Scanning electron microscope studies of miracidia suggest introgressive hybridization between Schistosoma haematobium and S. haematobium x S. mattheei in the Eastern Transvaal

Schistosoma haematobium miracidia were collected from a locality with a high prevalence of human infection with the animal parasite, S. mattheei, which hybridizes with S. haematobium, and from 2 localities with negligible infection rates. The terebratoria of the miracidia from these localities were compared with each other, with laboratory maintained S. haematobium and with four populations of S. mattheei by means of scanning electron microscopy. It was found that the terebratorial membrane of certain of the S. haematobium miracidia from the locality with a high S. mattheei prevalence in humans, resembled the more intricate membrane of S. mattheei. This suggests introgressive hybridization between S. haematobium and S. haematobium x S. mattheei.     

Do all human urinary infections with Schistosoma mattheei represent hybridization between S. haematobium and S. mattheei?

Enzyme electrophoresis indicated that all Schistosoma mattheei eggs passed in the urine of humans derive from S. mattheei females in copula with S. haematobium males. It appears that S. mattheei males do not reach sexual maturity in man; however, S. haematobiumxS. mattheei males possibly do.     

Variation in the morphology of the dorsal and dorso-lateral tegument of male Schistosoma haematobium from southern Africa

The teguments of Schistosoma haematobium males from three localities in the Eastern Transvaal and one in the eastern Caprivi were studied by means of scanning electron microscopy. Eastern Transvaal S. haematobium, which occurs sympatrically with S. mattheei, a bovine schistosome also infecting man and which hybridizes with S. haematobium, exhibited certain S. mattheei characteristics. The occurrence of these characteristics were neither related to the prevalence of human S. mattheei infections nor could they be attributed exclusively to phenotypic plasticity. The variation therefore may be geographical and possibly related to the phylogeny of the two species.     

Hybridisation between the digeneans Schistosoma haematobium and S. mattheei : viability of hybrids and their development in sheep

The F1 and F2 hybrids of Schistosoma haematobium male × S. mattheei female were studied with regard to infectivity to intermediate and definitive hosts, isoenzymes (phosphoglucomutase) of individual male worms, randomly amplified polymorphic DNAs of individual adult worms and scanning electron micrographs of the tubercles of male worms. The infection rate of the F1 hybrid miracidia in Bulinus globosus (41.7%) was greater than that achieved in B. wrighti (16.3%); the infection rate of the F2 in B. wrighti was 15.4%. In the definitive hosts: in sheep only male F1 hybrids (i.e. no females and no F2 worms)were recovered; but in hamsters both paired F1 worms and unpaired F1 males were recovered, as were one pair of worms and unpaired males of the F2 generation. The S. mattheei and S. haematobium male worms showed very distinctive PGM patterns, and the F1 hybrids showed additive patterns and a polymorphism with two distinct types of band patterns which are the result of polymorphism in the S. haematobium. The RAPD profiles of the F1 hybrids were also composite of the two parental species. Scanning electron micrographs of the tubercles of male S. haematobium showed them to be heavily spined, whereas those of S. mattheei males were devoid of spines. The F1 hybrids did show variation ranging from non-spined, some with partial spination, to those with heavily spined tubercles. Male worms of the F2 generation possessed tubercles either with or without spines. The potential significance of hybridisation in areas of sympatry between S. haematobium and S. mattheei is discussed.     

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

A 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 male X S. intercalatum femal 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.     

Interspecific variation within the 'hypervariable' region of the 18S ribosomal RNA gene among species of Schistosoma Weinland, 1858 (Digenea)

The 'hypervariable' region of the 18S ribosomal DNA gene, comprising 344 base pairs, was sequenced for five species of the genus Schistosoma (S. intercalatum, S. margrebowiei, S. mattheei, S. malayensis and S. rodhaini), representing three of the schistosome species groups. Heterobilharzia americana was also included as an outgroup. The sequences were compared with previously published sequences (S. haematobium, S. japonicum, S. mansoni and S. spindale). The analysis confirmed that the Asian schistosomes were distantly related to the other groups of schistosomes and showed that the amount of variation within the S. japonicum group (1.49%) was greater than that detected within the S. mansoni and S. haematobium groups (0% in each case).     

Studies on experimental mixed infections of Schistosoma mansoni and S. haematobium in hamsters

Golden hamsters were superinfected simultaneously with 100 Schistosoma haematobium cercariae, 1 and 3 weeks after initial infection with 100 S. mansoni cercariae. Results indicate that there was a higher degree of resistance to superinfection with S. haematobium at 1 week following initial infection with S. mansoni than that produced in the other two superinfections. This resistance was evidenced by a reduction in the number and size of worms of both species, decrease in S. haematobium egg extrusion per female and by a striking deviation in the egg distribution pattern of both species. Such an early host resistance was not recorded in previous works. Cross-mating was observed but no hybridization took place and the eggs produced were hatchable and typical of their species.     

Multispecies schistosomal infections of the female genital tract detected in cytology smears.

Egg morphology of Schistosomes affecting man in Africa is described and illustrated with particular reference to the appearance of the ova of S. haematobium, S. mansoni and S. mattheei in Papanicolaou stained smears.     

Further observations on immunization of sheep against Schistosoma mansoni and S. bovis using irradiation-attenuated schistosomula of homologous and heterologous species.

This paper describes further characteristics of the immunization of sheep against schistosomes using live, irradiation schistosomula. Sheep immunized with a non-virulent strain of Schistosoma mattheei were protected against a more virulent strain of the same species for over a year. As there was no evidence that the irradiated parasites were able to persist this long, it was concluded that the vaccine had induced a sterile resistance. Heterologous vaccination, using irradiated S. mattheei schistosomula to immunize against S. bovis or irradiated S. mansoni schistosomula to immunize against S. mattheei, failed to induce any protection.     

Identification of schistosome hybrids and larval parasites using rRNA probes

Observations have been made on the ribosomal RNA (rRNA) gene units of hybrid progeny produced by experimental crosses of S. haematobium × S. mattheei, S. mattheei × S. bovis and S. haematobium × S. intercalatum. Hybridisation of DNA probe pSM 889 to restriction endonuclease digested DNA extracted from adult worms showed that each parental form could be differentiated by differences in the rRNA gene unit. In each experimental cross the F1 hybrid generation produced a composite major banding pattern of the two parental species. No differences associated with the stage of development were detected in the major bands of hybridisation when DNA extracted from various life-cycle stages of S. mansoni and S. margrebowiei was digested with EcoR1 and hybridised with probe pSM 889. Prepatent infections of S. mansoni in Biomphalaria glabrata were detected 16 days post-infection utilising probe pSM 389 and dot blot analysis. Small numbers of intact cercariae dotted onto nitrocellulose were detected using probe pSM 389, 10 cercariae being the minimum number required for accurate determination.     

Schistosoma haematobium and S. japonicum: analysis of the ribosomal RNA genes and determination of the "gap" boundaries and sequences

We have determined the intragenic organization of the rRNA genes of Schistosoma haematobium and S. japonicum and found them to be similar to that of S. mansoni and other eukaryotes. An entire ribosomal repeat approximately 10 kbp in size from each species was isolated as a SalI fragment from a genomic library constructed in bacteriophage lambda. The segments encoding both the small and large rRNAs have been identified using three cloned EcoRI fragments of S. mansoni as probes. There were three EcoRI fragments (4.2, 3.0, 1.6 kbp) from S. haematobium and four EcoRI fragments (4.6, 2.3, 1.7, 1.0 kbp) from S. japonicum. As in a wide variety of organisms within the protostome phyla, the 28S rRNA in schistosomes contains a "gap" which separates it into two fragments. The length of the gap sequence in S. haematobium is 54 bases and it is identical to that in S. mansoni in both length and sequence. However, in S. japonicum the sequence is between 64-67 bases long. In each case, irrespective of the species, the gap is located at the same position within the 28S rRNA. Secondary structures of the gap sequence derived by computer analysis predict a conformation with the minimum free energy that has an UAAU tract in a hairpin loop for S. haematobium and an UAUU tract for S. japonicum.     

Further observations on the relationship and distribution of Schistosoma margrebowiei and S. leiperi in central southern Africa

A further survey in East Caprivi, Chobe National Park, Okavango swamps and Kavango was undertaken in June 1976. No evidence of lechwe schistosomes was found in droppings of African buffalo (Syncerus caffer) nor baboons (Papio ursinus) living in lechwe habitats. It was thought that they were not capable of spreading or maintaining these parasites outside the confines of the known distribution of Kobus sp. The role of goats was equivocal but probably they too are poor hosts. Kavango, an endemic area of S. haematobium and S. mansoni, was thought to be free of all animal schistosomes, thus confirming the hypotheses that (1) cattle and goats are poor hosts of the lechwe schistosomes and (2) S. mattheei was blocked from entering the territory by the presence of lechwe schistosomes in the surrounding areas. Evidence of schistosomes was not found in cattle and goats at Maun for the same reasons. The prevalence of S. mansoni at Maun has increased alarmingly over the past 20 years with a simultaneous disappearance of lechwe from the area. S. margrebowiei and S. leiperi eggs were found in lechwe and tsessebe droppings some 80 km north of Maun. A high proportion of children with negative excreta from "non-endemic" areas in East Caprivi had positive CFT and/or skin tests, suggestive of exposure to lechwe schistosomes resulting in a possible immunity to S. mansoni and S. haematobium.     

Observations on the infectivity and fecundity of a Sudanese isolate of Schistosoma bovis in albino mice

A Sudanese isolate of Schistosoma bovis from experimentally infected sheep was found to be highly pathogenic to albino mice. Eggs were first found in the livers 42 days after exposure. The distribution of eggs in the liver and small and large intestines changed little during the course of infection. Results are compared with others using an Iranian isolate of S. bovis, which causes only a mild infection, and S. mattheei.     

The mouse as a suitable host for an isolate of Schistosoma haematobium from Niger.

The host-parasite relationships of a Schistosoma haematobium isolate, originating from Niger, and the white mouse are described. Swiss OF1 albino mice were exposed individually to 200 cercariae and worms were recovered 9, 12, 16 and 20 weeks post infection. The mean worm returns ranged between 10.54 and 13.05% and did not alter significantly between 9 and 20 weeks post infection. The sex ratio of worms was always in favour of males; from 7.09:1 at 9 weeks after infection it decreased regularly to 3.28:1 at 20 weeks. Male worms reached a mean length of 8.72 mm at 20 weeks. From the 12th week post infection, a high number of eggs was found in the liver and gut. At 20 weeks, eggs were also found in the bladder. Viable eggs and infective miracidia were obtained. The infection of Bulinus truncatus from Niger succeeded with a mean rate of 61% after the first passage through mice. The isolate of S. haematobium was maintained in the laboratory during 3 successive passages through mice. These entirely new results are very probably linked to genetic characteristics peculiar to the S. haematobium populations from Niger.     

Schistosoma japonicum: the pathology of experimental infection

The pathology of experimental schistosomiasis japonica is reviewed and compared with the pathology of schistosomiasis japonica in man and to some aspects of schistosomiasis mansoni and schistosomiasis haematobia in experimental animals. The induction of granulomas around Schistosoma japonicum eggs depends upon cell mediated immunity, as do the reactions to Schistosoma mansoni and Schistosoma haematobium eggs. However, the modulation of the reaction to S. japonicum eggs can be greatly influenced by antibody, while antibody has no effect on the granulomas around S. mansoni eggs. Adult worm pairs of S. japonicum tend to cluster in the mesenteric venules, and most eggs are laid in a few sites. This leads to large, focal intestinal lesions similar to the discrete lesions produced by S. haematobium in the intestine and urinary tract but in contrast to the widespread, diffuse lesions produced by S. mansoni. Comparison with S. japonicum infection in humans is limited chiefly by our scant knowledge of the pathology produced by S. japonicum in infected persons. Most such comparisons are, in any case, limited by the marked differences in the reactions of various experimental host species to the infection and by differences in the reaction of a given host species to different strains of the parasite.     

Mating interactions between Schistosoma haematobium and S. mansoni

Schistosoma haematobium and S. mansoni are two medically important schistosomes, commonly occurring sympatrically in Africa and so potentially able to infect the same human host. Experiments were designed to study the mating behaviour of these two species in mixed infections in hamsters. Analysis of the data obtained showed that both heterospecific and homospecific pairs readily form. No significant difference was seen between the two species in their ability in forming pairs, however, S. mansoni showed a greater homospecific mate preference. Analysis of the data using the Mantel-Haenszel test suggests that mating competition does occur between S. haematobium and S. mansoni, the former being the more dominant species. Both species appeared to be able to change mate, with S. haematobium showing a greater ability in taking S. mansoni females away from S. mansoni males when introduced into a pre-established S. mansoni infection highlighting the competitiveness of S. haematobium. The significance of the results is discussed in relation to the epidemiological consequences occurring in Senegal, and other areas where both species are sympatric.     

Analysis and comparison of cercarial emergence rhythms of Schistosoma haematobium, S. intercalatum, S. bovis, and their hybrid progeny

In identical experimental conditions, the three schistosomes of the terminal spined egg group (S. haematobium, S. intercalatum and S. bovis) showed significant differences in their cercarial shedding patterns. The cercariae of the F1 hybrids, obtained by experimental crosses between these three species, showed the same circadian emergence rhythm (with one peak) as the cercariae of the parental species. However, the mean shedding time of these hybrid parasites (12:14 +/- 1 h 34 min for S. haematobium x S. intercalatum; 09:58 +/- 1 h 24 min for S. haematobium x S. bovis; 08:57 +/- 1 h 19 min for S. bovis x S. intercalatum) was always in advance to the one of their parental species (13:51 +/- 2 h 04 min for S. haematobium; 13:59 +/- 1 h 55 min for S. intercalatum; 10:09 +/- 2 h 04 min for S. bovis). These results are compared with those obtained from crosses between schistosomes with lateral spined eggs, and from crosses between intraspecific chronobiological variants of S. mansoni. They corroborate the genetic determinism of the cercarial emergence of schistosomes. Moreover, such significant differences between the hybrids and their parents in the times of cercarial emission may be of value in the epidemiological research and characterization of naturally occurring hybrids.     

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.     

Schistosoma haematobium (Egyptian strain): rate of development and effect of praziquantel treatment

This study investigates the development of the Egyptian strain of Schistosoma haematobium and the resultant immunohistopathology and biochemical changes in organs affected. In addition, the response of different developmental stages of S. haematobium worms to praziquantel (PZQ) was examined. Schistosoma haematobium-infected hamsters were classified into 4 groups and were treated at day 35, 55, 75, and 95 postinfection (PI), respectively. Each group was subdivided into 3 subgroups. Two of them were treated orally with PZQ (300 mg/kg or 500 mg/kg divided equally on 2 consecutive days), and the third group was left without treatment. Treated groups were killed 20 days posttreatment. Infection with S. haematobium became patent 73 days PI; tissue egg load and worm fecundity were higher at 95 days and maximal 115 days PI, with an oogram pattern comparable to that in Schistosoma mansoni infection. In the liver, small cellular granulomas were observed 75 days PI, with preponderance of CD4+ T-cell phenotypes. In the urinary bladder, only submucosal focal Brunn's-nest formation and angiogenesis without typical granulomas were observed. Ninety-five and 115 days PI, confluent granulomata with multiple eggs in the center were observed in the liver and urinary bladder, with a preponderance of CD8+ positive T cells in the liver and hyperplasia of the urinary bladder epithelium with cystitis cystica and papillae formation. One hundred percent worm eradication was recorded with the higher dose of PZQ in animals treated 75 and 95 days PI. In conclusion, in spite of the long prepatent period of the Egyptian strain of S. haematobium, sensitivity to PZQ was recorded soon after infection. Granulomata were similar to those of S. mansoni in the livers and urinary bladders, but they were confluent with multiple eggs in the centers, hyperplasia of the urinary bladder urothelium with cystitis cystica, papillae, and Brunn's-nest formation predictive of malignant changes with no hepatocyte dysplasia.     

Schistosoma mekongi and Schistosoma japonicum: Differences in the distribution of eggs in the viscera of mice

The difference in the distribution of Schistosoma eggs in the viscera has not been clearly elucidated in the two closely related species Schistosoma japonicum and Schistosoma mekongi. In this study, we quantitatively compared the distribution of eggs in mice infected with the two species. In S. mekongi-infected mice, 56.6% to 69.4% of total eggs were found in the distal small intestine 9 to 15 weeks after infection, while in S. japonicum-infected mice, 48.8% to 71.8% of eggs were found in the proximal small intestine during the same period. There were significantly more eggs in the liver in mice infected with S. japonicum than in those infected with S. mekongi. The number of adult worms recovered did not differ between the two species during the study period. The total number of eggs laid in the tissues also did not differ between the two species at 12 to 15 weeks postinfection, but in the earlier period the total number of eggs was significantly fewer in S. mekongi-infected than in S. japonicum-infected mice, suggesting the delayed maturation of the former compared with the latter. These results clearly show that S. japonicum and S. mekongi exhibit different oviposition behavior in their hosts.