V. R. Southgate

Schistosomiasis at Loum, Cameroun; Schistosoma haematobium, S. intercalatum and their natural hybrid

SummaryA survey of 500 schoolchildren in Loum in 1968 revealed an overall infection rate of 54.2% with Schistosoma intercalatum and this was the only species of schistosome encountered. In 1972 a number of children were found to be passing schistosome eggs in their urine and these eggs ranged in shape and size from the forms characteristic for S. haematobium to those of S. intercalatum. Preliminary laboratory studies demonstrated that hybridisation between the two species was occurring. Subsequent field surveys showed that the snail hosts for the two parasites (B. rohlfsi for S. haematobium and B. forskali for S. intercalatum) were both present in the river Mbette and its tributaries in Loum and the distribution of the two snail species coincided closely with the distribution of the schistosomes in the human population. Detailed study of a small group of children passing hybrid eggs in their urine revealed that few of them were passing eggs in their faeces and that those eggs which were found in faeces were not viable.Analysis of schistosome egg-shape by plotting cumulative size-frequency data on probability paper demonstrated that the graph obtained from a natural hybrid series was different from that given by a known mixture of the two separate species. The hybrid series included a number of exceptionally large eggs resembling those of S. bovis but isolation of these eggs and subsequent laboratory passage of the parasites showed that they were part of the series and were not evidence of the presence of a third species. Hybridisation experiments in the laboratory showed that the cross S. haematobium ♂ × S. intercalatum ♀ is fully viable but that the reverse mating is not successful, thus accounting for the failure of the faecal eggs recovered from children with hybrid infections. Histological results from laboratory passaged hybrids suggest that the Ziehl-positive staining reaction of the egg-shells of S. intercalatum may be a recessive character.The observations reported here indicate that S. haematobium has only recently become established in Loum and that it is, through introgressive hybridisation, replacing the indigenous S. intercalatum. A suggested explanation for the change in the parasite fauna is offered and this depends upon ecological changes resulting from forest clearance and agricultural development providing improved conditions for the spread of B. rohlfsi, the snail host for S. haematobium. It is suggested that, in contrast to recent reports on the spread of S. intercalatum, this species is in fact retreating and being replaced by S. haematobium in areas where forest clearance is taking place.In conclusion it is suggested that introgressive hybridisation of this kind may have been responsible for the evolution of certain characteristic local strains of African schistosomes.

Observations on Schistosoma bovis Sonsino, 1876

Summary Some biological characteristics of Schistosoma bovis Morocco, Sardinia and Iran in hamsters are described and discussed. Differences in infectivity, growth rates, maturation times, egg production and tissue deposition of eggs in the different strains are recorded. The egg shape and size of S. bovis Morocco, Sardinia, Iran, Kenya and S. mattheei are compared, as are the acid phosphatase, malate dehydrogenase and glucose 3-phosphate dehydrogenase isoenzymes. The compatibility/incompatibility of the same four strains of S. bovis to various members of the five species complexes of the genus Bulinus are recorded.

Observations on natural and experimental interactions between Schistosoma bovis and S. curassoni from West Africa

Surveys of 332 naturally infected bovines at eight abattoirs in Senegal, The Gambia and Mali were carried out to determine the prevalence of infection with Schistosoma bovis and S. curassoni and to pinpoint areas where the distribution of the species overlap. S. bovis was the commonest schistosome of cattle in Senegal and Mali being found in animals at seven abattoirs, the highest prevalence of 85.1% occurred at Mopti in Mali. S. bovis was the only bovine schistosome observed in The Gambia. S. curassoni was isolated from a cow at Bamako and shown to have similar glucose-6-phosphate dehydrogenase, phosphoglucomutase and acid phosphatase profiles to those described for a Senegalese isolate. Evidence of interaction of S. bovis with S. curassoni was found in cattle from Senegal, at Tambacounda and Kolda, and from Mali, at Bamako and Mopti. A mixed experimental infection of both species in a sheep showed the lack of any specific mate recognition system: identification of the worms was facilitated by analysis of acid phosphatase by isoelectric focusing in polyacrylamide gels. Viable hybrid parasites were produced in the laboratory and were maintained up until the F4 generation. Comparisons of egg morphology, surface structure of adult male worms and enzyme profiles have been made between experimental hybrid lines and field isolates. Possible mechanisms maintaining species integrity are discussed.

Enzymes in Schistosoma intercalatum and the relative status of the Lower Guinea and Zaire strains of the parasite.

Abstract Seven enzyme systems [phosphoglucomutases (PGM), glucose phosphate isomerases (GPI), glucose-6-phosphate dehydrogenases (G6PD), malate dehydrogenases (MDH), laetate dehydrogenases (LDH), acid phosphatases (AcP) and hexokinases] in extracts of adult worms from two isolates of Schistosoma intercalatum , one from Zaire and one from Cameroun, were compared by isoelectric focusing. Two systems (GPI and PGM) were also compared in extracts of cercariae. Distinctive differences between the strains were found in the LDH system and even more marked differences in the G6PD and PGM systems (the latter were apparent in both adult worm and cercarial extracts). These observations are discussed in conjunction with existing evidence on the results of intermediate host infection experiments and of experimental hybridization between the two strains of S. intercalatum . In turn these aspects are discussed in the light of what is known about other species of African Schistosoma . It is concluded that any definite decision on the relative status of the two strains of S. intercalatum is still premature.

The influence of Calicophoron microbothrium on the susceptibility of Bulinus tropicus to Schistosoma bovis.

A total of 480 snails were collected from 3 habitats on the Mau Escarpment, Kenya, and were identified asBulinus tropicus. Of the 351 snails examined alive in London, 75 were infected withCalicophoron microbothrium, 39 withC. microbothrium andSchistosoma bovis, 1 withS. bovis. 24 with other species of trematodes and 212 were uninfected. Examination of digestive glands ofB. tropicus either uninfected or infected with bothC. microbothrium andS. bovis demonstrated that it is possible to differentiate between parasite and host enzyme activity using glucose phosphate isomerase. However, malate dehydrogenase enables a much clearer differentiation between the enzyme activity of the schistosome and that of the amphistome. Laboratory snail infection experiments demonstrated that it is possible successfully to infectB. tropicus withS. bovis if the snails have previously been exposed to miracidia ofC. microbothrium.

On Schistosoma curassoni, S. haematobium and S. bovis from Senegal: development in Mesocricetus auratus, compatibility with species of Bulinus and their enzymes

A comparative study on the development of Senegalese isolates of Schistosoma curassoni, S. haematobium and S. bovis in hamsters is reported, together with the compatibility of these parasites with Bulinus spp. and enzymes of adult worms. The mean worm return from 35 hamsters exposed to 100 cercariae each of S. curassoni was 11·5%, and of these 54% were paired, the remainder were single males. The growth and maturation of the worms were recorded from 40 to 100 days. The cross-over point (when paired females are of the same length as paired males) was reached at 42 days post-infection when the worms averaged 13·7 mm in length. The majority of tissue eggs (84·5%) were recovered from the liver, compared with 11% in the colon, 2·5% in the caecum and 1·6% in the small intestine. Estimates of the fecundity of paired females averaged 167 eggs/day per female worm. Snail-infection experiments showed S. curassoni to be compatible with B. umbilicatus, marginally compatible with B. senegalensis and incompatible with B...

Mating behaviour in mixed infections of Schistosoma haematobium and S. intercalatum

Summary Experiments were designed to examine the mating behaviour of Schistosoma haematobium and S. intercalatum in mixed infections in hamsters. Individual worms were identified by electrophoretic analysis of glucose-6-phosphate dehydrogenase which was characteristic for each isolate, in addition the uterine eggs of individual females were examined. The results showed that a specific mate recognition system does not exist for S. haematobium and S. intercalatum. The application of statistical tests to the results suggests that S. haematobium male worms are better at pairing with female worms of either species than S. intercalatum male worms. The significance of the results is discussed in relation to an occurrence of natural hybridization between these species in Cameroun.

Schistosoma curassoni Brumpt, 1931 in sheep and goats in Senegal

A survey of 5722 sheep and 1752 goats at the abatoir in Dakar, Senegal showed that the overall prevalence of schistosomiasis was 2·1%. Of the 112 animals where identification of the schistosome species was possible, all were infected with Schistosoma curassoni, and 2·7% had mixed infections with S. bovis. The adult worms of S. curassoni are described, and on the basis of egg morphology this species is shown to be distinct from S. bovis and S. mattheei. Eggs of S. curassoni measured 146 μm ± 16·8 × 63·3 μm ± 4·5 from sheep and 149·4 μm ± 13·2 × 62·8 μm ± 4·9 from mouse liver, and appear to be indistinguishable from the eggs of S. haematobium Guede Chantier, Senegal which measure 153·1 μm ± 11·1 × 62·4 μm ± 12·1 from mouse liver. However, S. curassoni differs from S. haematobium in that it develops more quickly than S. haematobium in hamsters, uterine eggs becoming visible at least 20 days earlier; the adult worms of S. curassoni are nearly double the size of S. haematobium in hamsters at 70 days post infec...

On Schistosoma margrebowiei Le Roux, 1933: The morphology of the egg, miracidium and cercaria, the compatibility with species of Bulinus, and development in Mesocricetus auratus

SummaryThe morphology of the egg, miracidium, and cercaria of Schistosoma margrebowiei are described.The compatibility of S. margrebowiei with species of Bulinus has been examined. The parasite develops well in the diploid, tetraploid and octoploid snails of the truncatus/tropicus complex with overall infection rates of 38.1%, 30.1% and 29.1% respectively, and in reticulatus group snails (Bulinus wrighti) with an infection rate of 45.8% of those snails surviving the prepatent period. Only two species of the forskali complex (Bulinus bavayi, Aldabra; Bulinus beccarii, South Arabia) are slightly compatible, and snails of the africanus group are incompatible.The overall worm return from 24 hamsters exposed individually to 100 cercariae was 39.8%; 82.1% of the worms were paired, the remainder unpaired. The growth of the paired worms was recorded from 28–60 days. The prepatent period in hamsters is 33 days, in sheep 38 days. The mean egg production was 837 eggs/day in infections ranging from 33 to 60 days. Most eggs (80.9%) were deposited in the intestine, and only 18.2% were deposited in the liver. The parasite is pathogenic in hamsters, and peak death rate occurred in the 50–60 day group infections which coincided with the period of peak egg production.

Observations on an isolate of Schistosoma bovis from Tanzania.

The eggs ofSchistosoma bovis isolated from Misungwi, Tanzania measure 211.1 μm±18.4 long and 66.7 μm±5.4 wide. The parasite is naturally transmitted byBulinus africanus and is compatible in the laboratory with snails belonging to theB. truncatus, B. forskali, andB. reticulatus groups. The compatibility withB. africanus group snails is shared with isolates from Kenya and Sudan but not withS. bovis from more northern distributions. Enzyme analyses were carried out by isoelectric focusing. In adult worms, phosphoglucomutase (PGM), hexokinase (HK), malate dehydrogenase (MDH), and lactate dehydrogenase (LDH) proved to be monomorphic whereas two types of glucosephosphate isomerase (GPI), three types of glucose-6-phosphate dehydrogenase (G6PDH), and two types of acid phosphatase (AcP) were identified. Differences in the pI values of GPI and MDH of snail digestive glands and of larval parasites allowed the intramolluscan stages to be characterised. The GPI heterogeneity encountered was common both to the larval and adult parasites. The enzyme types identified inS. bovis are discussed both from an intra- and interspecific viewpoint.