The infectivity and pathogenicity of geographical strains of Schistosoma mansoni

Schistosomiasis Morbidity Hotspots: Roles of the Human Host, the Parasite and Their Interface in the Development of Severe Morbidity

Schistosomiasis is the second most important human parasitic disease in terms of socioeconomic impact, causing great morbidity and mortality, predominantly across the African continent. For intestinal schistosomiasis, severe morbidity manifests as periportal fibrosis (PPF) in which large tracts of macro-fibrosis of the liver, visible by ultrasound, can occlude the main portal vein leading to portal hypertension (PHT), sequelae such as ascites and collateral vasculature, and ultimately fatalities. For urogenital schistosomiasis, severe morbidity manifests as pathology throughout the urinary system and genitals, and is a definitive cause of squamous cell bladder carcinoma. Preventative chemotherapy (PC) programmes, delivered through mass drug administration (MDA) of praziquantel (PZQ), have been at the forefront of schistosomiasis control programmes in sub-Saharan Africa since their commencement in Uganda in 2003. However, despite many successes, 'biological hotspots' (as distinct from 'operational hotspots') of both persistent high transmission and morbidity remain. In some areas, this failure to gain control of schistosomiasis has devastating consequences, with not only persistently high infection intensities, but both "subtle" and severe morbidity remaining prevalent. These hotspots highlight the requirement to revisit research into severe morbidity and its mechanisms, a topic that has been out of favor during times of PC implementation. Indeed, the focality and spatially-structured epidemiology of schistosomiasis, its transmission persistence and the morbidity induced, has long suggested that gene-environmental-interactions playing out at the host-parasite interface are crucial. Here we review evidence of potential unique parasite factors, host factors, and their gene-environmental interactions in terms of explaining differential morbidity profiles in the human host. We then take the situation of schistosomiasis mansoni within the Albertine region of Uganda as a case study in terms of elucidating the factors behind the severe morbidity observed and the avenues and directions for future research currently underway within a new research and clinical trial programme (FibroScHot).

Could diet composition modulate pathological outcomes in schistosomiasis mansoni? A systematic review of in vivo preclinical evidence

Schistosomiasis and malnutrition are often overlapped in poor communities, resulting in disproportionately high mortality rates. Currently, fragmented data make it difficult to define the relationship between diet and schistosomiasis. Thus, we systematically review the preclinical evidence on the impact of diet in Schistosoma mansoni infection. From a structured search, we recovered 27 original articles. All studies used mice and most of them investigated hypoproteic (70.37%), hyperlipidic (22.22%) or vitamin-deficient (7.41%) diets. Diets based on carbohydrate, zinc or milk supplementation were investigated at a reduced frequency (3.70% each). Hypoproteic diets attenuated parasitic load and granulomatous inflammation, but also reduced host resistance to S. mansoni infection, determining higher mortality rates. By stimulating steatohepatitis, parasitic load and granulomatous inflammation, hyperlipidic diets increase organ damage and mortality in infected animals. Although a high-sugar diet and vitamin restriction potentiate and zinc supplementation attenuates S. mansoni infection, the current evidence for these diets remains inconclusive. Analysis of methodological quality indicated that the current evidence is at high risk of bias due to incomplete characterization of the experimental design, diet composition and treatment protocols. From the bias analysis, we report methodological limitations that should be considered to avoid systematic reproduction of inconsistent and poorly reproducible experimental designs.

Praziquantel decreases fecundity in Schistosoma mansoni adult worms that survive treatment: evidence from a laboratory life-history trade-offs selection study

Background

Mass drug administration of praziquantel is the World Health Organization’s endorsed control strategy for schistosomiasis. A decade of annual treatments across sub-Saharan Africa has resulted in significant reductions of infection prevalence and intensity levels, although ‘hotspots’ remain. Repeated drug treatments place strong selective pressures on parasites, which may affect life-history traits that impact transmission dynamics. Understanding drug treatment responses and the evolution of such traits can help inform on how to minimise the risk of drug resistance developing, maximise sustainable control programme success, and improve diagnostic protocols.

Methods

We performed a four-generation Schistosoma mansoni praziquantel selection experiment in mice and snails. We used three S. mansoni lines: a praziquantel-resistant isolate (R), a praziquantel-susceptible isolate (S), and a co-infected line (RS), under three treatment regimens: untreated, 25 mg/kg praziquantel, or 50 mg/kg praziquantel. Life-history traits, including parasite adult-worm establishment, survival, reproduction (fecundity), and associated morbidity, were recorded in mice across all four generations. Predictor variables were tested in a series of generalized linear mixed effects models to determine which factors had a significant influence on parasite life-history traits in definitive hosts under different selection regimes.

Results

Praziquantel pressure significantly reduced adult-worm burdens across all generations and isolates, including within R-lines. However, previous drug treatment resulted in an increase in adult-worm establishment with increasing generation from P1 to F3. The highest worm numbers were in the co-infected RS line. Praziquantel treatment decreased adult-worm burden, but had a larger negative impact on the mean daily number of miracidia, a proxy for fecundity, across all three parasite isolates.

Conclusions

Our predicted cost of resistance was not supported by the traits we measured within the murine host. We did not find evidence for negative adult worm density-dependent effects on fecundity. In contrast, of the adult worms that survived treatment, even low doses of praziquantel significantly reduced adult-worm fecundity. Such reductions in worm fecundity post treatment suggest that egg - based measures of drug efficacy, such as Kato-Katz, may overestimate the short-term effect of praziquantel on adult - worm burdens. These findings have important implications for S. mansoni transmission control, diagnostic protocols, and the potential for undetected selection toward drug resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s40249-017-0324-0) contains supplementary material, which is available to authorized users.

A Study of the Granulomatous Responses Induced by Different Strains of Schistosoma mansoni

The increased pathogenesis of the Schistosoma mansoni BH strain compared with the SJ strain has been attributed to the number of granulomas formed in experimental infections, which increase the mortality in definitive hosts. The aim of the present study was to investigate the development of granulomas around the eggs of the S. mansoni BH and SJ strains and to determine whether this host reaction was strain specific. Four experimental groups were analyzed. Two groups contained mice inoculated in the caudal vein with eggs from the S. mansoni BH or SJ strains and the other two contained mice that were infected with cercariae of the BH strain prior to being inoculated with eggs. The number of granulomas per tissue area in the lungs and liver, as well as the size of the granulomas, was analyzed to characterize the response to schistosome infection. The largest granulomatous responses were observed around eggs of the BH strain. Granulomas covered a larger area in the lungs of mice that were previously infected with cercariae and subsequently inoculated with eggs of the BH strain. These results indicated that specific granulomatous responses occurred following an infection with the BH and SJ strains of S. mansoni.

Experimental Evaluation of the Pathogenicity of Different Strains ofSchistosoma mansoni

The pathogenesis of three different Schistosoma mansoni strains from the Brazilian states of Minas Gerais (BH strain) and São Paulo (SJ and SD strains) was evaluated in experimentally infected mice. Observations of the most severe clinical cases among local patients treated (SD strain) in the city of Campinas (São Paulo, Brazil) formed the basis of this study. Mice were used as definitive hosts and were infected with cercariae from Biomphalaria tenagophila (SJ and SD strains) and Biomphalaria glabrata (BH strains). The parameters analyzed were as follows: number of S. mansoni eggs in mice feces; number of granulomas per tissue area in liver, spleen, lungs, pancreas, and ascending colon; measurements of hepatic and intestinal granulomas; number of adult worms; and measurements of trematode eggs. The comparison among the three strains indicated that the SD strain, isolated in Campinas, presented a higher worm recovery relative to the number of penetrating cercariae. In addition, when compared to the SJ and BH strains, the SD strain demonstrated similar pathogenicity to the BH strain, with a greater quantity of granulomas in the viscera, as well as larger granulomas and eggs. Furthermore, a greater quantity of trematode eggs was also shed in the feces.

Susceptibility of two strains of mice to the infection with Schistosoma mansoni: parasitological and biochemical studies

In this article, two strains of mice BALB/C and C57 were infected with Egyptian strain of Schistosoma mansoni. BALB/C mice appeared to harbor fewer parasites than did C57 mice. The hepatic and intestinal tissues of C57 mice were loaded with more eggs than that of BALB/C mice. Regardless the strain of mice, the number of eggs per gram of liver tissues was higher than in the intestinal tissues. Some biochemical parameters were measured in the liver of infected and non-infected mice; a significant decrease in the activities of alkaline phosphatase, catalase, glutathione-s-transferase, glutathione, and total lipids of infected mice compared to their matched control were observed. However, there was a significant increase in malondialdehyde level of infected mice compared to their matched group. Detailed discussion on the parasitological and biochemical differences between the two strains was presented.

Microsatellite typing reveals strong genetic structure of Schistosoma mansoni from localities in Kenya

Genetic diversity and population structure of seven populations of Schistosoma mansoni sampled in Kenya were assessed using five microsatellite markers. The mean number of alleles per locus, expected heterozygosity in Hardy-Weinberg equilibrium and pairwise F(ST) values ranged from 5.2 to 10.7, 0.5-0.8 and 3.6-27.3%, respectively. These data reveal that S. mansoni populations in Kenyan have relatively high levels of genetic diversity and is significantly differentiated. Our data combined with information on biogeography support the hypothesis that the strong genetic structure in Kenyan schistosomes is as a result of limited gene flow and large population sizes. Resistance to anthelminthics has not been reported among the Kenyan schistosomes, we hypothesize that this is probably due to the very little gene flow among populations, thereby limiting opportunities for the spread of rare alleles that might confer resistance to the drugs.

Behavior in Mus musculus of Schistosoma mansoni from mollusks treated with hydrocortisone

Twenty mice were exposed to cercariae from mollusks treated with hydrocortisone and another 20 mice received cercariae from non-treated mollusks. The behavior of the parasites from the two groups of mollusks was compared based on the ability of cercariae to penetrate mice, on the total number of worms recovered after eight weeks of infection, on the relationship between the number of penetrating cercariae and the number of recovered worms and on the number of eggs in the feces. Treating the mollusks with hydrocortisone did not alter the ability of cercariae to penetrate mice nor did it affect the total number of worms recovered. The number of female worms, the number of coupled worms and the number of eggs in the feces were greater in mice infected by cercariae from mollusks treated with hydrocortisone.

[Genetic selection of Biomphalaria glabrata and Biomphalaria tenagophila seeking the alteration of the susceptibility and resistance to Schistosoma mansoni]

Generations of Biomphalaria glabrata and Biomphalaria tenagophila selected genetically for resistance and susceptibility to Schistosoma mansoni of strains BH and SJ were used in a study of the trematode adaptation to the intermediate host. Descendants of the planorbids were obtained by self-fertilization of the mollusks that became susceptible or resistant after exposure to the miracidia of Schistosoma mansoni. For Biomphalaria glabrata they were obtained from the following generations: Parental, F1S (Susceptible), F1R (Resistant), F2S and F2R. For Biomphalaria tenagophila the studied generations were: Parental, F1S, F1R and F50S. The comparison of the infection rates presented by the different generations showed that the increase in susceptibility was more easily obtained in both species. The difficulty in increasing the resistance of the mollusks to Schistosoma mansoni has important epidemiologic implications.

[Schistosoma mansoni: a study of pathogenesis of Santa Rosa strain (Campinas, SP, Brasil) in mice]

The present paper reports the comparison between a newly isolated strain of S. mansoni in the city of Campinas (SP, Brazil) named SR, with the strains BH (Belo Horizonte, MG, Brazil) and SJ (São José dos Campos, SP, Brazil). The study was done using mice experimentally infected with 70 cercariae from sympatric snails: B. glabrata for the BH strain and B. tenagophila for the SJ and SR strains. The data obtained revealed that the new strain is less pathogenic than the other two, since it yielded fewer worms and shed eggs, a lower number of granulomas and a smaller granuloma size in the liver and intestine.

[Schistosoma mansoni: the action of lovastatin on the murine model]

Aimed to evaluate the lovastatin action on Schistosoma mansoni oviposition, infected mice with 100 plus minus 10 cercariae of the LE strain were used. Thirty days after infection the animals were treated with 100, 200 and 400mg/kg of lovastatin, per os, during five consecutive days and then sacrificed 7, 15, 30 or 60 days after treatment. We analyzed: distribution of worms in mesenteries and liver; mortality of worms in the liver; alteration of the oogram; eggs counting in the jejunum and liver; presence of intrauterine eggs and morphology of the worms from the treated and control groups (infected and not treated animals). Significant statistical differences were found between treated and control groups when the presence of intrauterine eggs was considered and also, alteration on the oogram, eggs at different stages of development in jejunum, liver and in the body length of males and females. The morphological study of the worms showed that the degenerative modifications occurred, mainly in the reproductive system, with reduction and alteration of the viteline follicles and the ovary of the females. Also modifications in the males testicles were observed. The results suggest that the drug under study reduces, considerably, the female S. mansoni, oviposition increases the worms' size, leads to alteration in the reproductive system of males and females. It also may induce death of a significant part of the worm populations at the dose of 400mg/kg.

[Relationship between pathogenicity of Schistosoma mansoni in mice and the susceptibility of the vector mollusk. IV--Infectiousness of miracidia]

OBJECTIVE

The infection ability of miracidia of BH and SJ strains of S. mansoni, obtained from mice infected with cercariae taken from Biomphalaria glabrata and Biomphalaria tenagophila, genetically selected for susceptibility is compared with the infection ability of miracidia obtained from mice infected with larvae from non-selected mollusks.

MATERIAL AND METHOD

Progeny of S. mansoni resulting from successive infections of selected mollusk sproduced various generations of selected miracidia. Selection of B. glabrata and B. tengophila was carried out by autofertilization of mollusks susceptible to the BH and SJ strains of S. mansoni. Five generations of mollusks (from parental down to F4), were used in the experiment. Tests for the infectiousness of the miracidia used 10 larvae: susceptibility was checked starting on day 30 after infection, for 90 days, through observation for the presence of cercariae.

RESULTS

The results showed that susceptibility of the selected mollusks in the face of the respective sympatric strains was not altered by the selection process of S. mansoni. However F4 miracidia of the BH strain were more infectant for non-selected B. glabrate than parental miracidia of the same strain. Miracidia of BH and SJ strains, parental generation, and BH strain, F3 generation, showed the same infectiousness in selected B. glabrata. Nevertheless, these mollusks had distinct infection rates from allopatric selected miracidia (SJ strain, F4 generation). The generation of successive infections of S. mansoni SJ in selected B. tenagophila resulted in the adaptation of the worm strain to the species of mollusk. B. tenagophila was never susceptible to the BH strain, even when selected mollusks and trematodes were employed. The susceptibility/infectiousness of the pair B. tenagophila-SJ S. mansoni strain was only changed by the selection process of the mollusks.

CONCLUSION

As non-selected B. glabrata were more intensively infected by selected BH miracidia than by non-selected BH miracidia, one is led to surmise that the greater pathogenicity of S. mansoni from susceptible mollusks implies the greater infection ability of these miracidia.

Schistosoma mansoni: maturation rate and drug susceptibility of different geographic isolates

The fecundities and drug susceptibilities of Schistosoma mansoni isolates from Senegal, Puerto Rico, and Kenya have been examined in mice. The Senegal parasite, obtained from the field in 1993, was shown to have a longer prepatent period (eggs first recovered in the faeces on Day 46 after infection) than those of two isolates, from Puerto Rico and Kenya, that had been maintained for a long period in the laboratory (faecal eggs recovered on Days 38 and 36 after infection, respectively). A Kenyan isolate, also collected from the field in 1994, was shown to mature more slowly than the laboratory-maintained Kenyan isolate. Tissue egg counts confirmed that early in infection the fecundity of the recently collected isolates from Senegal and Kenya was significantly lower than that of the long-term laboratory-maintained Kenyan isolate. Praziquantel and oxamniquine treatment of 8-week-old infections caused a significant (P < 0.001) reduction in worm burden in all isolates tested. However, the reduction in worm burden after praziquantel treatment of infections of the Senegal isolate (50% reduction) was significantly lower than the > 90% reductions in worm burdens after praziquantel treatment of mice infected with either of the Kenyan isolates (P < 0.001). The study confirms that despite being tolerant to praziquantel, the Senegal isolate is fully susceptible to oxamniquine. The praziquantel tolerance of the Senegal parasite is not solely attributed to the state of maturation of the parasite at the time of drug administration.

[Relationship between the pathogenicity of Schistosoma mansoni in mice and the susceptibility of the vector mollusk. III. Mortality, body and viscera weight]

The relationship between the development of hepatomegaly, splenomegaly, body weight and mortality rate and the degree of susceptibility of Biomphalaria glabrata and B. tenagophila at which infective cercariae developed was studied. The study utilized Swiss mice, SPF, as definitive hosts and populations of snails genetically selected for character susceptibility as intermediate hosts. Low body weight and smaller viscera were observed in infected mice with cercariae originating from snails that showed a high degree of susceptibility. The higher susceptibility of molluscs infected with S. mansoni corresponded to a lower survival of the infected mice. These results lead to the conclusion that the higher degree of adaptation of the parasite to its intermediate hosts, evidenced by the high indexes of susceptibility, leads to different behaviour on the part of this parasite in its definitive host.

Antischistosomal activity of acridanone-hydrazones in Cebus monkeys experimentally infected with the SJ strain of Schistosoma mansoni

In this study, four compounds were utilized at the dose of 12.5 mg/kg body weight, p.o., to treat Cebus monkeys experimentally infected with about 200 cercariae of Schistosoma mansoni (SJ strain), via transcutaneous route. The oograms performed with rectal snips, as well as stool examinations carried out periodically, showed no viable eggs of the parasite, from day 29 to 226 post-treatment. The perfusion undertaken after killing the animals showed absence of worms in the treated monkeys, whereas 83 worms were recovered from the control, thus corroborating the results obtained by means of oograms and coproscopy. These results confirm the efficacy of 9-acridanone-hydrazones previously tested against the LE strain of S. mansoni. The low curative dose and apparent absence of toxicity render these drugs an important therapeutic reserve, taking into consideration the reports on the resistance of S. mansoni to the modern drugs oxamniquine and praziquantel.

Genetic diversity of Schistosoma mansoni: quantifying strain heterogeneity using a polymorphic DNA element

Intraspecific genetic variation among 14 geographic isolates of Schistosoma mansoni was quantified using a molecular marker to examine individual genotypes. Genetic crosses demonstrated maternal inheritance of S. mansoni DNA element pSM750. This element revealed diagnostic banding profiles, which allowed accurate strain identification. Most strains had similarity indices greater than 0.75 indicating that within-strain variation in these laboratory parasite populations was low. However, individual parasites from the NMRI strain were quite diverse (S = 0.40). Genetic heterogeneity among strains was quantified using a phenogram of mean genetic distance. Strain diversity between two geographic regions was quantified using a similarity index and was estimated to be substantial among isolates collected from a single local site.

[Relationship between the pathogenicity of Schistosoma mansoni in mice and the susceptibility of the mollusk vector. II. Number of eggs in feces and number and size of granuloma in visceras]

The influence of the susceptibility of the vector snails of S. mansoni on the development of the pathogenicity of the worm to the host was studied. Off-spring of snails Biomphalaria glabrata and Biomphalaria tenagophila were used, selected with regard to the susceptibility to S. mansoni of the strains BH and SJ trait, respectively. Parenteral, F1, F2, F3 and F4 generation cercariae were used for the infection of Swiss mice, which were killed eight weeks after infection. The number of eggs in the feces and the number of granulomas in the liver, spleen and intestines were counted. The size of the granulomatous reactions was evaluated. The results led to the conclusion that greater susceptibility of B. tenagophila was associated with a larger egg production in the feces and a higher density of granulomas in the liver. There is a clear inverse relationship between the area of hepatic, splenic and intestinal granulomas and the rate of infection of the snails which served as intermediate hosts for the S. mansoni.

[Relationship between the pathogenicity of Schistosoma mansoni in mice and the susceptibility of the vector mollusc. I. Cercariae infectivity and worm burden]

The possible influence of the immune mechanisms of the molluscs on the development of the sporocysts and the resultant behavior of the adult worm in the vertebrate host were studied. Two strains of Schistosoma mansoni (BH and SJ) were used. These were genetically selected for their susceptibility and maintained, respectively, in Biomphalaria glabrata and Biomphalaria tenagophila, the natural invertebrate hosts. Experimental groups of mice infected with cercariae proceeding from the two species of molluscs, belonging to generations P, F1, F2, F3 and F4, were formed. The infection rates of the snails, the number of the penetrant cercariae and the worm burden in mice were evaluated. It was concluded that the higher susceptibility of Biomphalaria tenagophila results in a greater ability on the part of the cercarial to develop into adult worms. The higher susceptibility of B. glabrata results in greater power of penetration into mice tegument on the part of the cercariae.

The infection characteristics of a South African isolate of Schistosoma mansoni: a comparison with a Puerto Rican isolate in BALB/c mice and Mastomys coucha

The infection characteristics of a South African (RSA) laboratory isolate of Schistosoma mansoni were compared with those of a widely studied Puerto Rican (PR) isolate in BALB/c mice and Mastomys coucha over the period of 5-12 weeks after infection. The infectivity, maturation and, in particular, the fecundity of the two schistosomes were found to differ markedly. Tissue egg-accumulation rates of the RSA strain were substantially lower than those of the PR strain. Although the maturation of both parasite strains was somewhat delayed in M. coucha, the characteristics of each was essentially consistent in both rodent hosts. Notwithstanding the possible influence of prolonged laboratory passage on the behaviour of schistosome isolates, the results of the present study are nevertheless considered to support the idea that parasite strain differences may, to some extent, account for regional variations in the morbidity of intestinal schistosomiasis. The need to be cognizant of the characteristics of individual host-parasite combinations when comparing different schistosome species and strains, particularly with respect to both their abilities to elicit immunopathology and induce concomitant immunity and their susceptibilities to chemotherapy, is highlighted.

[Behavior of Schistosoma mansoni strain LE, after passage through an accidentally infected human host]

Uma auxiliar de laboratório infectou-se acidentalmente, com cercárias de Schistosoma mansoni, cepa LE, mantida rotineiramente em nossos laboratórios. Decorridos 5 meses, o exame parasitológico de fezes revelou 108 ovos/g . A pacientefoi tratada com oxamniquine, porém a infecção continuou ativa (6 ovos/g). Foi então obtido o isolado SSF mantido no modelo Biomphalaria glabrata - camundongo albino. Os resultados obtidos no estudo comparativo, entre o isolado SSF e a cepa LE, que lhe deu origem, mostraram que a duração do período pré-patente e o índice de infectividade em camundongos, bem como a resposta aos agentes esquistossomicidas (hycanthone, oxamniquine epraziquantel) não apresentaram diferenças estatisticamente significativas. Por outro lado, o número de miracídios obtidos dos intestinos e fígados dos camundongos infectados foi o dobro com a cepa LE, quando comparados com aquele do isolado SSF. Também a variação do peso dos animais foi bastante diferente. Concluiu-se que apenas uma passagem pelo hospedeiro humano não mudou substancialmente as características da cepa estudada.

Comparison of two Egyptian strains of Schistosoma mansoni in hamsters

In human infection with Schistosoma mansoni from Beni-Suef, the eggs were encountered more frequently in the urine of patients than in infection with S. mansoni from Giza, where eggs were passed into the stool. A comparative study of the two strains of S. mansoni from Beni-Suef and Giza has been carried out in golden hamster. Consistent strain differences were observed. The Beni-Suef strain proved to have lower worm recovery and different egg distribution patterns in tissues of infected hamsters. Worms of both sexes of this strain were larger in size and required a longer period to reach maturity. Hence, the prepatent period was prolonged. Significant differences between the two strains were also noted in the number of eggs per worm. A lower mortality rate and a longer survival time were encountered in hamsters infected with the Beni-Suef strain.

Schistosoma japonicum: migration of adult worm pairs within the mesenteric veins of mice

Mice infected with a single worm pair of a Japanese, a Philippine and a Formosan strain of Schistosoma japonicum were examined to determine the frequency of movement of worms within the mesenteric veins eight to 76 weeks after infection. By the 18th week after infection, one or more gross focal lesions developed in the gut, and 47 to 89% of eggs in the gut were concentrated in these lesions. In spite of this predominantly focal oviposition, oograms revealed first-stage eggs scattered throughout the gut, indicating frequent movement of the worms along the length of the intestine. Multiple gross lesions, often far from each other, were seen in half the mice. Our findings indicate that worms pairs move frequently, but that they are attracted to existing gross lesions, probably by substances released into the mesenteric blood from eggs or the surrounding inflammatory exudate.

Modification of the pathogenicity of Schistosoma mattheei for sheep by passage of the parasite in hamsters

Border Leicester X Suffolk sheep infected with a strain of S. mattheei maintained in hamsters do not develop the same pathological changes as Romney Marsh sheep infected with the same strain of parasite before hamster passage. To determine the cause of this reduced pathogenicity, five Romney Marsh sheep were each infected with 10 000 cercariae of the hamster-passaged parasite and five with 10 000 cercariae of a S. mattheei strain from Onderstepoort, South Africa, passaged exclusively through sheep. Striking pathological and parasitological differences were found between the two strains. Infection with the "sheep" strain was lethal, whereas infection with the "hamster" strain produced little evidence of clinical disease. By 13 weeks post-infection the mean body weight of the sheep infected with the sheep strain had declined by 15% compared with both the uninfected controls and the sheep infected with the hamster strain, and the mean PCV was lowered to 20% in the sheep strain infected animals. Egg production began at seven weeks with the sheep strain, faecal counts rising to more than 300 e.p.g., whereas only two of the sheep infected with the hamster strain passed eggs in the faeces (at nine weeks) and the maximum egg count was 50 e.p.g. Twice as many adult worms of the sheep strain were recovered, and, although the number of eggs found in the tissues "per worm pair" was not significantly different, overall egg production was higher for the sheep strain; also more of the sheep strain eggs were deposited in the intestines. Similar parasite differences were seen in a supplementary study in mice and it seemed that "attenuation" of the parasite had occurred, presumably due to its maintenance in hamsters. Histopathological observations and faecal egg counts both indicated an inability of hamster strain eggs to penetrate the intestinal lumen; this was probably important in reducing the pathogenicity of the hamster strain.

A comparison of the pathogenicity of two geographical strains of Schistosoma mansoni in rhesus monkeys

1. Six rhesus monkeys were exposed to 750 cercariae of a Puerto Rican strain ofS. mansoniand six to the same dose of a Tanzanian strain.

2. The Puerto Rican strain proved to be much more pathogenic and during the early stages of the infection the animals infected with this strain were obviously more severely affected than the Mwanza strain animals; two died only two weeks after eggs appeared in the faeces.

3. With the Puerto Rican strain there was a shorter prepatent period and a highly significant difference in the infectivity as determined by the worm recovery. The mean maturation rate was 81% in the two fatal infections and 63·5% in the four remaining monkeys as compared with a mean of 45·9% in the 6 monkeys infected with the Tanzanian strain.

4. There were also differences in the pattern of egg output in the faeces with higher densities with the Puerto Rican strain in the initial stages but lower densities in the later stages of the infection. But it was noted that faecal egg output was a poor index of the severity of the infection. In the severely ill monkeys the faecal egg output expressed as the total egg output in 24 hours was very misleading because the affected animals had poor appetites and they passed very small quantities of blood stained mucus containing very little faecal material.

5. Tissue egg counts were far more reliable as was shown in a preliminary experiment where during a period of one week approximately 100,000 eggs were excreted in the faeces as compared with more than a million eggs retained in the tissues.

6. Details are given of the egg densities in the various organs of the monkeys. The most important difference between the two strains was the number of eggs in the liver with a mean of 1,222 eggs per g. in the Puerto Rican strain monkeys as compared with only 158·5 eggs per g. with the Mwanza strain.

7. Both groups of monkeys developed a marked lowering of the haemoglobin levels but this was not a strain characteristic and the severity was not related to the tissue egg densities. There was raised white count which reached the highest level at the time when the haemoglobin levels were at their lowest. There was an increase in the eosinophilic count in some of the animals but again this was not a strain characteristic. There was no eosinophilia in the two animals that died.

8. Fluorescent antibody levels were recorded at different stages of the infection together with the blood protein levels. The F.A. titres were at the highest levels in the acute stages of the disease. There was a significant correlation between the peak F.A. titres and the eosinophil levels in the blood. The percentage of gamma globulins in the sera showed a peak at the end of the period of observation when the F.A. titres were decreasing.

9. These observations support the clinical impression of the relatively low pathogenicity ofS. mansoniin Tanzania and other parts of Central and East Africa as compared with Egypt and South America. However, the inherent pathogenicity of the strain may not be the most important factor in accounting for the differences in the epidemiology. Differences in the reaction of the human population and in the intensity of transmission are probably equally important. In Africa there is also the possibility that in some areas worm loads are reduced due to the heterologous immunity that is acquired as a result of repeated exposure to the prevalent “nonpathogenic” schistosomes of domestic and wild animals.