Micro-Geographical Heterogeneity in Schistosoma mansoni and S. haematobium Infection and Morbidity in a Co-Endemic Community in Northern Senegal

Background

Schistosoma mansoni and S. haematobium are co-endemic in many areas in Africa. Yet, little is known about the micro-geographical distribution of these two infections or associated disease within such foci. Such knowledge could give important insights into the drivers of infection and disease and as such better tailor schistosomiasis control and elimination efforts.

Methodology

In a co-endemic farming community in northern Senegal (346 children (0–19 y) and 253 adults (20–85 y); n = 599 in total), we studied the spatial distribution of S. mansoni and S. haematobium single and mixed infections (by microscopy), S. mansoni-specific hepatic fibrosis, S. haematobium-specific urinary tract morbidity (by ultrasound) and water contact behavior (by questionnaire). The Kulldorff''s scan statistic was used to detect spatial clusters of infection and morbidity, adjusted for the spatial distribution of gender and age.

Principal Findings

Schistosoma mansoni and S. haematobium infection densities clustered in different sections of the community (p = 0.002 and p = 0.023, respectively), possibly related to heterogeneities in the use of different water contact sites. While the distribution of urinary tract morbidity was homogeneous, a strong geospatial cluster was found for severe hepatic fibrosis (p = 0.001). Particularly those people living adjacent to the most frequently used water contact site were more at risk for more advanced morbidity (RR = 6.3; p = 0.043).

Conclusions/Significance

Schistosoma infection and associated disease showed important micro-geographical heterogeneities with divergent patterns for S. mansoni and S. haematobium in this Senegalese community. Further in depth investigations are needed to confirm and explain our observations. The present study indicates that local geospatial patterns should be taken into account in both research and control of schistosomiasis. The observed extreme focality of schistosomiasis even at community level, suggests that current strategies may not suffice to move from morbidity control to elimination of schistosomiasis, and calls for less uniform measures at a finer scale.     

Cytokine Interaction and Immune Responses during Schistosoma mansoni Infection

Of the estimated 200 million people infected with Schistosoma, a subset develop severe life-threatening disease. Adult Schistosoma mansoni are refractory to the immune response and are long-lived, causing chronic exposure to parasite antigen. Although the adult worms themselves are not antigenically inert, it is the parasite eggs that, by accumulating in the liver and traversing the intestinal wall, place a complex series of often-conflicting demands on the host's immune system. In this article, Laura Rosa Brunet, David Dunne and Edward Pearce discuss data from experimental studies in the mouse and field studies in endemic areas that combine to suggest that it is a failure to juggle this immunological conflict that results in severe disease.     

Epidemiology of mixed Schistosoma mansoni and Schistosoma haematobium infections in northern Senegal

Due to the large overlap of Schistosoma mansoni- and Schistosoma haematobium-endemic regions in Africa, many people are at risk of co-infection, with potential adverse effects on schistosomiasis morbidity and control. Nonetheless, studies on the distribution and determinants of mixed Schistosoma infections have to date been rare. We conducted a cross-sectional survey in two communities in northern Senegal (n=857) to obtain further insight into the epidemiology of mixed infections and ectopic egg elimination. Overall prevalences of S. mansoni and S. haematobium infection were 61% and 50%, respectively, in these communities. Among infected subjects, 53% had mixed infections and 8% demonstrated ectopic egg elimination. Risk factors for mixed infection - i.e. gender, community of residence and age - were not different from what is generally seen in Schistosoma-endemic areas. Similar to overall S. mansoni and S. haematobium infections, age-related patterns of mixed infections showed the characteristic convex-shaped curve for schistosomiasis, with a rapid increase in children, a peak in adolescents and a decline in adults. Looking at the data in more detail however, the decline in overall S. haematobium infection prevalences and intensities appeared to be steeper than for S. mansoni, resulting in a decrease in mixed infections and a relative increase in single S. mansoni infections with age. Moreover, individuals with mixed infections had higher infection intensities of both S. mansoni and S. haematobium than those with single infections, especially those with ectopic egg elimination (P<0.05). High infection intensities in mixed infections, as well as age-related differences in infection patterns between S. mansoni and S. haematobium, may influence disease epidemiology and control considerably, and merit further studies into the underlying mechanisms of Schistosoma infections in co-endemic areas.     

Bladder Morbidity and Hepatic Fibrosis in Mixed Schistosoma haematobium and S. mansoni Infections: A Population-Wide Study in Northern Senegal

Background

The global distribution map of schistosomiasis shows a large overlap of Schistosoma haematobium- and S. mansoni-endemic areas in Africa. Yet, little is known about the consequences of mixed Schistosoma infections for the human host. A recent study in two neighboring co-endemic communities in Senegal indicated that infection intensities of both species were higher in mixed than in single infections. Here, we investigated the relationship between mixed Schistosoma infections and morbidity in the same population. So far, this has only been studied in children.

Methods

Schistosoma infection was assessed by microscopy. Schistosoma-specific morbidity was assessed by ultrasound according to WHO guidelines. Multivariable logistic regression models were used to identify independent risk factors for morbidity.

Principal Findings

Complete parasitological and morbidity data were obtained from 403 individuals. Schistosoma haematobium-specific bladder morbidity was observed in 83% and S. mansoni-specific hepatic fibrosis in 27% of the participants. Bladder morbidity was positively associated with S. haematobium infection intensity (OR = 1.9 (95% CI 1.3–2.9) for a 10-fold increase in intensity). Moreover, people with mixed infections tended to have less bladder morbidity than those with single S. haematobium infections (OR = 0.3 (95% CI 0.1–1.1)). This effect appeared to be related to ectopic S. mansoni egg elimination in urine. Hepatic fibrosis on the other hand was not related to S. mansoni infection intensity (OR = 0.9 (95% CI 0.6–1.3)), nor to mixed infections (OR = 1.0 (95% CI 0.7–1.7)).

Conclusions/Significance

This is the first population-wide study on the relationship between mixed Schistosoma infections and morbidity. Mixed infections did not increase the risk of S. mansoni-associated morbidity. They even tended to reduce the risk of S. haematobium-associated morbidity, suggesting a protective effect of S. mansoni infection on bladder morbidity. These unexpected results may have important consequences for schistosomiasis control in co-endemic areas and warrant further investigation.     

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.     

Susceptibility of Iraqi fresh water snails to infection with Schistosoma haematobium and Schistosoma mansoni Egyptian strains

A great number of Egyptian workers and farmers are seeking settlement in Iraq and some of them proved to have either Schistosoma Haematobium (S.h.) or Schistosoma mansoni (S.m) or even mixed infection. Besides, there is the possibility that some of the Iraqi fresh water snails may prove to be susceptible to infection by one or both of the Schistosoma Egyptian strains. The present study deals with investigations on the susceptibility of Iraqi B. truncatus, Gyranaulus ehrenbergi, Physa c.f. fontinalis, Lymnea lagetis, Melanoides tuberculata and Melanopsis nodes by these parasites. Egyptian S. haematobium but not Egyptian S. mansoni infect Iraqi B. truncatus and both proved to be unable to infect any of the other snails included in the study. Yet, the number of cercariae shedded by B. truncatus snails infected with the Egyptian S. haematobium strain, was much less that the number of cercariae shedded by these snails when infected with the Iraqi S. Haematobium strain.     

Exposure,hazard, and vulnerability all contribute to Schistosoma haematobium re-infection in northern Senegal

BackgroundInfectious disease risk is driven by three interrelated components: exposure, hazard, and vulnerability. For schistosomiasis, exposure occurs through contact with water, which is often tied to daily activities. Water contact, however, does not imply risk unless the environmental hazard of snails and parasites is also present in the water. By increasing reliance on hazardous activities and environments, socio-economic vulnerability can hinder reductions in exposure to a hazard. We aimed to quantify the contributions of exposure, hazard, and vulnerability to the presence and intensity of Schistosoma haematobium re-infection.Methodology/Principal findingsIn 13 villages along the Senegal River, we collected parasitological data from 821 school-aged children, survey data from 411 households where those children resided, and ecological data from all 24 village water access sites. We fit mixed-effects logistic and negative binomial regressions with indices of exposure, hazard, and vulnerability as explanatory variables of Schistosoma haematobium presence and intensity, respectively, controlling for demographic variables. Using multi-model inference to calculate the relative importance of each component of risk, we found that hazard (Ʃw_{i =} 0.95) was the most important component of S. haematobium presence, followed by vulnerability (Ʃw_i = 0.91). Exposure (Ʃw_i = 1.00) was the most important component of S. haematobium intensity, followed by hazard (Ʃw_i = 0.77). Model averaging quantified associations between each infection outcome and indices of exposure, hazard, and vulnerability, revealing a positive association between hazard and infection presence (OR = 1.49, 95% CI 1.12, 1.97), and a positive association between exposure and infection intensity (RR 2.59–3.86, depending on the category; all 95% CIs above 1)Conclusions/SignificanceOur findings underscore the linkages between social (exposure and vulnerability) and environmental (hazard) processes in the acquisition and accumulation of S. haematobium infection. This approach highlights the importance of implementing both social and environmental interventions to complement mass drug administration.     

Interpreting low praziquantel cure rates of Schistosoma mansoni infections in Senegal

Praziquantel is currently the drug of choice for the treatment of schistosomiasis. Selective treatment of Schistosoma mansoni infections in various endemic countries usually present cure rates of >70% when using the standard dose of 40 mg kg(-1) body weight of praziquantel. However, unusually low cure rates (18-38%) have been reported from Senegal, raising fears for emergence of resistance (or tolerance) to praziquantel. One major problem is the precise quantitative interpretation of cure rates, which allows an unequivocal distinction between drug failure and normal drug performance. This article reviews studies on praziquantel treatment of population by standardizing the data through an innovative meta-analysis and provides empirical evidence concerning the extent to which the reported low cure rates from Senegal are atypical.     

The effect of treatment on the age-antibody relationship in children infected with Schistosoma mansoni and Schistosoma haematobium

The effect of praziquantel treatment on the age-antibody relationship was studied in 174 children aged between 6 and 17 years from a schistosome endemic area in Zimbabwe. The children were co-infected with Schistosoma mansoni and S. haematobium with infection prevalences of 74% and 53% respectively. Antibody levels for the isotypes IgA, IgE, IgM, IgG1, IgG2, IgG3 and IgG4, directed against soluble egg antigen were measured using an indirect ELISA assay. Treatment resulted in a significant increase in levels of IgG2 and IgG3 while levels of IgA decreased significantly. In untreated children there were significant decreases in levels of IgG4. Treatment also resulted in significant alteration in the age-antibody profiles for the isotypes IgE, IgM, IgG1 and IgG2 in treated children but not in untreated children. The results are discussed in the context of factors believed to give rise to the age-antibody relationship; i.e. age-related exposure patterns, age-related development of acquired immunity, age-related hormonal changes and age-related changes in innate susceptibility to infection.     

Unique T Cells with Unconventional Cytokine Profiles Induced in the Livers of Mice during Schistosoma mansoni Infection

During infection with Schistosoma, serious hepatic disorders are induced in the host. The liver possesses unique immune systems composed of specialized cells that differ from those of other immune competent organs or tissues. Host immune responses change dramatically during Schistosoma mansoni infection; in the early phase, Th1-related responses are induced, whereas during the late phase Th2 reactions dominate. Here, we describe unique T cell populations induced in the liver of mice during the period between Th1- and Th2-phases, which we term the transition phase. During this phase, varieties of immune cells including T lymphocytes increase in the liver. Subsets of CD4⁺ T cells exhibit unique cytokine production profiles, simultaneously producing both IFN-γ and IL-13 or both IFN-γ and IL-4. Furthermore, cells triply positive for IFN-γ, IL-13 and IL-4 also expand in the S. mansoni-infected liver. The induction of these unique cell populations does not occur in the spleen, indicating it is a phenomenon specific to the liver. In single hepatic CD4⁺ T cells showing the unique cytokine profiles, both T-bet and GATA-3 are expressed. Thus, our studies show that S. mansoni infection triggers the induction of hepatic T cell subsets with unique cytokine profiles.     

Evidence for the presence of active cytochrome P450 systems in Schistosoma mansoni and Schistosoma haematobium adult worms

Extracts of the adult worms of both Schistosoma mansoni and Schistosoma haematobium can metabolise some typical P450 substrates but to differing degrees. S. mansoni worm extracts displayed a approximately 12-fold higher specific activity for an aminopyrine substrate than rat liver microsomes. At 4 mM substrate concentration the demethylation reaction with N-nitrosodimethylamine (NDMA) (5 nmol HCHO/mg protein/min) was only half that of rat liver microsomes, whereas in extracts of S. haematobium, no detectable activity was found towards NDMA. Using ethylmorphine as substrate the demethylation activity of S. mansoni extracts (1.82 nmol HCHO/mg protein/min) was 5.5-fold lower than that of rat liver microsomes. Benzphetamine demethylase activity was also readily detectable in S. mansoni worm extracts at 6.79 nmol HCHO/mg protein/min compared with 10.20 nmol HCHO/mg protein/min in the case of rat liver microsomes. When aniline was used as substrate, surprisingly, no activity was found in worm extracts of either S. mansoni or S. haematobium, whereas rat liver microsomes showed high activity towards this amine. The anti-P450 2E1 and 2B1/2 cross-reacted with both worm homogenates and gave a specific band corresponding to a protein of molecular weight of approximately 50.0 kDa. A study with anti-P450 IVA antibody revealed that while this protein was strongly expressed in S. haematobium worm extracts, no immunoreactivity was observed with extracts of S. mansoni. Immunoblotting analyses with anti-P450 IIIA and P450 1A1 did not detect immunoreactive protein in either S. mansoni or S. haematobium.     

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.     

Schistosoma mansoni and Schistosoma haematobium: identification and characterization of glycoconjugate antigens in the hemolymph of infected vector snails

Two carbohydrate epitopes were identified by monoclonal antibodies (KCS and E2) and characterized with respect to their immunoreactivity, monosaccharide structure, and location. Immunofluorescence demonstrated the presence of both epitopes on the surfaces of sporocysts, cercariae, and miracidia of Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum. However, spatial distribution and density of expression varied among species and developmental stages, and neither epitope was detectable on adult worm surfaces. Both glycans were found in the hemolymph of infected, but not uninfected, intermediate snail hosts. The presence of epitopes in hemolymph, as well as in schistosome eggs, is species-specific for KCS, recognizing only S. mansoni, and partly specific for E2, which reacted predominantly with S. haematobium. Immunoaffinity purification of target antigens for KCS and E2 from hemolymph of infected Biomphalaria and Bulinus, respectively, followed by carbohydrate composition analysis revealed a high content of fucose in both glycans. Methylation analysis demonstrated exclusively terminal fucose for the target antigen of KCS and terminal as well as internal fucose for the one of E2. Removal of terminal fucose abolished reactivity with both monoclonal antibodies. Both glycans are different from previously characterized schistosome carbohydrates. Their biological function(s) remain to be defined.     

Relation of tegumentary phosphohydrolase to purine and pyrimidine transport in Schistosoma mansoni.

Inhibition of the saturable influx of 0.05 mM 14C-labeled adenine or adenosine by AMP in adult Schistosoma mansoni in vitro suggested hydrolysis of this nucleotide at the tegumental surface of the parasite. Adenosine liberated as a result of AMP hydrolysis was the inhibitor of uptake of labeled adenine or adenosine. Inhibition of adenosine uptake by AMP or ATP was relieved by paranitrophenyl phosphate or ammonium molybdate supporting the hypothesis of nucleotide hydrolysis at the tengumental surface. Addition of glucose-1-phosphate, glucose-6-phosphate, NaF, or cysteine did not relieve AMP inhibition of adenosine uptake indicating substrate and inhibitor specificity for the surface enzyme(s). AMP, ATP, UMP, and p-nitrophenyl are hydrolyzed, at least in part, by the same enzyme(s). Apparent absorption of labeled AMP was preceded by hydrolysis, with labeled adenosine as the actual compound absorbed, although there was a small diffusion component for absorption of intact AMP. The site of nucleotide hydrolysis in close proximity to absorption sites provides a kinetic advantage for uptake of products of adenine nucleotide hydrolysis but not for products of uracil nucleotide hydrolysis.