Schistosomiasis continues to exert a burden on public health in many communities ofthe tropics and sub-tropics even where National Control Programmes operate. This isdue to the fact that chemotherapy alone does not prevent re-infection. Wehypothesized at the outset of this project that mollusciciding, using localknowledge of snail breeding sites, could lead to an additional benefit to thepopulations at risk of infection. Mollusciciding is no longer a feature of manyschistosomiasis control programmes but our results press home the fact thatadditional measures against the snail population can have measurable benefits. Themajor observations of this study are that the snail numbers were reducedsignificantly at all sites along the Kambu River (Intervention area), and that bothprevalence and intensity of infection after chemotherapy remained lower in theDarajani community, close to Kambu river, than in the non-intervention area,Ulilinzi village, close to Thange River during the follow-up period. The combinationof chemotherapy followed by intermittent mollusciciding, therefore, had asignificant protective effect not only for the Darajani community but also forneighbouring communities also relying on the Kambu River for their waterrequirements, i.e. an estimated population of over 30,000 .
In an area where environmental conditions favour the proliferation of intermediatehost snails, Biomphalaria pfeifferi, complete elimination of snails may beimpossible to achieve, especially in stream habitats. Further, after molluscicidingsome snails may be found at some habitats, especially where suitable ponds or poolsform. Formation of such pools is usually caused by fast currents during rains thatwash away sand at some points along the riverbed, leaving deep furrows. The locationof such pools may change from one rainy season to the next. It is thereforenecessary to identify such sites and check for snails regularly. This is in markedcontrast to a study in the Gambia, where mollusciciding of seasonal rainwater poolsprogressively reduced snail population density and markedly reduced transmission ofS. haematobium.
Where snails occur in streams, as in the present study area, rainfall plays animportant role in reducing snail populations and the population “startsafresh” after each rainy season, but the effect is only temporary asre-population occurs rapidly. This is partly because close to the source of suchstreams several “snail pockets” may exist and some of them may be ofvery small size and yet support a thriving snail population. It may not be possibleto locate and treat all these reservoirs of snails. Thus after mollusciciding isstopped snail populations will soon recover. This study has shown that takingadvantage of rainfall by timed mollusciciding helps to keep snail populations lowfor a prolonged period. It may, however, not be straightforward to estimate whenrains may cause depletion of snails, since during this study the rainfall did notfollow the same pattern from year to year. During the March-May long rainy seasonsthe rains were generally low. The El Niño rains from November 1997 to January1998, in contrast, were devastating for snail habitats in both Kambu and Thangerivers, resulting in depletion of monthly snail recoveries in 1998 until August forKambu and September for Thange.
Focal mollusciciding using the compression sprayers hardly caused any noticeableeffect on non-target organisms, especially fish, but area-wide mollusciciding didkill some fish (data not included). Kambu River is not an important area for fishingnor does the community rely on fish as a source of food, but it joins a major river,the Athi, which is an important source of fish. Due to dilution, the molluscicideeffect was not felt in the Athi River during the area wide mollusciciding (the lateDr RF Sturrock, personal communication).
Several important questions are associated with the use of molluscicide to preventreinfection rather than relying on repeated treatment of exposed individuals tomaintain low reinfection levels. The cost effectiveness of the molluscicidingapproach is particularly important. In the present study, the cost of a single roundof chemotherapy of Darajani community was approximately 1.8 times higher than theannual mollusciciding cost (data not shown), but would obviously be different atpresent due to the reduction in praziquantel cost. Had all persons at risk beentreated annually, the relative cost would have been considerably higher, and such anapproach would not have prevented re-infection [2, 7]. It is also important to ask whether or not intermittent molluscicidingcan have a demonstrable effect on morbidity. In this context, it has been shown thatthe mollusciciding of the River Kambu was associated with both a lack of reinfectionamongst a cohort of school children from the nearby Mbeetwani community, and also aregression of hepatosplenomegaly over a three year period [24, 25]. A lack of re-infection amongst the Mbeetwani cohort led to observationsconcerning the effects of malaria exposure on hepatosplenic disease associated withschistosomiasis [26, 27]. Taken together, these two sets of studies indicate that whilst reductionin infection is clearly demonstrable through a combination of mollusciding andchemotherapy, any causally related effect on morbidity is likely to be a function ofvarious factors including the presence of co-infections of different species.Defining the nature of this ‘function’ will require a mixed-methodsapproach in future studies to ensure that a more complete understanding ofschistosome ecology is available to inform the development of optimised controlprogrammes. With some countries shifting schistosomiasis control strategy frommorbidity control to elimination [28, 29], transmission control becomes important and mollusciciding could becomeessential to achieve this and there is need to develop new molluscicide formulationsor new strategies of application adapted to the local conditions [29, 30].