Transmission of Schistosoma bovis in Mkulwe (Mbozi District, Mbeya region, southern highlands of Tanzania)

Various populations of laboratory bred bulinid snails were exposed to miracidia of Schistosoma bovis from Mbozi. The parasite is naturally transmitted by Bulinus globosus in the area. Laboratory infection revealed a good relationship with B. forskalii and B. globosus from Mbozi and a population of B. forskalii from Dar es Salaam (infection rates 100%, 63.6% and 41.7% respectively). Populations of B. globosus and B. nasutus from Dar es Salaam were refractory. It appears that both snail species (B. globosus and B. forskalii) present in Mbozi district transmit S. bovis.     

A review of the biology and transmission ecology of African bovine species of the genus Schistosoma

The present paper reviews the information available concerning the biology and transmission ecology of the African bovine species Schistosoma bovis, S. mattheei, S. margrebowiei and S. leiperi. Criteria for species identification (egg morphology, intermediate host spectra, definitive host-parasite relationships, etc.) are listed and the geographical distribution of the four species and factors determining the relative occurrence within their overall distributional ranges are described. S bovis and S. mattheei occur north and south of 10 degrees S, respectively, and S. margrebowiei occurs mainly, and S. leiperi only, in southern central Africa. Definitive host-related factors (susceptibility, water contact pattern, ect.) providing the background for being a primary definitive host and the primary definitive host spectra for the four schistosome species are described. The primary definitive host spectrum for S. margrebowiei and S. leiperi comprise lechwe, puku and waterbuck, for S. mattheei lechwe, puku, waterbuck plus cattle, and for S. bovis cattle and possibly also some of the listed antelope species. In addition, wild bovines and cattle may provide a reservoir of S. mattheei and S. margrebowiei in humans, but wild bovines and domestic stock play no major role in the transmission of other human species of schistosomes. The intermediate snail host spectra of S. mattheei and S. leiperi only comprise members of the Bulinus africanus species complex; S. bovis is transmitted by members of the B. truncatus, B. africanus and B. forskalii species groups, and S. margrebowiei is transmitted by members of the B. forskalii species group and possibly also by members of the B. tropicus and B. truncatus species groups. Factors determining the transmission ecology of the four schistosome species, and thereby the epidemiology of bovine schistosomiasis, are discussed. Influential factors comprise environmental conditions mediated via the effect of these on the size of the snail host population and on the rate of the intramolluscan development, behavioural patterns of the definitive host population and the course of the infection in the definitive host as related to aspects of susceptibility and level of endemicity. The epidemiological pattern (prevalence and intensity of infection, seasonality of transmission, etc.) is described and exemplified, and it is finally concluded that the increasing water conservation and changing methods of husbandry may result in bovine schistosomiasis becoming a major veterinary problem in Africa.     

Studies of the relationships between Schistosoma and their intermediate hosts. IV. The genus Bulinus and Schistosoma bovis from Morocco

Schistosoma bovis from Morocco was used in infection experiments with several populations of Bulinus truncatus. The snails from Libya, Malawi, Morocco and Senegal were very compatible with the schistosome since the infection rates were approximately 90%, the mortality was low, and a very high production of cercariae, approximately 1.2 million per 100 exposed snails, was observed. Only a very few B. truncatus (2n = 36) from Rhodesia became infected. B. permembranaceus and B. forskalii were refractory.     

Further studies on the compatibility between s. intercalatum from cameroun and za?re and species of bulinus.

The results from the infection experiments with Schistosoma intercalatum from Cameroun and from Za?re and the intermediate hosts, belonging to B. forskalii and B. globosus, could be divided into groups according to the degree of compatibility with the schistosomes. This was indicated by the total cercariae production per 100 exposed snails (TCP/100 exp. snails). B. forskalii from Kinshasa, Za?re were the snails which were most compatible with S. intercalatum from Cameroun, and B. globosus populations tested were refractory. The TCP/100 exposed snails was about 300,000 for the populations of B. forskalii from Kinshasa compared with the low production for B. forskalii from Cameroun of 125,000. B. wrighti from South Arabia produced 155,000 cercariae per 100 exposed snails. B. cernicus from Mauritius could be separated into two types according to the compatibility with S. intercalatum from Cameroun, the TCP/100 exposed snails was 31,000 and 267,000, respectively. B. globosus from Kinshasa, Za?re, was very compatible with S. intercalatum from Za?re and the TCP/100 exposed snails was very high, and 300,000. An albino strain of B. globosus from Rhodesia was the most compatible snail having a TCP/100 exposed snails of 2.4 million cercariae. Other strains of B. globosus from Cameroun and Togo were refractory and less susceptible. It was also possible to infect B. africanus and the two tested populations from Kenya and Tanzania, producing 15,000 and 179,000 cercariae per 100 exposed snails, respectively.     

Experimental taxonomy of Bulinus (Gastropoda: Planorbidae): the West and North African species reconsidered, based upon an electrophoretic study of several enzymes per individual

Horizontal starch gel electrophoresis was used to score mobilities of seven different enzymes in species of Bulinus from North and West Africa. An account of the intra- and inter-population variation observed was given. Based on the data obtained it was suggested that enzymic data should be used as taxonomic characters in parallel with morphological and anatomical characters. It is suggested that enzymic characters could be used to place taxa into synonymy if the data are collected in such a way that one can write up an enzyme profile for the single individual. The nine taxa of Bulinus known to occur in North and West Africa were revised, the taxon B. jousseaumei was synonymized with B. globosus , and the two taxa B. guernei and B. rohlfsi were synonymized with B. truncatus. As a result of the revision only six species could be recognized as valid from the area, these being B. forskalii, B. globosus, B. senegalensis, B. truncatus, B. ugandae and B. umbilicatus. Finally, some enzymic characters are suggested that may be used for a reliable identification of these morphologically very difficult taxa.     

Host choice and penetration by Schistosoma haematobium miracidia

Schistosome parasites commonly show specificity to their intermediate mollusc hosts and the degree of specificity can vary between parasite strains and geographical location. Here the role of miracidial behaviour in host specificity of Schistosoma haematobium on the islands of Zanzibar is investigated. In choice-chamber experiments, S. haematobium miracidia moved towards Bulinus globosus snail hosts in preference to empty chambers. In addition, miracidia preferred uninfected over patent B. globosus. This preference should benefit the parasite as patent snails are likely to have mounted an immune response to S. haematobium as well as providing poorer resources than uninfected snails. Miracidia also discriminated between the host B. globosus and the sympatric, non-host species Cleopatra ferruginea. In contrast, S. haematobium did not discriminate against the allopatric Bulinus nasutus. Penetration of the host by miracidia was investigated by screening snails 24 h after exposure using polymerase chain reaction (PCR) with S. haematobium specific DraI repeat primers. There was no difference in the frequency of penetration of B. globosus versus B. nasutus. These responses to different snail species may reflect selection pressure to avoid sympatric non-hosts which represent a transmission dead end. The distribution of B. nasutus on Unguja is outside the endemic zone and so there is less chance of exposure to S. haematobium, hence there will be little selection pressure to avoid this non-host snail.     

Identification of snails within the Bulinus africanus group from East Africa by multiplex SNaPshot trade mark analysis of single nucleotide polymorphisms within the cytochrome oxidase subunit I

Identification of populations of Bulinus nasutus and B. globosus from East Africa is unreliable using characters of the shell. In this paper, a molecular method of identification is presented for each species based on DNA sequence variation within the mitochondrial cytochrome oxidase subunit I (COI) as detected by a novel multiplexed SNaPshotTM assay. In total, snails from 7 localities from coastal Kenya were typed using this assay and variation within shell morphology was compared to reference material from Zanzibar. Four locations were found to contain B. nasutus and 2 locations were found to contain B. globosus. A mixed population containing both B. nasutus and B. globosus was found at Kinango. Morphometric variation between samples was considerable and UPGMA cluster analysis failed to differentiate species. The multiplex SNaPshotTM assay is an important development for more precise methods of identification of B. africanus group snails. The assay could be further broadened for identification of other snail intermediate host species.     

Cercarial shedding patterns of Schistosoma bovis and S. haematobium from single and mixed infections of Bulinus truncatus

The cercarial shedding of Schistosoma bovis and S. haematobium were studied in single and mixed infections in the snail host Bulinus truncatus. The two species displayed a distinctive diurnal cercarial emergence with an earlier shedding pattern for S. bovis than S. haematobium (the average emergence peaks were respectively at 0800 h and 1200 h). In mixed infections, each species kept its own cercarial shedding rhythm with no marked alterations. The cercarial emergence pattern is proposed as a new method to identify natural mixed infections in the snail intermediate hosts. The interactions between the two parasites are discussed.     

Spatial and temporal population genetic survey of Bulinus globosus from Zanzibar: an intermediate host of Schistosoma haematobium

In Zanzibar, the freshwater hermaphroditic snail Bulinus globosus is the intermediate host of the human parasite Schistosoma haematobium . To shed light on the patterns of genetic variation within Bu. globosus , variation at six polymorphic microsatellite loci was assessed to quantify spatial genetic structure within eight populations and temporal changes in a further four populations of the snails. Limited genetic variation was observed within populations, possibly reflecting both partial-selfing and fluctuations in population size. Although a statistically significant heterozygote deficiency was observed, Bu. globosus appears to be a preferential out-crosser, which was consistent with the absence of genotypic linkage disequilibria. Genetic variation across populations was substantial, illustrating significant isolation-by-distance on a regional scale and that gene flow was restricted to nearby populations. The temporal survey showed that levels of genetic variation and inbreeding changed over a 1-year period, consistent with field observations of seasonal change. The results strongly support a snail demographic model of population expansion and contraction throughout the year with habitats re-colonized by aestivating or surviving snails from local refugia. This model might help explain co-evolution of snails and schistosomes in Zanzibar as local populations of Bu. globosus have varying degrees of susceptibility to S. haematobium .     

Susceptibility of freshwater snails to the amphistome Calicophoron microbothrium and the influence of the species on susceptibility of Bulinus tropicus to Schistosoma haematobium and Schistosoma mattheei infections

The susceptibility of Bulinus tropicus, B. globosus, Biomphalana pfeifferi, Lymnaea natalensis, and Melanoides tuberculata to Calicophoron microbothrium was examined. Bulinus tropicus had the highest prevalence (65.0%), followed by B. pfeifferi (37.5%), B. globosus (6.8%), and M. tuberculata (5.9%). Lymnaea natalensis was refractory to infection. Bulinus tropicus snails infected with C. microbothrium alone or coinfected with either Schistosoma haematobium or S. mattheei 0, 7, 14, and 21 days after exposure to C. microbothrium produced C. microbothrium cercariae only.     

The suitability of several aquatic snails as intermediate hosts for Angiostrongylus cantonensis.

Sixteen species of aquatic snails of four families were tested by quantitative technique under standardized conditions for their suitability as intermediate hosts for Angiostrongylus cantonensis. These species were the planorbid snails Biomphalaria glabrata, Biomphalaria alexandrina, Planorbis planorbis, Planorbis intermixtus, Bulinus truncatus, Bulinus contortus, Bulinus africanus, Bulinus tropicus and Helisoma sp.; the lymnaeid snails Lymnaea natalensis, Lymnaea tomentosa, Lymnaea stagnalis, and Stagnicola elodes; the physid snail Physa acuta (an Egyptian and a German strain) and the ampullariid snails Marisa cornuarietis and Lanistes carinatus. All these snail species proved to be susceptible to infection with A. cantonensis, and first stage larvae reached the infective third stage in all of them. However, the rate and intensity of infection varied with different species. B. glabrata was the most susceptible snail species with a 100% infection rate and an average percentage recovery of third stage larvae of 26.1. This was followed by S. elodes and B. africanus, with a 100% infection rate and an average percentage recovery of third stage larvae of 15.6 and 14.6 respectively. The rest of snail species proved to be less susceptible. For comparative evaluation of the suitability of the various snail species as intermediate hosts of A. cantonensis a "Capacity Index" was determined. This index should provide a useful method for the evaluation of the suitability of various snails as intermediate hosts of nematode parasites under standardized conditions in the laboratory.     

The effect of Schistosoma haematobium infection on the growth and fecundity of three sympatric species of bulinid snails

Three species of bulinid snails were monitored to determine the effect of infection with 2 sympatric strains of Schistosoma haematobium on longevity, growth, and reproductive output, from the onset of cercarial production until death. Bulinus senegalensis was least affected by infection, with total fecundity reduced by less than 34%. Infected Bulinus truncatus showed an acceleration in growth accompanied by a 63% reduction in fecundity, although the majority of snails continued to oviposit at a low level. The longest-lived snails in this study, Bulinus globosus, showed decreased growth and survival when infected. In addition a significant proportion of infected individuals of this species failed to oviposit, and those that retained some reproductive capacity produced fewer embryos than controls. Total fecundity of B. globosus was reduced almost 90% by infection with S. haematobium, yet long-term monitoring of individuals showed that reproductive activity recovered when parasite productivity was low. Results from this and similar studies indicate that the time of infection by a trematode in relation to reproductive maturity of the molluscan host is important in determining the subsequent effects on host growth and fecundity.     

The cercarial output from three Nigerian bulinids infected with two strains of Schistosoma haematobium

The daily cercarial output of two Nigerian strains of Schistosoma haematobium in sympatric Bulinus truncatus, B. globosus and B. senegalensis was measured at weekly intervals from the start of emission to the snails' death. In all cases cercariae were released throughout the life of the host, with no cases of "self cure". Patterns of output through the course of infections in B. truncatus and B. senegalensis were similar to those reported for S. haematobium by other workers, with daily production of cercariae rising to a peak within a few weeks of the onset of shedding, then declining until the host's death. In the longer lived B. globosus production was significantly higher, but declined to very low levels after the initial peak; in some individuals cercarial output remained very low, while others showed a second period of high cercarial emission. The relative compatibility of each host-parasite combination is discussed.     

Dynamics of the intramolluscan larval development of Schistosoma haematobium: replication of daughter sporocysts and cercarial production

During the intramolluscan larval development of Schistosoma haematobium (Algerian strain) in Bulinus truncatus, two replication processes of daughter sporocysts occur. Replication by direct sporocystogenesis appears more important than sporocystogenesis post cercariogenesis. These mechanisms assure a periodic renewal of the sporocyst stock in the snail host and seem to be synchronized with the development of cercarial generations. The succession of several generations of cercariae is responsible for the alternation of high and low periods of productivity. The scheme proposed for the intramolluscan development of S. haemtobium is compared with those described for S. mansoni and S. bovis and interpreted in terms of demographic strategies adapted to a better exploitation of the snail host.     

Schistosoma haematobium in Lake Malaŵi: susceptibility and molecular diversity of the snail hosts Bulinus globosus and B. nyassanus

Intermediate hosts of Schistosoma haematobium, the causative agent of urinary schistosomiasis, in Lake Mala?i include: Bulinus globosus, a member of the B. africanus group and B. nyassanus, a diploid member of the B. truncatus/tropicus species complex. We compared genetic variability between isolates of S. haematobium from the southern part of the lake (Cape Maclear), where both B. globosus and B. nyassanus play a role as intermediate hosts, and isolates from the northern part, where only B. globosus is host. Data show that the S. haematobium isolates from these two areas of Lake Mala?i cannot be distinguished using nuclear or mitochondrial sequences and are capable of cross-infections.     

Host-parasite relationship of Bulinus forskalii (Ehrenberg) and Schistosoma intercalatum Fisher 1934, from Cameroun.

The susceptibility of 13 different B. forskalii strains from Zaire, Cameroun, Gabon, Senegal, Rhodesia and Tanzania and B. reticulatus wrighti Aden has been tested to infection with Schistosoma intercalatum from Cameroun. The most common prepatent period was 23-28 days. B. forskalii from Zaire, Cameroun and Gabon were excellent intermediate hosts with infection rates varying from 59-0 to 97-4% and with very low death rates, 0-28-4%. The strains from Senegal and Rhodesia had infection rates from 50-0 to 58-4%. B forskalii from Tanzania was refractory. 53-0% of the exposed B. reticulatus wrighti was infected. The total cercariae production per snail varied from 1000-11000. The duration of the infection varied from 8 to 174 days.     

Schistosoma bovis: patterns of cercarial emergence from snails of the genera Bulinus and Planorbarius

A comparative experimental study of the rhythmic shedding of two geographic strains of Schistosoma bovis cercariae by Bulinus truncatus and Planorbarius metidjensis did not show any differences in emergence patterns. In our opinion, the results support the hypothesis that cercarial emission rhythms are determined primarily by the parasite in these snail/parasite associations.     

Intermediate host antigens associated with the cercariae of Schistosoma haematobium.

Bulinus (Physopsis) africanus antigens were shown to be associated with the cercarial glycocalyx of Schistosoma haematobium using immunofluorescence and the Cercarienhüllen Reaktion. It is proposed that this snail antigen may sensitise the definitive host and that resistance to further invasion by cercariae could be induced in this manner.     

Hybridization between different strains of Schistosoma intercalatum Fisher, 1934 from Cameroun and Za?re

The offspring from hybridization experiments between Schistosoma intercalatum, Cameroun (intermediate host Bulinus forskalii) and S. intercalatum, Za?re (intermediate host B. globosus) was investigated. Reciprocal crosses between the two forms were performed. F1- and F2-miracidia developed in both parental intermediate hosts. The minimum prepatent period was 24 days at 26 degrees C, an average of the prepatent periods for the two parental strains. The total cercariae production per 100 exposed snails decreased sharply from 121 000 for the parental strain from Cameroun in B. forskalii to 21 000 and 5000 from the F1- and F2-generations, respectively. The figures for S. intercalatum, Za?re in B. globosus were 270 000, 134 000 and 3000, respectively. F1- and F2-generations were from the cross between males from Cameroun and females from Za?re. The viablility of the four cross-breeding experiments carried out (two with each combinations of the parental strains) were low. It was only possible to obtain F2-cercariae. No miracidia were hatched from eggs produced by the F2-gerneation.     

Snail predation by larvae of Sepedon scapularis Adams (Diptera: Sciomyzidae), a potential biocontrol agent of snail intermediate hosts of schistosomiasis in South Africa

Malacophagous larvae of the fly Sepedon scapularis Adams were shown experimentally to be effective predators of three species of aquatic pulmonate snails tested as prey: Bulinus africanus (Krauss) an important intermediate host of Schistosoma haematobium (Bilharz), Bulinus tropicus (Krauss) and the invasive species Physa acuta Draparnaud. Survival of S. scapularis larvae from instar to instar was negatively affected by the size of prey snails, since larvae tended to be asphyxiated by the mucous secretions of the snails, or by the larval hydrofuge hairs becoming entangled in snail faeces. In experiments to test the choice of S.scapularis larvae for different species and sizes of snails, B.africanus was significantly disfavoured compared with the other two snail species. Small snails (< 3 mm) were more frequently killed by all three larval instars of S.scapularis. First instars killed few, if any, large snails (> 7 mm), whereas second and third instars preyed effectively on all sizes of snails. Third instars killed significantly more snails than younger instars. For larvae offered only one species of snail as prey, the mean total number of snails killed per larva during its entire development was 49 B.africanus, 45 B.tropicus or 34 P.acuta. It is concluded that S.scapularis is potentially useful as a biological control agent for use against indigenous Bulinus and exotic Physa snails.