Experimental control of Biomphalaria pfeifferi, the intermediate host of Schistosoma mansoni, by the ampullariid snail Lanistes varicus.

The biological control of the snail hosts of the trematodes that cause human schistosomiasis appears to be a promising method for achieving sustainable reductions in the transmission of the parasites. The possibility of using the Ghanaian strain of an ampullariid snail, Lanistes varicus, for the biological control of the main snail host of Schistosoma mansoni , Biomphalaria pfeifferi, has now been investigated in laboratory-based experiments. Adult and 2-week-old L. varicus were found to feed voraciously on the egg masses and juveniles of B. pfeifferi (from the Tono irrigation canals in northern Ghana). When single L. varicus were exposed to 20-200 egg masses, they consumed all of the masses over 24 h (if adult) or about 50% of them over 4 days (if 2-week-old juveniles). The effect of the secretions of the ampullariid on the reproduction, growth and mortality of B. pfeifferi was also investigated, by maintaining the two snail species in the same aquarium but separated by nylon netting. The presence of L. varicus in the same aquarium reduced the number of egg masses produced by each B. pfeifferi, although, curiously, the presence of a single L. varicus in the aquarium appeared to have more of an impact, on the egg-mass deposition by 20 B. pfeifferi, than the presence of five or more of the ampullariids. It appears that, under laboratory conditions at least, the Ghanaian stain of L. varicus has the potential to limit populations of B. pfeifferi.

The relative susceptibilities of juvenile and adult Bulinus globosus and Bulinus truncatus to the molluscicidal activities in the fruit of Ghanaian Blighia sapida, Blighia unijugata and Balanites aegyptiaca.

The relative susceptibilities of juvenile (2- to 3-week-old) and adult (5- to 7-week-old) Bulinus globosus and Bu. truncatus to the molluscicidal activities of aqueous preparations of the powdered, dried fruits of Blighia sapida, Bl. unijugata and Balanites aegyptiaca were investigated under laboratory conditions. For the extract prepared from the freeze-dried, semi-ripe fruits of Bl. sapida, the concentrations killing 95% of the snails after an exposure for 24 h (LC95) were, respectively, 232.7 and 161.2 ppm for the juveniles and adults of Bu. globosus and 187.6 and 140.2 ppm for the juveniles and adults of Bu. truncatus. The corresponding values for Bl. unijugata were 136.9 and 98.7 ppm for Bu. globosus and 132.2 and 98.5 ppm for Bu. truncatus, respectively. Extracts of the freeze-dried, ripe fruit of Ba. aegyptiaca were much more active, however, giving LC95 of just 16.9 and 19.7 ppm for the juveniles and adults of Bu. globosus, and 14.2 and 12.0 ppm for the juveniles and adults of Bu. truncatus, respectively. In general, similar results were obtained when the fruit were oven-dried rather than freeze-dried but the potency of Balanites extracts produced from oven-dried fruit was relatively low. That the juvenile snails appeared less susceptible than the adults to the molluscicidal activity in each Blighia extract is interesting. This observation may have implications for snail control, in terms of the optimal frequency, timing and concentration of molluscicides applied in a particular habitat.

Evolutionary relationships and biogeography of Biomphalaria (Gastropoda: Planorbidae) with implications regarding its role as host of the human bloodfluke, Schistosoma mansoni.

The wide geographic distribution of Schistosoma mansoni, a digenetic trematode and parasite of humans, is determined by the occurrence of its intermediate hosts, freshwater snails of the genus Biomphalaria (Preston 1910). We present phylogenetic analyses of 23 species of Biomphalaria, 16 Neotropical and seven African, including the most important schistosome hosts, using partial mitochondrial ribosomal 16S and complete nuclear ribosomal ITS1 and ITS2 nucleotide sequences. A dramatically better resolution was obtained by combining the data sets as opposed to analyzing each separately, indicating that there is additive congruent signal in each data set. Neotropical species are basal, and all African species are derived, suggesting an American origin for the genus. We confirm that a proto-Biomphalaria glabrata gave rise to all African species through a trans-Atlantic colonization of Africa. In addition, genetic distances among African species are smaller compared with those among Neotropical species, indicating a more recent origin. There are two species-rich clades, one African with B. glabrata as its base, and the other Neotropical. Within the African clade, a wide-ranging tropical savannah species, B. pfeifferi, and a Nilotic species complex, have both colonized Rift Valley lakes and produced endemic lacustrine forms. Within the Neotropical clade, two newly acquired natural hosts for S. mansoni (B. straminea and B. tenagophila) are not the closest relatives of each other, suggesting two separate acquisition events. Basal to these two species-rich clades are several Neotropical lineages with large genetic distances between them, indicating multiple lineages within the genus. Interesting patterns occur regarding schistosome susceptibility: (1) the most susceptible hosts belong to a single clade, comprising B. glabrata and the African species, (2) several susceptible Neotropical species are sister groups to apparently refractory species, and (3) some basal lineages are susceptible. These patterns suggest the existence of both inherent susceptibility and resistance, but also underscore the ability of S. mansoni to adapt to and acquire previously unsusceptible species as hosts. Biomphalaria schrammi appears to be distantly related to other Biomphalaria as well as to Helisoma, and may represent a separate or intermediate lineage.

The sugar chemoreception niches of Bulinus (physopsis) globosus (Morelet) and Bulinus rohlfsi (Clessin), the two most important snail hosts of Schistosoma haematobium (Bilharz) in west Africa.

1. The behavioural responses of two species of freshwater pulmonate snails [Bulinus (P.) globosus and Bulinus rohlfsi] to sugar gradients were investigated by means of diffusion olfactometers. 2. Both snail species proved to be very discriminating in their responses. Of the 17 sugars tested, 35.3%, namely D(-)glucuronic, maltotriose, maltose, cellobiose, D(-)arabinose, D(+)mannose proved to be statistically significant attractants or arrestants to B. rohlfsi. Only 23.5% of these sugars (maltotriose, maltose), D(+) mannose and D(+) xylose were significant attractants or arrestants to B. (P.) globosus. 3. Glucuronic acid was a significant repellent to B. rohlfsi but none of the sugars was a repellent to B. (P.) globosus. 4. The results are compared with those obtained for other snail species and their relevance to the ecology and control of the snails are discussed.

A study of the sugar chemoreception niches of two bulinid snail hosts of schistosomiasis.

Components of the sugar chemoreception niches of two host snails of urinary schistosomiasis, namely Bulinus globosus (Morelet) and Bulinus rohlfsi (Clessin), were measured by using a buccal mass olfactometer. Among the polysaccharides tested, amylose was found to be the strongest phagostimulant for adults and juveniles of both snail species. Other phagostimulants identified were maltose and xylose for B. rohlfsi and maltotriose for B. globosus. The a(1-4)-glucosidic linkage and the presence of glucose residues were found to be key factors in the stimulus recognition system of the snails. The possible use of these findings in the design of controlled-release formulations for the selective removal of schistosome host snails is considered. The ecological implications of these studies are also examined.

The incidence of the head louse (Pediculus humanus capitis) among pupils of two schools in Accra.

Of 319 pupils examined by visual inspection for head lice in two large schools of contrasting socioeconomic environments in Accra, Ghana, 158 were found to be infected, with infection rates being higher in the school of lower socio-economic status. Younger pupils (7 years) had significantly lower infection rates than older one (11 years). Hair plaiting, communal use of toilet facilities and crowding have been found to be involved in the epidemiology of the head louse. A curious discrepancy was found between the number of pupils who admitted they had lice and those who actually had them. Relative intensities of infestation remain very low in 75% of the cases examined. Mothers have been found to exhibit a high sense of responsibility in treating their children. A call has been extended to the Health Authorities to initiate a national delousing programme for the control of this ectoparasite.