vaerinq ...
Proceedings of ‘Worlshop on Medical and Vaerinq Maiacology in Afiica’, Harare, Zimbabwe, November 8-12,1999,
p. 91-104.
POPULATION DYNAMICS OF SCHISTOSOME INTERMEDIATE HOST SNAILS IN
THE SENEGAL RIVER AT R.ICJ3ARD-TOLL (DELTA OF THE SENEGAL RIVER
BASIN): A MALACOLOGICAL COLONIZATION
O.T. DIAW
Service de Parmicologie, L. N. E. R V/IS. R A., Dakar, Sénégal
ABSTRACT
Intestinal schistosomiasis was unknown in Richard-TOI1 until the recent outbreak in 1989-1990.
This was the first sign that ecological changes resulting &om the dam conshuction were having an
impact on the prevalence of schistosomiasis in the Senegal river basin. Surveys conducted in the
delta before construction of the dams in 1986 and 1990 showed that the area was f?ee of
Biomphulariu pfeifteri and populations of Bulinus spp. were very restricted iq distribution. The
objectives of the present study were to investigatethe dynamics of populations of schistosomiasis
snail hosts, particularly fluctuations in the density of B. pfe#ëri and Bulinus spp. in the Senegal
river at Richard-Tell. The study was done over a period of 30 months fiom July 1994 to December
1996.Biomphdaria pfe@eri, Bulinus truncatus, B. globosus, B. forskalii and Lymnaea ruztalensis
were the most commoniy cokcted species in the Senegal river at Richard-Tell. A total of 3,124
snails was collected. Biomphdizria pfi$ëri was more abundant (55.O?‘L of this total) than B.
truncatus (39.6%), B. globosus (3.7%) or B.forskalii (1 .O%). April, May and June were the months
when these snails were most abundant. Similarly, the period from April-August was when
schistosome infection rate in snails was hi@est (52-88 %). The overali infection rate in B. pfeifleri
was 52?4, B. truncatus 5% and 5. giobosus 14%. The infections in Bulinus spp. were not identified
to species and hence some coild he animai schistosomes. The prevalence of .S. haematobium in
Richard-Tell is not ven, high and B. ~runcutus is not involved in its transmission, but this situation
bas to be reconsiderzd since B. trum~utus was seen to be infected with S. haematobium in the delta.
This study bas permitted tk &ual process of colonization of the Senega! river and other
waterbodies at Richard-TOI1 by saails, particularly B. ,&$èri and Euhus spp., to be followed. This
bas enabled a reconsideration oftheir roles in the transmission of S. mansoni and S. haemotobium in
Richard-Tell,, the delta and the Senegal river basin. The Senegal river may be considered as the
major dispersa1 channel for these snails.
Keywords: schistosome, Bulinw Jobosus. B. truncatus. B forskalii. Biomphalaria pfeifferi.
Senegal river. Richard-Toll, Senegaf
IN-I’RODI JCTION
During the last decades major &anges in water management have resuited in an increase
in snail habitats in the Senegag river hasin and this bas facilitated transmission of
schistosomiasis particularly in the rtelta. ‘l%e recent outbreak of intestinal schistosomiasis in
Richard-Toll, previously unknown in the area, was the first sign that ecological changes
were having an impact on the prevatexe of schistosomiasis in the Senegal river basin. This
epidemiological situation is wel‘1 *locumenred and described elsewhere (Vercruysse et ai.
92
Dynamics of schktosome itiermediate hosts in the Senegal river
1985; Malek & Chaine 1989; Talla et al. 1990; Stelman et ul. 1993; Verle et al. 1994;
Piquet et ul. 1996). Before the construction of the Diama and Manantally dams in 1986 and
1990 respectively, previous malacological surveys in the delta were assessed. These studies
showed that a11 the waterbodies of the area, particularly the Senegal River, were fi-ee of
Biomphalaria pfei@èri, intermediate host of Schistosoma mansoni (Lemasson & Diaw
1977; Diaw 1980).
In 1976 and early 1980 many surveys conducted aiong the Senegal river basin,
particularly in the delta, showed that urinary schistosomiasis (S. huematobium) was
restricted to small foci, whereas the intestinal disease, Schistosomu mansoni, was unknown
in northem Senegal. Biomphalariapfezfiri, the intermediate host of S. marzsoni, was only
seen occasionally and was not well established and never in the river (Chaine & Malek
1983; Lemasson & Diaw 1977; Diaw 1980).
In relation to the outbreak of S. mansoni, a cross-sectional malacological survey was
carried out in order to study the density of B. pfezJ&i, its distribution and infection rates to
local& transmission sites (Diaw et al. 1991a). In Richard-Toll, the Senegal river, the
Taouey marigot, the Taouey canal and the irrigation canais and drains of the Senegal Sugar
Company (CSS) constituted the principal waterbodies. Transmission of S. mansoni was
taking place in a11 the districts of Richard-Toll, mainly ïn these waterhodies but with the
exception of the Senegal river where 31 sites were screened and a11 found free of B.
pfezjki. This same situation was described for the Senegal river at Dagana and Rosso from
1980 to 1990. It was not until May and July 1991, during a cross-sectional survey at four
sites along the banks of the Senegal river at Richard-Toll, that the first B. pfezj’& were
collected (32 in May and 29 in July). At the same time, 182 Minus sp. and 208 Lymnaea
nutalensis were coilected (Diaw et al. 1998a).
After these findings, other surveys conducted in the Senegal river have revealed the
permanence of snail populations in the river, particularly of B. pfeifferi, B, truncaturs and
B.globows, a sign that snails were colonizing the river.
‘
The objectives of the present study were to investigate the dynamics of populations of
schistosomiasis snail hosts, particularly fluctuations in the density of B. pfezJëri and
Bulinus spp. in the Senegal river at Richard-Toll, scene of a recent malacological
colonization. This paper reports the findings of monthly surveys at the ten sites on the river
from July 1994 to December 1996.
MA=RIAL, AND METHODS
St* area
The town of Richard-Toi1 lies on the northem bank of the Senegal river which flows
westwards for another 100 km to its mouth on the Atlantic Ocean. The Senegal river is the
second iargest river in West Afiica and forms the border bctween Senegal and Mauritania
(Fig. 1). Richard-Toi1 has been described in demi1 elsewhere as bas the epidemiology of
Schistosoma nzansoni there (Vercruysse et al. 1985; Diaw et al. 1990; Talla et ai. 1990;
Talla et al. 1992; Stelma et al. 1993; Vercruysse et al. 1994; Piquet et al. 1996).
Tbe area and climate are typically Sahelian. There is a hmited rainy season from July to
September with a long dry period from October to June. The average annuai rainfail is 1t400
mm. Temperatures reach 40°C in summer but fa11 to 18-25” C in winter. In Richard-Ta11 the
O.T. Diaw
hydrographie network is dense (Figure 2) and is formed by (i) the Senegal river which
borders the whole of the town; the Taouey marigot, an old meandering river which
connected Lac de Guiers with the Senegal river; the Taouey canal, a man-made canal which
is straight and deep and replaces the Taouey marigot and now connects the Senegal river
with the Lac de Guiers; the irrigation canais and drains of the Senegal Sugar Company.
hf~ URITA 4!2//.4
Ii
/
/,’
/
FIG. 1. Map of the Senegal river delta showing tire study area.
Ail these waterbodies are well frequented by people and constitute four epidemiological
systems. They are the sources of domestic water for the population of Richard-Toil. Along
the banks of the river, most water contact takes place at well defined sites that are used for
domestic purposes (laundry, washing, washing utensils, fetching water, watering animais,
etc.) or recreational activities by chiidren (piaying, swimming, etc.).
Water contact is intense at a11 these sites which are sometimes very crowded. The water
level is subject to considerable fiuctuations. Generaliy, there is not much vegetation along
the banks of the river but at some sites Graminae, Cyperus as well as Pnlygonum, Typh,
Pottunogeton, Pistia and N~*mphea were found.
Snail surveys: Selectiort df snail sampling sites ad sampling procedures
A preliminary survey was cilnducted on the Senegal river at Richard-Tell along the
banks of the different quaners of Ndiaw, Ndiangue, Escale and Gaya to identify the water
contact sites where people were mvolved in domestic and recreational activities (Fig. 2).
94
Dynamics of schktosome intermediate hosts in the Senegal river
The results of this survey and those of previous surveys allowed the selection of ten sites on
the river:
- 3 sites: nos. 1 - 3 - in the Ndiaw quarter
- 3 sites: nos. 4 - 6 - in the Ndiangue quarter
- 2 sites: nos. 7 - 8 - in the Escale quarter
- 2 sites: nos. 9 - 10 - in the Gaya quarter.
Snail searches were performed every month fiom July 1994 to December 1996 (30
months). At monthly samplings, snails were collected at each site by two technicians for ten
minutes using scoop nets. Only potential intermediate host snails of schistosomes were
collected. The surface water temperaiure was recorded at each site as was the vegetation
and other site conditions. At fhe end of the searches at the ten sites, the snaiis collccted
were taken to a field laboratory at Richard-Toi1 where they were washed, identified (Brown
1994) and counted. Afterwards they were placed individually in glass tubes with I!I 10 ml of
fïltered water and exposed to direct sunlight or electric light for 30-40 minutes to induce
cercarial shedding. Cercariae were checked under a dissecting microscope and only those of
Schistosomu were recorded (Frandsen & Christensen 1984). Snails which did not release
cercariae were kept ovcrnight and a second examination for cercariae shedding u;as done
before they were destroyed.
O.T. Diaw
9 5
REXJLTS
Snailfindings in the Senegal river
Biomphahria pfeifseri, Bulinus globosus, B. truncatus and B. forskalii are the species
recorded in the Senegal river at Richard-Toll. Biomphalaria pfe@eri was the dominant
species; it was present at all ten water contact sites selected and during the survey. The
second most common species was Bulinus truncatus - also present at a11 sites and for the
total period of the survey. The other two species, Bulinus globosus and B. forskalii were
present at iow densities, were not present at a11 sites and not for the whole survey. Bulinus
globosw was absent from site 8 and B. forskalii was not found at sites 2 and 7.
During the survey a totai of 3,124 snails was collected at the ten selected water contact
sites: BiomphaIariapfe@ëri
with 1,736 specimens was dominant and represented 55.5% of
the total collected in the river during the survey. Bulinus truncatus was the second species
with 1,238 specimens and accounted for 39.6% of the total; B. globosus accounted for 3.7%
(118 specimens) and B. forshlii for 1% (32 specimens).
Snail population dynamics at Richard- Tell
Snail counts varied seasonally with peak densities occurring during different periods:
July-August 1994, April-June 1996, May and August 1995 for B. pfeijfki and July-August
1994, May 1995 and January and June 1996 for Bulinus spp. (Fig. 3). This correlated with
the time over which the water temperature increased from 20” to 30” C. Sites 3,4, 5, and 6
produced the highest numbers of B. pfezjëri and Bulinu~ spp. which together represented
21.8, 15.4, 12.7 and 1 1.2% of total snail counts respectively.
.
.~~-
-
-m-- 13. truncatus ; B.globosus
~B.pf*itMi - - - - Temperature
2 5 0
3 5
3 0
2 0 0
25 -
,o
I 0
JASONDJFklAMJJASONDJFMAMJJASOND
1994
195
1996
FIG. 3. Total number of snails c&cted in ten sampiing sites in the Senegal river together with
average water temperature from Jr : i y I 994 tO December 1996.
96
Qynamics of schistosome intermediate hosts in the Senegal river
During the survey, the process of colonization of the river was demonstrated by the
increasing numbers of snails collected per year from 1994 to 1996. From July to December
1994 a total of 571 snails was collected representing 18.2% of the total for the whole
survey. This fauna comprised 233 B. pfeijki (12.8%), 348 Bulinus spp. (25.0%), i.e. 332 B.
truncatw, 13 B. forskalii and 3 B. globosus. From January to December 1995, the second
period, 1,038 snails were collected representing 33% of the total recorded during the
survey. This comprised 612 B. pfeifléri (35.2%) and 426 Bulinus spp. (30.6%), i.e. 385 B.
truncatus, 37 B. globosus and 4 B. forskalii. During the last period, January to December
1996, 1,515 snails were collected (48.4% of the total recorded during the survey): 901 B.
pfeifseri (5 1.9%) and 614 Bulinus spp. (44.2%), i.e. 52 1 B. truncatq 78 B. globosus and 15
B. forskdii. Table 1 and Figs. 5 & 7 show how snail abundance varied between sites with
highest numbers at sites 4 and 5 and the lowest at sites 2 and 10.
TABLE 1. Number of snails, number infected with schistosomes and the infection rate (%) of
Biomphalaria pfeiffri, Bulinus truncatus, B. globosus and B. forskalii collected in each of ten
sampling
_ - sites in the Senegal river at Richard-Toi1 fiom July 1994 to December 1996.
Biomphalaria
Bulinus
Bulinus
Bulinus
pfelferi
mimatus
globosus
forskalii
07.r$
Site
no.
1
88 30 34 98 0 0 0 0 0 3 0
0 189 6
2
58 17 29 96 2 2 1 0 0 0 0
0 155 4.9
3
173
79 45.6 170
5
2.9 1 0 0 8 0
0 352 11.2
4
398 215
54 237 15
6.3 40
5 12.2 7
1 14.2 682 21.8
5
261
137 52.4 192 14
7.2
25
3
12 4 0
0 482 15.4
6
254
159 62.5 124 10 8 17
5 29.4 4 0
0 399 12.7
7 99
45 45.4 73 3
4.1 24
4 16.6 0 0
0 196 6.27
8
142
61 42.9 111 7
4.3 0 0 0 2 0
0 255 8.16
9
177
109 61.5 73 3
4.1 3 0 0 2 0
0 257 8.22
10 86
47 54.6
44
2
3.1 7 0 0 2 0
0 157 5
Total 1736 899 51.7 1238 2
4.9 118
17 14.4 32
1
3.1 3124
Although the variation in total counts for individual species differed signifkantly
between the ten sites, similar pattems could be detected at a11 of them. Biomphahria
pfeifferi and B. truncatus had a scattered distribution in the river and were found at a11 sites
but at differing densities; B. globosus and B. ,forskalii, however, presented a different
picture. Bulinus globosus occurt-ed at ail sites except 1 and 8. The highest number coltected
per period was 28 specimens in November 1996 and per site, 40 specimens at site 4.
Bulinus forskdii was less common with a total of only 32 specimens. It occurred at low
densities at all sites except 2 and 7. The highest number collected per period was 11 in July
1996 and per site, 8 specimens at site 3.
The months in which the largest numbers of snails were collected were August 1994
(229 specimens), May 1995 (2841, April 1996 (258) and May 1996 (229) (Fig. 3).
O.T. Diaw
97
Infestation of snails: transmission of schistosomiasis
In Senegal, B. pfeifsi is the intermediate host for Schistosoma mansoni and B. globosus
the snail host for S. haematobium and, together with B. truncatus and B. forskzdii, of S.
bovis as well (Diaw & Vassiliades 1987; Diaw 1998). Bulinus truncatus was never found
infected with S. haematobiurn in the study area.
The overall infection rate of B. pfez&ri was 51.7% (899/1,736). Fig. 4 shows that in
1994, peak infection rates were fiom August to September (48-53%). In 1995, the peak was
in March (62%) but the highest infection rate (86%) was in August. A similar pattem was
recorded in 1996, i.e. high infection rates from March to August with a peak in May (73%).
Maximum rates thus occurred in March 1995 (62.2%), August 1995 (86.4%), April 1996
(67.6%) and May 1996 (73%).
300 r-.-.---..- - _. -
---- ---_ l(-jo
_-- .-... .-..- ^. ._~..
0 Uninfected m Infected -Infection rate (%) L 9.
-
250 ;
- 80
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
1994
1995
1996
FIG. 4. Total number of Biamphalariap&z@eri
collected in the ten sampling sites in the Senegal river,
number infected with schistosomes and the infection rate.
Fluctuations in infection: :ate were observed not only from month to month (Fig. 4) but
also from site to site (Fig. 6). Schistosome-infecte B. pfeifseri were found during almost
the whole survey period, exzept October 1994 and February 1996 when snail densities were
low. Nevertheless, transmkion took place at a11 sites, sometimes with very high infection
rates ranging from 29% to 62.5%. Sites 4, 5, 6. 9 and 10 had the highest rates with 62.5%
(159/254), 61.5% (109/1?7), 54.6% (47/86), 54.0% (215’398) and 52.4% (137/261)
respectively. Eight of the ten selccted sites had infection rates above 42%, demonstrating
that transmission was very intense.
In 1996 the pattem w& the same with high infection rates from March to August and a
peak in May (73%) but high rates were aiso recorded in April(67.6%).
98
Llynamics of schistosome intermediate hosts in the Senegal river
500
400
u)
.Z
g 300
‘i;. 200
z
100
0
1 2
5 6 7 8
Site
FIG. 5. Number of Biomphalaria pfeifferi, Bulinus truncatus, B. glohosus and 8. forskxlii collected in
each of ten sampling sites in the Senegal river fiom July 1994 to December 1996.
I
a
i 30 üm
20 I
1
2
3
4
5
6
7
8
9
1 0
Site
-------.
-
-
m Infected ~Total --A- Infection rate
FIG. 6. Number of Biomphalaria pferfiri collected in each of‘ ;CR sampling sites in the Senepal iver
from Juiy 1994 to December 1996, the number of snails infecied with schistosomes and the inf’ction
rate (%),
Schistosome-infected B. pfezj+ëri were found dur& ehe whole surve~ escepting October
1994 and February 1996 which were periods with very iow snail dcnsities (4 and 75
specimens respectively). Nevertheless, S. rnu~~~or~i tran~n~ission took piace at a11 sites anJ
O.T. Diaw
infection rates were high, ranging from 29% to 62,5%. Sites 4, 5, 6, 9 and 10 had the
highest infection rates with 54% (215/398), 52.4% (137/261), 62.5% (159/254), 61.5%
(109/177) and 54.6% (47/86) respectively. Eight of the ten selected sites had infection rates
above 42% which demonstmte that transmission was very intense. Tbe lowest infection rate
was 9.6% in November 1995.
Concerning Bulinus spp., the overall infection rate for B. globosus was 14.4% (17/118),
4.9% (61/1,238) for B. truncutus and 3.1% (1/32) for B. forskdii. However, the infections
in these Bulinus spp. were not identified to species and they could be S. haematobium or
animal schistosomiasis (S. bovi.s). The epidemiological role of these Bulinus spp. in the
delta is well documented (Vercruysse, Southgate Bi Rollinson 1985; Diaw & Vassiliades
1987; Piquet et ul. 1996) and more recently the status of B. truncutus in the delta has had to
be reconsidered since they were found naturally i&cted with S. haematobium (Emould
1996; Diaw 1998). The periods of high transmission from B. truncatus were: July to
September 1994 (5-8%), March-August 1995 (12.5 %), July 1995 (6%), May 1996 (14%)
and December 1996 (8%).
Transmission was recorded at a11 ten sites except site 1. The overall infection rate varied
from 2-8% with sites 4, 5, 6 and 8 having the highest infection rates, i.e. 6.3% (15/237),
7.2% (14/192), 8.0% (10024) and 6.3% (7/111) respectively.
The periods of high transmission fiom Buhus glob0su.s were May 1994, May, July and
September 1996 with 50%, 7% and 14% respectivefy. Four sites were involved: sites 4,5,6
and 7 with infection rates of 12.5% (5/40), 12% (3/25), 29.4% (5/17) and 16.6% (4/24)
respectively.
The only infected B. forskalii was found in September 1996 (1/2) giving an overall
infection rate of 3.1% (1/32). This snail came froc site 4 with an infection rate of 14.2%
(117).
DISCUSSION
The results of this survey resemble those of other similar studies in West Afiica
(McCullough 1957; Onabamiro 1972; Sodeman 1979; Saladin et al. 1980; Coulibaly &
Madsen 1990; Istifanus, Fabiyi & Ndifon 1996; Diaw 1998). Biomphahria pfeifseri had
never been recorded in the Senegal river befze 1991 but since 1988-89, after the
construction of the dams, it bas colonized waterbodies in the delta (the Lampsar marigot
and irrigation canais), particularly irrigation canais and drains of CSS, Taouey marigot and
Taouey canal at Richard-Toi1 (Diaw et af. 199 1 a & 1991 b; Verle et al. 1994).
Data from surveys before the Diama dam was bu& and those alter its opening, cross-
sectional surveys in 1990, 1991 and 1992, a longitudit~al survey in 1992-94 and the present
study from 1994 to 1996, allow the process of cokkration of the Senegal river to be
followed in terms of the number of species of schktosomiasis intermediate host snails
becoming established and the changing sizes of thGr populations. Several physical and
chemical changes to the environment since the opening of the dams have favoured the
establishment of B. pfeifferi in the delta (cf.. the ou-h& of S. mansoni at Richard-Toll and
its extension to the delta) and now in the Senegal river as well. These changes have aiso
favoured Buhur spp., particularly B. globosus whicn is increasing in population size and
extending its habitat.
p. 91-104.
POPULATION DYNAMICS OF SCHISTOSOME INTERMEDIATE HOST SNAILS IN
THE SENEGAL RIVER AT R.ICJ3ARD-TOLL (DELTA OF THE SENEGAL RIVER
BASIN): A MALACOLOGICAL COLONIZATION
O.T. DIAW
Service de Parmicologie, L. N. E. R V/IS. R A., Dakar, Sénégal
ABSTRACT
Intestinal schistosomiasis was unknown in Richard-TOI1 until the recent outbreak in 1989-1990.
This was the first sign that ecological changes resulting &om the dam conshuction were having an
impact on the prevalence of schistosomiasis in the Senegal river basin. Surveys conducted in the
delta before construction of the dams in 1986 and 1990 showed that the area was f?ee of
Biomphulariu pfeifteri and populations of Bulinus spp. were very restricted iq distribution. The
objectives of the present study were to investigatethe dynamics of populations of schistosomiasis
snail hosts, particularly fluctuations in the density of B. pfe#ëri and Bulinus spp. in the Senegal
river at Richard-Tell. The study was done over a period of 30 months fiom July 1994 to December
1996.Biomphdaria pfe@eri, Bulinus truncatus, B. globosus, B. forskalii and Lymnaea ruztalensis
were the most commoniy cokcted species in the Senegal river at Richard-Tell. A total of 3,124
snails was collected. Biomphdizria pfi$ëri was more abundant (55.O?‘L of this total) than B.
truncatus (39.6%), B. globosus (3.7%) or B.forskalii (1 .O%). April, May and June were the months
when these snails were most abundant. Similarly, the period from April-August was when
schistosome infection rate in snails was hi@est (52-88 %). The overali infection rate in B. pfeifleri
was 52?4, B. truncatus 5% and 5. giobosus 14%. The infections in Bulinus spp. were not identified
to species and hence some coild he animai schistosomes. The prevalence of .S. haematobium in
Richard-Tell is not ven, high and B. ~runcutus is not involved in its transmission, but this situation
bas to be reconsiderzd since B. trum~utus was seen to be infected with S. haematobium in the delta.
This study bas permitted tk &ual process of colonization of the Senega! river and other
waterbodies at Richard-TOI1 by saails, particularly B. ,&$èri and Euhus spp., to be followed. This
bas enabled a reconsideration oftheir roles in the transmission of S. mansoni and S. haemotobium in
Richard-Tell,, the delta and the Senegal river basin. The Senegal river may be considered as the
major dispersa1 channel for these snails.
Keywords: schistosome, Bulinw Jobosus. B. truncatus. B forskalii. Biomphalaria pfeifferi.
Senegal river. Richard-Toll, Senegaf
IN-I’RODI JCTION
During the last decades major &anges in water management have resuited in an increase
in snail habitats in the Senegag river hasin and this bas facilitated transmission of
schistosomiasis particularly in the rtelta. ‘l%e recent outbreak of intestinal schistosomiasis in
Richard-Toll, previously unknown in the area, was the first sign that ecological changes
were having an impact on the prevatexe of schistosomiasis in the Senegal river basin. This
epidemiological situation is wel‘1 *locumenred and described elsewhere (Vercruysse et ai.
92
Dynamics of schktosome itiermediate hosts in the Senegal river
1985; Malek & Chaine 1989; Talla et al. 1990; Stelman et ul. 1993; Verle et al. 1994;
Piquet et ul. 1996). Before the construction of the Diama and Manantally dams in 1986 and
1990 respectively, previous malacological surveys in the delta were assessed. These studies
showed that a11 the waterbodies of the area, particularly the Senegal River, were fi-ee of
Biomphalaria pfei@èri, intermediate host of Schistosoma mansoni (Lemasson & Diaw
1977; Diaw 1980).
In 1976 and early 1980 many surveys conducted aiong the Senegal river basin,
particularly in the delta, showed that urinary schistosomiasis (S. huematobium) was
restricted to small foci, whereas the intestinal disease, Schistosomu mansoni, was unknown
in northem Senegal. Biomphalariapfezfiri, the intermediate host of S. marzsoni, was only
seen occasionally and was not well established and never in the river (Chaine & Malek
1983; Lemasson & Diaw 1977; Diaw 1980).
In relation to the outbreak of S. mansoni, a cross-sectional malacological survey was
carried out in order to study the density of B. pfezJ&i, its distribution and infection rates to
local& transmission sites (Diaw et al. 1991a). In Richard-Toll, the Senegal river, the
Taouey marigot, the Taouey canal and the irrigation canais and drains of the Senegal Sugar
Company (CSS) constituted the principal waterbodies. Transmission of S. mansoni was
taking place in a11 the districts of Richard-Toll, mainly ïn these waterhodies but with the
exception of the Senegal river where 31 sites were screened and a11 found free of B.
pfezjki. This same situation was described for the Senegal river at Dagana and Rosso from
1980 to 1990. It was not until May and July 1991, during a cross-sectional survey at four
sites along the banks of the Senegal river at Richard-Toll, that the first B. pfezj’& were
collected (32 in May and 29 in July). At the same time, 182 Minus sp. and 208 Lymnaea
nutalensis were coilected (Diaw et al. 1998a).
After these findings, other surveys conducted in the Senegal river have revealed the
permanence of snail populations in the river, particularly of B. pfeifferi, B, truncaturs and
B.globows, a sign that snails were colonizing the river.
‘
The objectives of the present study were to investigate the dynamics of populations of
schistosomiasis snail hosts, particularly fluctuations in the density of B. pfezJëri and
Bulinus spp. in the Senegal river at Richard-Toll, scene of a recent malacological
colonization. This paper reports the findings of monthly surveys at the ten sites on the river
from July 1994 to December 1996.
MA=RIAL, AND METHODS
St* area
The town of Richard-Toi1 lies on the northem bank of the Senegal river which flows
westwards for another 100 km to its mouth on the Atlantic Ocean. The Senegal river is the
second iargest river in West Afiica and forms the border bctween Senegal and Mauritania
(Fig. 1). Richard-Toi1 has been described in demi1 elsewhere as bas the epidemiology of
Schistosoma nzansoni there (Vercruysse et al. 1985; Diaw et al. 1990; Talla et ai. 1990;
Talla et al. 1992; Stelma et al. 1993; Vercruysse et al. 1994; Piquet et al. 1996).
Tbe area and climate are typically Sahelian. There is a hmited rainy season from July to
September with a long dry period from October to June. The average annuai rainfail is 1t400
mm. Temperatures reach 40°C in summer but fa11 to 18-25” C in winter. In Richard-Ta11 the
O.T. Diaw
hydrographie network is dense (Figure 2) and is formed by (i) the Senegal river which
borders the whole of the town; the Taouey marigot, an old meandering river which
connected Lac de Guiers with the Senegal river; the Taouey canal, a man-made canal which
is straight and deep and replaces the Taouey marigot and now connects the Senegal river
with the Lac de Guiers; the irrigation canais and drains of the Senegal Sugar Company.
hf~ URITA 4!2//.4
Ii
/
/,’
/
FIG. 1. Map of the Senegal river delta showing tire study area.
Ail these waterbodies are well frequented by people and constitute four epidemiological
systems. They are the sources of domestic water for the population of Richard-Toil. Along
the banks of the river, most water contact takes place at well defined sites that are used for
domestic purposes (laundry, washing, washing utensils, fetching water, watering animais,
etc.) or recreational activities by chiidren (piaying, swimming, etc.).
Water contact is intense at a11 these sites which are sometimes very crowded. The water
level is subject to considerable fiuctuations. Generaliy, there is not much vegetation along
the banks of the river but at some sites Graminae, Cyperus as well as Pnlygonum, Typh,
Pottunogeton, Pistia and N~*mphea were found.
Snail surveys: Selectiort df snail sampling sites ad sampling procedures
A preliminary survey was cilnducted on the Senegal river at Richard-Tell along the
banks of the different quaners of Ndiaw, Ndiangue, Escale and Gaya to identify the water
contact sites where people were mvolved in domestic and recreational activities (Fig. 2).
94
Dynamics of schktosome intermediate hosts in the Senegal river
The results of this survey and those of previous surveys allowed the selection of ten sites on
the river:
- 3 sites: nos. 1 - 3 - in the Ndiaw quarter
- 3 sites: nos. 4 - 6 - in the Ndiangue quarter
- 2 sites: nos. 7 - 8 - in the Escale quarter
- 2 sites: nos. 9 - 10 - in the Gaya quarter.
Snail searches were performed every month fiom July 1994 to December 1996 (30
months). At monthly samplings, snails were collected at each site by two technicians for ten
minutes using scoop nets. Only potential intermediate host snails of schistosomes were
collected. The surface water temperaiure was recorded at each site as was the vegetation
and other site conditions. At fhe end of the searches at the ten sites, the snaiis collccted
were taken to a field laboratory at Richard-Toi1 where they were washed, identified (Brown
1994) and counted. Afterwards they were placed individually in glass tubes with I!I 10 ml of
fïltered water and exposed to direct sunlight or electric light for 30-40 minutes to induce
cercarial shedding. Cercariae were checked under a dissecting microscope and only those of
Schistosomu were recorded (Frandsen & Christensen 1984). Snails which did not release
cercariae were kept ovcrnight and a second examination for cercariae shedding u;as done
before they were destroyed.
O.T. Diaw
9 5
REXJLTS
Snailfindings in the Senegal river
Biomphahria pfeifseri, Bulinus globosus, B. truncatus and B. forskalii are the species
recorded in the Senegal river at Richard-Toll. Biomphalaria pfe@eri was the dominant
species; it was present at all ten water contact sites selected and during the survey. The
second most common species was Bulinus truncatus - also present at a11 sites and for the
total period of the survey. The other two species, Bulinus globosus and B. forskalii were
present at iow densities, were not present at a11 sites and not for the whole survey. Bulinus
globosw was absent from site 8 and B. forskalii was not found at sites 2 and 7.
During the survey a totai of 3,124 snails was collected at the ten selected water contact
sites: BiomphaIariapfe@ëri
with 1,736 specimens was dominant and represented 55.5% of
the total collected in the river during the survey. Bulinus truncatus was the second species
with 1,238 specimens and accounted for 39.6% of the total; B. globosus accounted for 3.7%
(118 specimens) and B. forshlii for 1% (32 specimens).
Snail population dynamics at Richard- Tell
Snail counts varied seasonally with peak densities occurring during different periods:
July-August 1994, April-June 1996, May and August 1995 for B. pfeijfki and July-August
1994, May 1995 and January and June 1996 for Bulinus spp. (Fig. 3). This correlated with
the time over which the water temperature increased from 20” to 30” C. Sites 3,4, 5, and 6
produced the highest numbers of B. pfezjëri and Bulinu~ spp. which together represented
21.8, 15.4, 12.7 and 1 1.2% of total snail counts respectively.
.
.~~-
-
-m-- 13. truncatus ; B.globosus
~B.pf*itMi - - - - Temperature
2 5 0
3 5
3 0
2 0 0
25 -
,o
I 0
JASONDJFklAMJJASONDJFMAMJJASOND
1994
195
1996
FIG. 3. Total number of snails c&cted in ten sampiing sites in the Senegal river together with
average water temperature from Jr : i y I 994 tO December 1996.
96
Qynamics of schistosome intermediate hosts in the Senegal river
During the survey, the process of colonization of the river was demonstrated by the
increasing numbers of snails collected per year from 1994 to 1996. From July to December
1994 a total of 571 snails was collected representing 18.2% of the total for the whole
survey. This fauna comprised 233 B. pfeijki (12.8%), 348 Bulinus spp. (25.0%), i.e. 332 B.
truncatw, 13 B. forskalii and 3 B. globosus. From January to December 1995, the second
period, 1,038 snails were collected representing 33% of the total recorded during the
survey. This comprised 612 B. pfeifléri (35.2%) and 426 Bulinus spp. (30.6%), i.e. 385 B.
truncatus, 37 B. globosus and 4 B. forskalii. During the last period, January to December
1996, 1,515 snails were collected (48.4% of the total recorded during the survey): 901 B.
pfeifseri (5 1.9%) and 614 Bulinus spp. (44.2%), i.e. 52 1 B. truncatq 78 B. globosus and 15
B. forskdii. Table 1 and Figs. 5 & 7 show how snail abundance varied between sites with
highest numbers at sites 4 and 5 and the lowest at sites 2 and 10.
TABLE 1. Number of snails, number infected with schistosomes and the infection rate (%) of
Biomphalaria pfeiffri, Bulinus truncatus, B. globosus and B. forskalii collected in each of ten
sampling
_ - sites in the Senegal river at Richard-Toi1 fiom July 1994 to December 1996.
Biomphalaria
Bulinus
Bulinus
Bulinus
pfelferi
mimatus
globosus
forskalii
07.r$
Site
no.
1
88 30 34 98 0 0 0 0 0 3 0
0 189 6
2
58 17 29 96 2 2 1 0 0 0 0
0 155 4.9
3
173
79 45.6 170
5
2.9 1 0 0 8 0
0 352 11.2
4
398 215
54 237 15
6.3 40
5 12.2 7
1 14.2 682 21.8
5
261
137 52.4 192 14
7.2
25
3
12 4 0
0 482 15.4
6
254
159 62.5 124 10 8 17
5 29.4 4 0
0 399 12.7
7 99
45 45.4 73 3
4.1 24
4 16.6 0 0
0 196 6.27
8
142
61 42.9 111 7
4.3 0 0 0 2 0
0 255 8.16
9
177
109 61.5 73 3
4.1 3 0 0 2 0
0 257 8.22
10 86
47 54.6
44
2
3.1 7 0 0 2 0
0 157 5
Total 1736 899 51.7 1238 2
4.9 118
17 14.4 32
1
3.1 3124
Although the variation in total counts for individual species differed signifkantly
between the ten sites, similar pattems could be detected at a11 of them. Biomphahria
pfeifferi and B. truncatus had a scattered distribution in the river and were found at a11 sites
but at differing densities; B. globosus and B. ,forskalii, however, presented a different
picture. Bulinus globosus occurt-ed at ail sites except 1 and 8. The highest number coltected
per period was 28 specimens in November 1996 and per site, 40 specimens at site 4.
Bulinus forskdii was less common with a total of only 32 specimens. It occurred at low
densities at all sites except 2 and 7. The highest number collected per period was 11 in July
1996 and per site, 8 specimens at site 3.
The months in which the largest numbers of snails were collected were August 1994
(229 specimens), May 1995 (2841, April 1996 (258) and May 1996 (229) (Fig. 3).
O.T. Diaw
97
Infestation of snails: transmission of schistosomiasis
In Senegal, B. pfeifsi is the intermediate host for Schistosoma mansoni and B. globosus
the snail host for S. haematobium and, together with B. truncatus and B. forskzdii, of S.
bovis as well (Diaw & Vassiliades 1987; Diaw 1998). Bulinus truncatus was never found
infected with S. haematobiurn in the study area.
The overall infection rate of B. pfez&ri was 51.7% (899/1,736). Fig. 4 shows that in
1994, peak infection rates were fiom August to September (48-53%). In 1995, the peak was
in March (62%) but the highest infection rate (86%) was in August. A similar pattem was
recorded in 1996, i.e. high infection rates from March to August with a peak in May (73%).
Maximum rates thus occurred in March 1995 (62.2%), August 1995 (86.4%), April 1996
(67.6%) and May 1996 (73%).
300 r-.-.---..- - _. -
---- ---_ l(-jo
_-- .-... .-..- ^. ._~..
0 Uninfected m Infected -Infection rate (%) L 9.
-
250 ;
- 80
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
1994
1995
1996
FIG. 4. Total number of Biamphalariap&z@eri
collected in the ten sampling sites in the Senegal river,
number infected with schistosomes and the infection rate.
Fluctuations in infection: :ate were observed not only from month to month (Fig. 4) but
also from site to site (Fig. 6). Schistosome-infecte B. pfeifseri were found during almost
the whole survey period, exzept October 1994 and February 1996 when snail densities were
low. Nevertheless, transmkion took place at a11 sites, sometimes with very high infection
rates ranging from 29% to 62.5%. Sites 4, 5, 6. 9 and 10 had the highest rates with 62.5%
(159/254), 61.5% (109/1?7), 54.6% (47/86), 54.0% (215’398) and 52.4% (137/261)
respectively. Eight of the ten selccted sites had infection rates above 42%, demonstrating
that transmission was very intense.
In 1996 the pattem w& the same with high infection rates from March to August and a
peak in May (73%) but high rates were aiso recorded in April(67.6%).
98
Llynamics of schistosome intermediate hosts in the Senegal river
500
400
u)
.Z
g 300
‘i;. 200
z
100
0
1 2
5 6 7 8
Site
FIG. 5. Number of Biomphalaria pfeifferi, Bulinus truncatus, B. glohosus and 8. forskxlii collected in
each of ten sampling sites in the Senegal river fiom July 1994 to December 1996.
I
a
i 30 üm
20 I
1
2
3
4
5
6
7
8
9
1 0
Site
-------.
-
-
m Infected ~Total --A- Infection rate
FIG. 6. Number of Biomphalaria pferfiri collected in each of‘ ;CR sampling sites in the Senepal iver
from Juiy 1994 to December 1996, the number of snails infecied with schistosomes and the inf’ction
rate (%),
Schistosome-infected B. pfezj+ëri were found dur& ehe whole surve~ escepting October
1994 and February 1996 which were periods with very iow snail dcnsities (4 and 75
specimens respectively). Nevertheless, S. rnu~~~or~i tran~n~ission took piace at a11 sites anJ
O.T. Diaw
infection rates were high, ranging from 29% to 62,5%. Sites 4, 5, 6, 9 and 10 had the
highest infection rates with 54% (215/398), 52.4% (137/261), 62.5% (159/254), 61.5%
(109/177) and 54.6% (47/86) respectively. Eight of the ten selected sites had infection rates
above 42% which demonstmte that transmission was very intense. Tbe lowest infection rate
was 9.6% in November 1995.
Concerning Bulinus spp., the overall infection rate for B. globosus was 14.4% (17/118),
4.9% (61/1,238) for B. truncutus and 3.1% (1/32) for B. forskdii. However, the infections
in these Bulinus spp. were not identified to species and they could be S. haematobium or
animal schistosomiasis (S. bovi.s). The epidemiological role of these Bulinus spp. in the
delta is well documented (Vercruysse, Southgate Bi Rollinson 1985; Diaw & Vassiliades
1987; Piquet et ul. 1996) and more recently the status of B. truncutus in the delta has had to
be reconsidered since they were found naturally i&cted with S. haematobium (Emould
1996; Diaw 1998). The periods of high transmission from B. truncatus were: July to
September 1994 (5-8%), March-August 1995 (12.5 %), July 1995 (6%), May 1996 (14%)
and December 1996 (8%).
Transmission was recorded at a11 ten sites except site 1. The overall infection rate varied
from 2-8% with sites 4, 5, 6 and 8 having the highest infection rates, i.e. 6.3% (15/237),
7.2% (14/192), 8.0% (10024) and 6.3% (7/111) respectively.
The periods of high transmission fiom Buhus glob0su.s were May 1994, May, July and
September 1996 with 50%, 7% and 14% respectivefy. Four sites were involved: sites 4,5,6
and 7 with infection rates of 12.5% (5/40), 12% (3/25), 29.4% (5/17) and 16.6% (4/24)
respectively.
The only infected B. forskalii was found in September 1996 (1/2) giving an overall
infection rate of 3.1% (1/32). This snail came froc site 4 with an infection rate of 14.2%
(117).
DISCUSSION
The results of this survey resemble those of other similar studies in West Afiica
(McCullough 1957; Onabamiro 1972; Sodeman 1979; Saladin et al. 1980; Coulibaly &
Madsen 1990; Istifanus, Fabiyi & Ndifon 1996; Diaw 1998). Biomphahria pfeifseri had
never been recorded in the Senegal river befze 1991 but since 1988-89, after the
construction of the dams, it bas colonized waterbodies in the delta (the Lampsar marigot
and irrigation canais), particularly irrigation canais and drains of CSS, Taouey marigot and
Taouey canal at Richard-Toi1 (Diaw et af. 199 1 a & 1991 b; Verle et al. 1994).
Data from surveys before the Diama dam was bu& and those alter its opening, cross-
sectional surveys in 1990, 1991 and 1992, a longitudit~al survey in 1992-94 and the present
study from 1994 to 1996, allow the process of cokkration of the Senegal river to be
followed in terms of the number of species of schktosomiasis intermediate host snails
becoming established and the changing sizes of thGr populations. Several physical and
chemical changes to the environment since the opening of the dams have favoured the
establishment of B. pfeifferi in the delta (cf.. the ou-h& of S. mansoni at Richard-Toll and
its extension to the delta) and now in the Senegal river as well. These changes have aiso
favoured Buhur spp., particularly B. globosus whicn is increasing in population size and
extending its habitat.