Long term study on the effect of mollusciciding with niclosamide in stream habitats on the transmission of schistosomiasis mansoni after community-based chemotherapy in Makueni District, Kenya

Abstract

Background: Schistosoma mansoni infection is a persistent public health problem in many Kenyan communities. Although praziquantel is available, re-infection after chemotherapy treatment is inevitable, especially among children. Chemotherapy followed by intermittent mollusciciding of habitats of Biomphalaria pfeifferi, the intermediate host snail, may have longer term benefits, especially if timed to coincide with natural fluctuations in snail populations.

Methods: In this cohort study, the Kambu River (Intervention area) was molluscicided intermittently for 4 years, after mass chemotherapy with praziquantel in the adjacent community of Darajani in January 1997. The nearby Thange River was selected as a control (Non-intervention area), and its adjacent community of Ulilinzi was treated with praziquantel in December 1996. Snail numbers were recorded monthly at 9-10 sites along each river, while rainfall data were collected monthly, and annual parasitological surveys were undertaken in each village. The mollusciciding protocol was adapted to local conditions, and simplified to improve prospects for widespread application.

Results: After the initial reduction in prevalence attributable to chemotherapy, there was a gradual increase in the prevalence and intensity of infection in the non-intervention area, and significantly lower levels of re-infection amongst inhabitants of the intervention area. Incidence ratio between areas adjusted for age and gender at the first follow-up survey, 5 weeks after treatment in the non-intervention area and 4 months after treatment in the intervention area was not significant (few people turned positive), while during the following 4 annual surveys these ratios were 0.58 (0.39-0.85), 0.33 (0.18-0.60), 0.14 (0.09-0.21) and 0.45 (0.26-0.75), respectively. Snail numbers were consistently low in the intervention area as a result of the mollusciciding. Following termination of the mollusciciding at the end of 2000, snail populations and infections in snails increased again in the intervention area.

Conclusion: The results of this study demonstrate that in the Kenyan setting a combination of chemotherapy followed by intermittent mollusciciding can have longer term benefits than chemotherapy alone.

Figure 1

The two study streams. The stippled line indicates the Nairobi–Mombasa road and stars the sampling sites. Sites were numbered consequtively from T1 (S2° 28′ 24.96″; E38° 3′ 12.12″) to T11 (S2° 23′ 54.00″; E38° 9′ 20.64″) in the Thange stream (Non-intervention) and from K1 (S2° 34′ 1.17″ E38° 7′ 3.46″) to K11 (S2° 35′ 56.71″; E38° 9′ 47.14″) in the Kambu stream (Intervention). Ulilinzi and Darajani show the approximate position of schools in the two communities.

Figure 2

Numbers of Biomphalaria pfeifferi (open columns) and Schistosoma mansoni infected snails (black columns) per month per site in two streams, Thange a) and Kambu b), during the pre-intervention years (1993 and 1994) and the first year of mollusciciding, together with rainfall c).

Figure 3

Total number of Biomphalaria pfeifferi collected in each site (Figure1) during the two pre-intervention years, 1993 and 1994. A few of these sites were excluded from the follow-up analysis.

Figure 4

Mean total number of Biomphalaria pfeifferi (a) and number of infected B. pfeifferi (b) collected per site per year during 1994 to 2005, together with rainfall (c). Error bars represents 95% CL.

Figure 5

Prevalence (a) of Schistosoma mansoni infection and heavy infections (black bars) and intensity of infection (b) by age group in the intervention and non-intervention area at base line. Error bars represents 95% CL.

Figure 6

Prevalence (a) of Schistosoma mansoni infection and heavy infections (black bars) and intensity of infection (b) as geometric mean by year in the intervention and non-intervention areas. Error bars represents 95% CL. The baseline was from October 1996, while surveys 1–4 were from November in the Intervention area and the following January in the Non-intervention area during the years 1997/1998, 1998/1999, 1999/2000 and 2000/2001, respectively.