Epidemic Schistosoma mansoni in a known S. haematobium area

An epidemic of bloody diarrhoea was observed in 1997 and 1998 in Bessoum, north Cameroon. Of 1176 inhabitants, 16.7% went for medical consultation to a public health centre located 10 km away. This epidemic was probably due to the recent introduction of Schistosoma mansoni in an ancient focus of S. haematobium, following the development of irrigated farming. The prevalences amongst primary school children were 76.6% for S. haematobium in urine, and 60.9% for S. mansoni in stool. S. mansoni was found in urine in 28.1%. This epidemic emergence may be secondary to interspecific competition between the 2 schistosomes.     

Experimental schistosomiasis in primates in Tanzania. Preliminary observations on the susceptibility of the baboon Papio anubis to Schistosoma haematobium and Schistosoma mansoni

Laboratory infection of animals with Schistosoma haematobium is generally unsatisfactory as adult worms invariably inhabit the portal venous system rather than the vesical plexus as in man. However, it was thought that certain primates might prove more valuable for experimental studies of schistosomiasis than the usual laboratory animals. Baboons, Papio anubis, were therefore exposed to cercariae of S. haematobium and the pattern of egg excretion in stools and urine was followed quantitatively. Histological studies of various organs were made and it was found that although eggs were excreted in the faeces, they were also passed in the urine and that tissue changes in the bladder were similar to those found in human infections. It is suggested that the sequelae of S. haematobium infection found in man might develop in baboons and that the animal may be useful for studying their development in the laboratory.     

The recognition of Schistosoma mansoni surface antigens by antibodies from patients infected with S. mansoni and S. haematobium

Polypeptide surface antigens of Schistosoma mansoni recognized by schistosomiasis patients have been identified and their strain and species specificity investigated. Antibodies from individuals infected with S. mansoni were used in immunoprecipitation assays of 125I-labelled schistosomulum surface antigens. All individuals surveyed from St. Lucia strongly precipitated antigens of approximately Mr 38,000 to 32,000 and 20,000. These antigens were shown by two-dimensional gel electrophoresis to be the same as those recognized by experimentally immunized mice. Although individuals showed a highly heterogeneous response against total polypeptide antigens synthesized in vitro by cell-free translation of adult S. mansoni mRNA, all individuals recognized the same surface antigens. Immunoprecipitation with sera from patients infected with S. mansoni in many different parts of Africa resulted in generally the same antigens being precipitated, although a very high molecular weight antigen(s), not strongly recognized by the St. Lucian sera was also precipitated by most of the African patient sera. One serum from Ghana precipitated the high molecular weight antigen but not the low molecular weight antigens, raising the possibility of the existence of S. mansoni strain(s) exhibiting some diversity in surface antigens. The surface of S. mansoni schistosomula was found to bind strongly antibodies from individuals infected with S. haematobium, demonstrating that most surface antigens are cross-reactive. Immunoprecipitation demonstrated, however, that of the polypeptide surface antigens only the very high molecular weight antigen was recognized by anti-S. haematobium antibodies and that the 38,000 to 32,000 and 20,000 Mr antigens were species-specific. Immunoprecipitation of the polypeptide antigens derived from purified adult surface membranes demonstrated recognition of the same 32,000, 25,000 and 20,000 Mr antigens recognized by chronically infected mice. Again these antigens were found to be species-specific.     

Non-human vertebrate hosts of Schistosoma haematobium and Schistosoma mansoni

The author reviews the results of experimental infections of various species of mammals, other than man, with S. haematobium and S. mansoni, and discusses investigations in Africa and Brazil into the possibility of the natural infection of non-human vertebrates with these two parasites. Only a few species, besides monkeys, could be easily infected with S. haematobium in the laboratory, while-outside man-natural infection with this parasite appears to be practically non-existent. On the other hand, many animals are good experimental hosts for S. mansoni, and at least 21 species of mammals have been found infected with this parasite in Africa and America. It is thus possible to state, provisionally, that man is the only reservoir of S. haematobium, but the question still remains open where S. mansoni is concerned. Further research is suggested in order to assess the importance of non-human reservoirs in the epidemiology of bilharziasis.     

Assessment of severity of disease caused by Schistosoma haematobium and S. mansoni in the Egypt-49 project area

The impact of bilharziasis on a community has been evaluated in terms of the stages and grades of severity of the disease; egg counts in faeces and urine were correlated with the clinical severity. At the time this study was carried out, the over-all prevalence of S. haematobium infection was 37.6%, that of S. mansoni infection 29.8% and that of mixed infections 17.1%.     

Schistosoma mansoni in the Kenyan baboon (Papio anubis): the development and predictability of resistance to homologous challenge.

Groups of baboons were exposed to primary infections of either 500 or 2,000 Schistosoma mansoni cercariae per baboon (c.p.b.). Five from each infection level and five uninfected baboons were challenged with 2,500 c.p.b. at one of four intervals of time after primary infection and killed ten weeks later, together with unchallenged appropriate primary infection controls. Primary faecal egg excretion was related to the cercarial dose, showing some systematic fluctuations during the 78 weeks of the experiment. Challenge infections increased faecal egg excretion in certain cases only. Faecal and tissue egg production were usually suppressed in the challenge worms. In contrast to less heavily infected, challenge-control baboons bearing primary infections, the challenged baboons had minimal gross pathology and there were no deaths due to acute schistosomiasis from the challenge infection. Over-all resistance to reinfection was low and unrelated to the age or intensity of the primary infections. However, seven baboons yielded less than 50% of the expected challenge worms. An in vitro assay, measuring anti-schistosomula antibody and peripheral leucocyte cytotoxic activity, successfully identified the in vivo immune status at thetime of challenge of 14/18 baboons tested. The in vivo significance of the immunological mechanism upon which the test is based is discussed in relation to possible future baboon and human studies.     

The epidemiology of Schistosoma haematobium and S. mansoni infections in the Egypt-49 project area. 4. Measurement of the incidence of bilharziasis

The measurement of incidence, or the rate at which people become positive, for Schistosoma haematobium and S. mansoni was carried out in four parts of the Egypt-49 project area near Alexandria. For S. haematobium, rates as high as 22.8% per year were found for children 0-6 years old in a rural area; in the same area, the incidence of S. mansoni was 8.5% per year. The true incidence is underestimated because many cases become negative spontaneously. This loss rate of S. haematobium cases is 0.476 per year for children 0-4 years old, and 0.049 per year for those aged 5 and 6 years; for S. mansoni, the rates are 0.580 and 0.327, respectively. Despite the error, incidence is the most accurate and sensitive method of assessing the success of control operations, and is an important measurable parameter in epidemiology.     

Multiplex real-time PCR for the detection and quantification of Schistosoma mansoni and S. haematobium infection in stool samples collected in northern Senegal

A multiplex real-time PCR assay for the detection and quantification of Schistosoma mansoni and S. haematobium DNA in faecal samples was developed and evaluated as an alternative diagnostic method to study the epidemiology of schistosomiasis. Primers and probes targeting the cytochrome c oxidase gene were designed for species-specific amplification and were combined with an internal control. Using positive control DNA extracted from adult Schistosoma worms and negative control samples (n=150) with DNA from a wide range of intestinal microorganisms, the method proved to be sensitive and 100% specific. For further evaluation, duplicate stool specimens with varying S. mansoni egg loads were collected in northern Senegal from pre-selected individuals (n=88). The PCR cycle threshold values, reflecting parasite-specific DNA loads in faeces, showed significant correlation with microscopic egg counts both for S. mansoni in stool and S. haematobium in urine. The Schistosoma detection rate of PCR (84.1%) was similar to that of microscopy performed on duplicate stool samples (79.5%). The simple faecal sample collection procedure and the high throughput potential of the multiplex real-time PCR provide a powerful diagnostic tool for epidemiological studies on schistosomiasis in remote areas, with possibilities for extension to other helminths or protozoa using additional molecular targets.     

Stimulation of hamster and human lymphocyte cultures by soluble egg antigens (SEA) of Schistosoma haematobium and S. mansoni.

Soluble egg and adult worm antigen preparations of Schistosoma haematobium and S. mansoni were tested in hamster and human lymphocyte cultures. Primed lymphocytes from infected donors showed a marked blastogenic response to the homologous antigens. A cross-reactivity to the heterologous antigens was seen especially in S. mansoni infected hamsters.     

Lack of resistance to Schistosoma japonicum in mice immunized with irradiated S. mansoni cercariae

Mice immunized with irradiated Schistosoma mansoni cercariae were resistant to challenge with S. mansoni cercariae (mean resistance 53%) but not to challenge with S. japonicum cercariae (mean resistance -5%). Furthermore, the antibodies induced by vaccination with irradiated S. mansoni cercariae were more reactive with S. mansoni than with S. japonicum schistosomula. These results support the concept that the resistance induced by vaccination with irradiated cercariae is immunologically specific.     

Hybrids between Schistosoma haematobium and S. mattheei and their identification by isoelectric focusing of enzymes

Some biological features of F₁ hybrids between South African strains of Schistosoma haematobium and S. mattheei are described and compared with those of both parental species. The distinctive patterns of the G6PD and PGM isoenzymes, resolved by isoelectric focusing, of both species and of the hybrid are defined and the results of enzyme analyses of parasites isolated from human infections in the Transvaal are reported. These show that hybridization does occur naturally in man and that the shape of the eggs produced is not necessarily a guide to the genetic constitution of the enclosed larvae. The experimentally produced F₁ hybrids exhibit heterosis in their increased infectivity to both snails and hamsters, in their more rapid growth and earlier maturation and in the increased daily egg production per female worm when compared with both of the parental species. The possible practical implications of this are discussed.     

Egg output stability and the epidemiology of Schistosoma haematobium. II. An analysis of the epidemiology of endemic S. haematobium

On the basis of observations of school children described in Part I, a model for the infective process of S. haematobium in man in the Misungwi area of Tanzania is put forward. On the basis of steady challenge by cercariae, effective concomitant immunity, exponential death of worm pairs, the observed variance of egg output at age 10, and constant ranking by infection level in early life, the behaviour of egg output patterns with age in the community is predicted. Expected curves of output are compared with those observed in cross-sectional survey of over 4,000 people at Misungwi. Agreement is close. This provides additional evidence for the occurrence of concomitant immunity to S. haematobium in man and suggests that immunity cannot be disregarded in epidemiological models of the infection. It is suggested that there are 3 phases in an age-cohort's experience of endemic infection: acquisition and increase of infection, a phase of decreasing worm load combined with concomitant immunity, and a steady state phase, with loss of immunity and reinfection balancing, in later life. The need for better data on the nature of human immunity to schistosomiasis becomes apparent.