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. 2017 Apr;96(4):850-855.
doi: 10.4269/ajtmh.16-0614. Epub 2017 Jan 23.

Divergent Effects of Schistosoma haematobium Exposure on Intermediate-Host Snail Species Bulinus nasutus and Bulinus globosus from Coastal Kenya

Affiliations

Divergent Effects of Schistosoma haematobium Exposure on Intermediate-Host Snail Species Bulinus nasutus and Bulinus globosus from Coastal Kenya

H Curtis Kariuki et al. Am J Trop Med Hyg. 2017 Apr.

Abstract

AbstractSchistosoma haematobium infection causes urogenital schistosomiasis, a chronic inflammatory disease that is highly prevalent in many parts of sub-Saharan Africa. Bulinid snails are the obligate intermediate hosts in the transmission of this parasite. In the present study, Bulinus globosus and Bulinus nasutus snails from coastal Kenya were raised in the laboratory and exposed to miracidia derived from sympatric S. haematobium specimens to assess the species-specific impact of parasite contact and infection. The snails' subsequent patterns of survival, cercarial shedding, and reproduction were monitored for up to 3 months postexposure. Schistosoma haematobium exposure significantly decreased the survival of B. globosus, but not of B. nasutus. Although both species were capable of transmitting S. haematobium, the B. globosus study population had a greater cumulative incidence of cercarial shedders and a higher average number of cercariae shed per snail than did the B. nasutus population. The effects of prior parasite exposure on snail reproduction were different between the two species. These included more numerous production of egg masses by exposed B. nasutus (as compared with unexposed snails), contrasted to decreased overall egg mass production by parasite-exposed B. globosus. The interspecies differences in the response to and transmission of S. haematobium reflect clear differences in life histories for the two bulinid species when they interact with the parasite, which should be taken into account when planning control interventions aimed at reducing each host snails' contribution to local transmission of Schistosoma infection.

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Figures

Figure 1.
Figure 1.
Kaplan–Meier survival curves for 100 unexposed (control) and 100 Schistosoma haematobium-exposed snail populations from each of two bulinid species: (A) Bulinus globosus and (B) Bulinus nasutus. Daily survival was monitored over 26 weeks; the indicated percent survival indicates the number of remaining snails. The separate curves show respective control (darker line) and S. haematobium-exposed (lighter line) groups for each species. Relative to the survival of unexposed snails of the same species, survival of exposed B. globosus was significantly reduced (P < 0.001 by log-rank test), whereas survival of exposed B. nasutus was not.
Figure 2.
Figure 2.
Numbers of Bulinus globosus and Bulinus nasutus snails shedding Schistosoma haematobium cercariae each day over the course of the experiment. The x axis indicates days since exposure to miracidia. The y axis indicates the number of snails shedding each day. darker line = B. globosus; lighter line = B. nasutus.
Figure 3.
Figure 3.
Average daily cercarial production per snail is shown for each week over the first 14 weeks of the study. The total number of Schistosoma haematobium cercariae produced each day by exposed Bulinus globosus and Bulinus nasutus snails was divided by the respective number of surviving snails. The daily rate shown was the mean daily rate for each study week. dark bars = B. globosus; light bars = B. nasutus.
Figure 4.
Figure 4.
Average egg mass production per Schistosoma haematobium-exposed and unexposed snails. The number of egg masses produced by exposed and control groups of (A) Bulinus globosus and (B) Bulinus nasutus snails was counted each week and averaged over the number of surviving snails per group. dark color = control snails; light color = exposed snails; bars = average egg masses per snail; lines = cumulative mean egg masses per snail over the period of observation. The two panels use different scales for the x axis and the y axis.

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