Human resistance to Schistosoma infections: age or experience?

Schistosomiasis

Schistosomiasis is a major cause of morbidity in the world and almost 800 million people worldwide are at risk for schistosomiasis; it is second only to malaria as a major infectious disease. Globally, it is estimated that the disease affects more than 250 million people in 78 countries of the world and is responsible for some 280,000-500,000 deaths each year. The three major schistosomes infecting humans are Schistosoma mansoni, S. japonicum, and S. haematobium. This chapter covers a wide range of aspects of schistosomiasis, including basic biology of the parasites, epidemiology, immunopathology, treatment, control, vaccines, and genomics/proteomics. In this chapter, the reader will understand the significant toll this disease takes in terms of mortality and morbidity. A description of the various life stages of schistosomes is presented, which will be informative for both those unfamiliar with the disease and experienced scientists. Clinical and public health aspects are addressed that cover acute and chronic disease, diagnosis, current treatment regimens and alternative drugs, and schistosomiasis control programs. A brief overview of genomics and proteomics is included that details recent advances in the field that will help scientists investigate the molecular biology of schistosomes. The reader will take away an appreciation for general aspects of schistosomiasis and the current research advances.

Associations of water contact frequency, duration, and activities with schistosome infection risk: A systematic review and meta-analysis

BACKGROUND

Schistosomiasis is a water-borne parasitic disease which affects over 230 million people globally. The relationship between contact with open freshwater bodies and the likelihood of schistosome infection remains poorly quantified despite its importance for understanding transmission and parametrising transmission models.

METHODS

We conducted a systematic review to estimate the average effect of water contact duration, frequency, and activities on schistosome infection likelihood. We searched Embase, MEDLINE (including PubMed), Global Health, Global Index Medicus, Web of Science, and the Cochrane Central Register of Controlled Trials from inception until May 13, 2022. Observational and interventional studies reporting odds ratios (OR), hazard ratios (HR), or sufficient information to reconstruct effect sizes on individual-level associations between water contact and infection with any Schistosoma species were eligible for inclusion. Random-effects meta-analysis with inverse variance weighting was used to calculate pooled ORs and 95% confidence intervals (CIs).

RESULTS

We screened 1,411 studies and included 101 studies which represented 192,691 participants across Africa, Asia, and South America. Included studies mostly reported on water contact activities (69%; 70/101) and having any water contact (33%; 33/101). Ninety-six percent of studies (97/101) used surveys to measure exposure. A meta-analysis of 33 studies showed that individuals with water contact were 3.14 times more likely to be infected (OR 3.14; 95% CI: 2.08-4.75) when compared to individuals with no water contact. Subgroup analyses showed that the positive association of water contact with infection was significantly weaker in children compared to studies which included adults and children (OR 1.67; 95% CI: 1.04-2.69 vs. OR 4.24; 95% CI: 2.59-6.97). An association of water contact with infection was only found in communities with ≥10% schistosome prevalence. Overall heterogeneity was substantial (I2 = 93%) and remained high across all subgroups, except in direct observation studies (I2 range = 44%-98%). We did not find that occupational water contact such as fishing and agriculture (OR 2.57; 95% CI: 1.89-3.51) conferred a significantly higher risk of schistosome infection compared to recreational water contact (OR 2.13; 95% CI: 1.75-2.60) or domestic water contact (OR 1.91; 95% CI: 1.47-2.48). Higher duration or frequency of water contact did not significantly modify infection likelihood. Study quality across analyses was largely moderate or poor.

CONCLUSIONS

Any current water contact was robustly associated with schistosome infection status, and this relationship held across adults and children, and schistosomiasis-endemic areas with prevalence greater than 10%. Substantial gaps remain in published studies for understanding interactions of water contact with age and gender, and the influence of these interactions for infection likelihood. As such, more empirical studies are needed to accurately parametrise exposure in transmission models. Our results imply the need for population-wide treatment and prevention strategies in endemic settings as exposure within these communities was not confined to currently prioritised high-risk groups such as fishing populations.

Associations of IL13 gene polymorphisms and immune factors with Schistosoma haematobium infection in schoolchildren in four schistosomiasis-endemic communities in Ghana

BACKGROUND

Schistosomiasis remains a major public health issue with over 90% of the prevalence rates recorded in Sub-Saharan Africa. In this study, the relationships between different interleukin gene polymorphisms (IL-13-591A/G, IL-13-1055C/T, IL-13-1258A/G) and Schistosoma haematobium infection levels were evaluated; as well as the host plasma antibodies and cytokine profiles associated with schistosomiasis infection.

METHODOLOGY

A total of 469 school children aged 6 to 19 years from four schistosomiasis-endemic communities in Ghana were involved. Single urine and stool samples were obtained from each pupil, processed via sedimentation and Kato-Katz, and examined via microscopy for Schistosoma and soil-transmitted helminth (STH) eggs. Next, venous blood samples were drawn from 350 healthy pupils, and used to measure antibody and plasma cytokine levels by ELISA. Single nucleotide polymorphisms in the IL-13 gene were genotyped on 71 selected blood samples using the Mass Array technique.

PRINCIPAL FINDINGS AND CONCLUSION

The overall prevalence of urinary schistosomiasis was 21.11%. Community-level prevalences were 17.12%, 32.11%, 20.80%, and 15.32% for Asempaneye, Barikumah, Eyan Akotoguah, and Apewosika respectively. Generally, higher S. haematobium infection prevalence and intensity were recorded for participants with genotypes bearing the IL13-1055C allele, the IL13-591A, and the IL13-1258A alleles. Also, higher S. haematobium infection prevalence was observed among participants in the 12-14-year age group with the IL13-1055C, IL13-591A, and IL13-1258A alleles. Interestingly, higher STH prevalence was also observed among participants with the IL13-1055C, IL13-591A, and IL13-1258A alleles. Furthermore, the age-associated trends of measured antibodies and cytokines of S. haematobium-infected school-children depicted a more pro-inflammatory immune profile for pupils aged up to 1l years, and an increasingly anti-inflammatory profile for pupils aged 12 years and above. This work provides insight into the influence of IL-13 gene polymorphisms on S. haematobium, and STH infections, in school-aged children (SAC).

Schistosomiasis

Schistosomiasis is a major cause of morbidity in the world; it is second only to malaria as a major infectious disease. Globally, it is estimated that the disease affects over 250 million people in 78 countries of the world and is responsible for some 280,000 deaths each year. The three major schistosomes infecting humans are Schistosoma mansoni, S. japonicum, and S. haematobium. This chapter covers a wide range of aspects of schistosomiasis, including basic biology of the parasites, epidemiology, immunopathology, treatment, control, vaccines, and genomics/proteomics. In this chapter, the reader will understand the significant toll this disease takes in terms of mortality and morbidity. A description of the various life stages of schistosomes is presented, which will be informative for both those unfamiliar with the disease and experienced scientists. Clinical and public health aspects are addressed that cover acute and chronic disease, diagnosis, current treatment regimens and alternative drugs, and schistosomiasis control programs. A brief overview of genomics and proteomics is included that details recent advances in the field that will help scientists investigate the molecular biology of schistosomes. The reader will take away an appreciation for general aspects of schistosomiasis and research advances.

Intestinal Schistosomiasis among Primary Schoolchildren in Two On-Shore Communities in Rorya District, Northwestern Tanzania: Prevalence, Intensity of Infection and Associated Risk Factors

In Tanzania, Schistosoma mansoni is of great public health importance. Understanding the prevalence and infection intensity is important for targeted, evidence-based control strategies. This study aimed at studying the prevalence, intensity, and risk factors of S. mansoni among schoolchildren in the study area. A cross-sectional study was conducted in Busanga and Kibuyi villages. Sampled 513 schoolchildren provided stool specimens which were examined using kato-katz method. Pretested questionnaire was used to collect sociodemographic data and associated risk factors. The prevalence of S. mansoni infection was 84.01%, with geometric mean egg intensity of 167.13 (95% CI: 147.19–189.79) eggs per gram of stool (epg). Other parasites detected were Ascaris lumbricoides (1.4%) and hookworms (1.4%). The geometric mean infection intensity in Busanga and Kibuyi were 203.70 (95% CI: 169.67–244.56) and 135.98 (95% CI: 114.33–161.73) epg, respectively. Light, moderate, and heavy infection intensities were 34.11%, 39.91%, and 25.99%, respectively. Village of residence, parent's level of education, toilet use, and treatment history were predictors of infection. The high prevalence and infection intensity in this study were associated with village, parent's level of education, inconsistent toilet use, and treatment history. To control the disease among at-risk groups, these factors need to be considered in designing integrated schistosomiasis control interventions.

Intestinal schistosomiasis and geohelminths of Ukara Island, North-Western Tanzania: prevalence, intensity of infection and associated risk factors among school children

Background

Schistosoma mansoni and soil-transmitted helminths (STH) are among the most prevalent and highly neglected tropical diseases in Tanzania. However, little is known on the distribution of these infections in rural settings, especially in the island areas on Lake Victoria. Identifying the local risk factors of S. mansoni and soil-transmitted helminths is one step towards understanding their transmission patterns and will facilitate the design of cost-effective intervention measures. The present study was therefore conducted to determine the prevalence, intensity of infection and risk factors associated with S. mansoni and soil-transmitted helminth infections among school children in Ukara Island.

Methods

This was a cross sectional study which enrolled 774 school children aged 4-15 years in 5 primary schools in Ukara Island, North-Western Tanzania. Single stool samples were collected, processed using the Kato Katz technique and examined for eggs of S. mansoni and geohelminths under a light microscope. A pre-tested questionnaire was used to collect socio-demographic information.

Results

Overall, 494/773 (63.91%, 95% CI; 45.19-90.36) of the study participants were infected with S. mansoni and the overall geometrical mean eggs per gram (GM-epg) of feaces were 323.41epg (95% CI: 281.09 – 372.11). The overall prevalence of soil-transmitted helminth (STH) was 6.73% (n = 52/773, 95% CI = 4.39 – 10.32) with the most prevalent species being hookworms, 5.69% (n = 44/773, 95% CI; 3.68 – 8.79). Location of school in the study villages (P < 0.0001), parent occupation, fishing (P < 0.03) and reported involvement in fishing activities (P < 0.048) remained significantly associated with the prevalence and intensity of S.mansoni infection.

Conclusion

Schistosoma mansoni infection is highly prevalent in the islands whereas the prevalence of soil-transmitted helminths is low. The risk of infection with S. mansoni and the intensity of infection increased along the shorelines of Lake Victoria. These findings call for the need to urgently implement integrated control interventions, starting with targeted mass drug administration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0612-5) contains supplementary material, which is available to authorized users.

Inbreeding within human Schistosoma mansoni: do host-specific factors shape the genetic composition of parasite populations?

The size, structure and distribution of host populations are key determinants of the genetic composition of parasite populations. Despite the evolutionary and epidemiological merits, there has been little consideration of how host heterogeneities affect the evolutionary trajectories of parasite populations. We assessed the genetic composition of natural populations of the parasite Schistosoma mansoni in northern Senegal. A total of 1346 parasites were collected from 14 snail and 57 human hosts within three villages and individually genotyped using nine microsatellite markers. Human host demographic parameters (age, gender and village of residence) and co-infection with Schistosoma haematobium were documented, and S. mansoni infection intensities were quantified. F-statistics and clustering analyses revealed a random distribution (panmixia) of parasite genetic variation among villages and hosts, confirming the concept of human hosts as 'genetic mixing bowls' for schistosomes. Host gender and village of residence did not show any association with parasite genetics. Host age, however, was significantly correlated with parasite inbreeding and heterozygosity, with children being more infected by related parasites than adults. The patterns may be explained by (1) genotype-dependent 'concomitant immunity' that leads to selective recruitment of genetically unrelated worms with host age, and/or (2) the 'genetic mixing bowl' hypothesis, where older hosts have been exposed to a wider variety of parasite strains than children. The present study suggests that host-specific factors may shape the genetic composition of schistosome populations, revealing important insights into host-parasite interactions within a natural system.

The Schistosoma japonicum self-cure phenomenon in water buffaloes: potential impact on the control and elimination of schistosomiasis in China

Schistosomiasis japonica, caused by Schistosoma japonicum, is an important zoonotic disease in China, the Philippines and small pockets of Indonesia. In addition to infecting people, S. japonicum can infect over 40 species of wild and domestic animals which have varying impacts on human infection. It is now generally accepted that bovines, particularly water buffaloes, are the major reservoir for human infection in China as they are naturally infected with schistosomes and deposit more eggs into the environment than humans or any other animal host. This complicates control efforts and the economic burden associated with schistosomiasis morbidity and mortality has taken its toll on both human and livestock populations. Over the last 50years, the schistosomiasis control program in China has made great strides in reducing prevalence and morbidity, and the Chinese authorities now aim to eliminate the disease nationwide in the next decade. Current Chinese control strategies place particular importance on interventions targeting bovines including: praziquantel treatment, barrier farming to prevent grazing in transmission areas, their replacement with mechanized tractors and possible bovine vaccination. A number of studies have shown that in the period following S. japonicum infection, the worm burden drops sharply in water buffaloes and some other animal hosts such as pigs. This is due to a self-cure phenomenon whereby there is parasite clearance by both immune and non-immune factors. Here we review studies investigating the self-cure effect, paying particular attention to S. japonicum infection in water buffaloes, and discuss its potential impact on the future schistosomiasis control and elimination efforts in China. Further understanding of the mechanism of self-cure in water buffaloes could be important for future schistosome vaccine design and delivery.

A very high infection intensity of Schistosoma mansoni in a Ugandan Lake Victoria Fishing Community is required for association with highly prevalent organ related morbidity

Background

In schistosomiasis control programmes using mass chemotherapy, epidemiological and morbidity aspects of the disease need to be studied so as to monitor the impact of treatment, and make recommendations accordingly. These aspects were examined in the community of Musoli village along Lake Victoria in Mayuge district, highly endemic for Schistosoma mansoni infection.

Methodology and Principal Findings

A cross sectional descriptive study was undertaken in a randomly selected sample of 217 females and 229 males, with a mean age of 26 years (SD ±16, range 7–76 years). The prevalence of S. mansoni was 88.6% (95% CI: 85.6–91.5). The geometric mean intensity (GMI) of S. mansoni was 236.2 (95% CI: 198.5–460.9) eggs per gram (epg) faeces. Males had significantly higher GMI (370.2 epg) than females (132.6 epg) and age was also significantly associated with intensity of infection. Levels of water contact activities significantly influenced intensity of infection and the highest intensity of infection was found among people involved in fishing. However, organomegaly was not significantly associated with S. mansoni except for very heavy infection (>2000 epg). Liver image patterns C and D indicative of fibrosis were found in only 2.2% and 0.2%, respectively. S. mansoni intensity of infection was associated with portal vein dilation and abnormal spleen length. Anaemia was observed in 36.4% of the participants but it was not associated with S. mansoni infection intensity. Considering growth in children as one of the morbidity indicators of schistosomiasis, intensity of S. mansoni was significantly associated with stunting.

Conclusion

Although organ-related morbidity, with the exception of periportal fibrosis, and S. mansoni infections were highly prevalent, the two were only associated for individuals with very high infection intensities. These results contrast starkly with reports from Ugandan Lake Albert fishing communities in which periportal fibrosis is more prevalent.

Schistosoma mansoni infection is one of the Neglected Tropical Diseases (NTDs) that perpetuate poverty, especially in Sub Saharan Africa. It is associated with hepatomegaly, splenomegaly or hepatosplenomegaly, liver fibrosis and anaemia. Control of schistosomiasis is now a priority in most endemic countries in Africa as a component of integrated control of NTDs using mass drug administration (MDA). Other than the new WHO strategic plan to eliminate schistosomiasis as a public health problem in WHO Africa region by 2020, the major target in the control of schistosomiasis has for a long time been reduction of its related morbidity. Epidemiological and morbidity studies are key in monitoring the impact of an intervention. However, epidemiology of schistosomiasis and its related morbidity have been shown to vary in different endemic areas and communities. We report on the epidemiology of S. mansoni infection and related morbidity in a community in Mayuge District along Lake Victoria in Uganda.

Puberty and Age-related Changes in Susceptibility to Schistosome Infection

Recent data from outbreaks of schistosomiasis in immunologically naive populations have refuelled the debate concerning the nature or existence of protective, acquired immunity to schistosomiasis in humans. Data from endemic communities provide some compelling evidence for an abrupt change in reinfection rates that coincides with puberty. We suggest that the hormonal changes of adrenarche may hold the key to understanding the relative resistance to infection found in adults.

IL-10 blocks the development of resistance to re-infection with Schistosoma mansoni

Despite effective chemotherapy to treat schistosome infections, re-infection rates are extremely high. Resistance to reinfection can develop, however it typically takes several years following numerous rounds of treatment and re-infection, and often develops in only a small cohort of individuals. Using a well-established and highly permissive mouse model, we investigated whether immunoregulatory mechanisms influence the development of resistance. Following Praziquantel (PZQ) treatment of S. mansoni infected mice we observed a significant and mixed anti-worm response, characterized by Th1, Th2 and Th17 responses. Despite the elevated anti-worm response in PBMC's, liver, spleen and mesenteric lymph nodes, this did not confer any protection from a secondary challenge infection. Because a significant increase in IL-10-producing CD4⁺CD44⁺CD25⁺GITR⁺ lymphocytes was observed, we hypothesised that IL-10 was obstructing the development of resistance. Blockade of IL-10 combined with PZQ treatment afforded a greater than 50% reduction in parasite establishment during reinfection, compared to PZQ treatment alone, indicating that IL-10 obstructs the development of acquired resistance. Markedly enhanced Th1, Th2 and Th17 responses, worm-specific IgG1, IgG2b and IgE and circulating eosinophils characterized the protection. This study demonstrates that blocking IL-10 signalling during PZQ treatment can facilitate the development of protective immunity and provide a highly effective strategy to protect against reinfection with S. mansoni.

Schistosomes are zoonotic parasitic helminths that infect hundreds of millions of people worldwide. Despite effective chemotherapy, schistosomiasis- the disease caused by these parasites, still plagues tropical regions of the world. This is due, in part, to poor resistance to reinfection resulting in high re-infection rates following treatment. This lack of resistance is intriguing, as effective treatment relies upon drug-induced parasite damage combined with host immune mediated killing. Furthermore, it has been widely reported that post-treatment, individuals develop and retain elevated levels of anti-parasite immune responses. We therefore asked why resistance to re-infection is so poor, despite the development of significant anti-worm responses post-treatment.

It is essential that immune responses are controlled by various immunosuppressive mechanisms to prevent immune-mediated pathologies. However, a robust immunoregulatory response may obstruct the development of protective immunity. Thus, a balanced immune response providing a non-pathogenic yet effective immune response may be required for the development of effective resistance to reinfection. Understanding the immunological mechanisms of resistance to re-infection and the role of effector and regulatory responses may aid in the development of more effective vaccines and treatment strategies for schistosomaisis. This study suggests that combining chemotherapy with drugs that block IL-10 might provide an improved strategy to elicit acquired immunity to this parasite.

The effect of host age on feeding performance of fleas

We asked whether the age of a rodent host affects the feeding performance of fleas. We predicted that fleas would perform better on young and old than on adult rodents. To test this prediction, we determined bloodmeal size, rate of digestion and time of survival after a single bloodmeal inXenopsylla ramesisfeeding onMeriones crassusof different ages. Fleas took less blood from subadult and adult than from juvenile and old animals. Fleas digested blood of old hosts at the highest rate and blood of juvenile hosts most slowly. After a bloodmeal, fleas survived the longest if fed on a juvenile host. The effect of host gender on bloodmeal size and survival after a bloodmeal was manifested only in (a) subadult and adult hosts and (b) subadult hosts, respectively. Host gender affected the rate of digestion differently among digestion stages and host age cohorts. We explain the observed patterns in flea performance in terms of changes in the host's immune defences and nutritional quality of blood during the host's individual life. Our observations suggested that a host of a particular age could not be unequivocally predicted to be more or less beneficial for a parasite than a younger or an older host.

Patterns in parasite epidemiology: the peak shift

A characteristic relationship between infection and host age, with levels of infection reaching a peak in particular age classes, has been reported for many parasites. However, several field studies have shown that this relationship is not invariant: if age-infection data are compared across host populations, the peak level of infection is higher and occurs at a younger age when the transmission rate is high, and is lower and occurs at an older age when it is low. This pattern is called the ;peak shift'. Here, Mark Woolhouse reviews the evidence for and the implications of the peak shift. The peak shift is consistent with the predictions of mathematical models that assume gradually acquired protective immunity, and this interpretation is supported by experimental studies using animals. This agreement between theory, experimental evidence and field studies strongly suggests that acquired immunity has a major impact on epidemiological patterns not only for parasites such as malaria, where the importance of acquired immunity is not in doubt, but also for many parasitic helminths, where the role of acquired immunity is less widely accepted.

Schistosome immunology: more questions than answers

The past 30 years have seen research on the immunology of schistosomiasis move to encompass studies of responses in naturally exposed human populations, in addition to the studies in animal model systems. While animal systems still retain an important place in research on the immunology of schistosomiasis, recent debate has centred on aspects of human immunological responses that may or may not be associated with resistance or susceptibility to infection. In this article, Paul Hagan, Patricia Ndhlovu and David Dunne take stock of the present state of knowledge and offer their views on prospects for the future.

Schistosoma haematobium treatment in 1-5 year old children: safety and efficacy of the antihelminthic drug praziquantel

Background

Morbidity due to schistosomiasis is currently controlled by treatment of schistosome infected people with the antihelminthic drug praziquantel (PZQ). Children aged up to 5 years are currently excluded from schistosome control programmes largely due to the lack of PZQ safety data in this age group. This study investigated the safety and efficacy of PZQ treatment in such children.

Methods

Zimbabwean children aged 1–5 years (n = 104) were treated with PZQ tablets and side effects were assessed by questionnaire administered to their caregivers within 24 hours of taking PZQ. Treatment efficacy was determined 6 weeks after PZQ administration through schistosome egg counts in urine. The change in infection levels in the children 1–5 years old (n = 100) was compared to that in 6–10 year old children (n = 435).

Principal Findings

Pre-treatment S. haematobium infection intensity in 1–5 year olds was 14.6 eggs/10 ml urine and prevalence was 21%. Of the 104 children, 3.8% reported side effects within 24 hours of taking PZQ treatment. These were stomach ache, loss of appetite, lethargy and inflammation of the face and body. PZQ treatment significantly reduced schistosome infection levels in 1–5 year olds with an egg reduction rate (ERR) of 99% and cure rate (CR) of 92%. This was comparable to the efficacy of praziquantel in 6–10 year olds where ERR was 96% and CR was 67%.

Interpretation/Significance

PZQ treatment is as safe and efficacious in children aged 1–5 years as it is in older children aged 6–10 years in whom PZQ is the drug of choice for control of schistosome infections.

Urogenital schistosomiasis is an important, but neglected, infectious disease affecting over 100 million people, mainly in Africa. Children carry the heaviest burden of infection with children as young as 1 year old showing signs of infection. Children aged 5 years and below are currently excluded from schistosome control programmes for several reasons, including operational difficulties associated with accessing preschool children, misconceptions about their level of exposure to infective water and lack of safety data on the drug of choice for schistosome control, praziquantel, in children aged 5 years and below. This study was one of a small number of studies recently funded by the World Health Organization to investigate the need for praziquantel treatment in preschool children (aged 1–5 years) and to subsequently assess the safety and efficacy of the drug praziquantel in this age group. This study confirmed that preschool children carry significant levels of schistosome infection, exceeding those carried by their parents/guardians, highlighting the urgent need for their immediate inclusion in schistosome control programmes. The study also showed that praziquantel treatment is as safe and efficacious in children aged 1–5 years as it is in older children aged 6–10 years who are currently the target for mass drug administration.

Evidence of long term benefit of morbidity reduction due to praziquantel treatment against schistosoma mansoni in kigungu fishing village in entebbe, Uganda

Praziquantel (PZQ) is efficacious against Schistosoma mansoni. This was prospective cohort study. This study was carried out at Kigungu fishing village, Entebbe, Uganda. The goal of the study was to establish cost effective regiment for mass drug administration (MDA) of Praziquentel in the morbidity reduction of S.mansoni infection. In January 2004, nine hundred and forty five (945) participants were registered in this study. Our analysis was based on examining microscopically three slides prepared from each of 945 stool specimens delivered by each of the participant using modified Kato/Katz method. These included male and female, children and adults living in Kigungu fishing village in Entebbe Uganda. In total 901, cohorts were re-examined for infections clearance six months later in July 2004 and 18 months later in June 2005, 625 cohorts were again re-evaluated for S.mansoni infections after the baseline study. At baseline, (448) of 945 (47.5%) cohorts were S. mansoni positive. All these participants were treatment with a single oral dose of praziquantel at 40mg/kg. At the same time, 495 (52.5%) were S. mansoni negative. Of the 625 (66.3%) cohorts who came back for final review, 80 (12.8%) were still positive for S. mansoni while 210 (33.6%) remained negative after the base line treatment with praziquantel. On the other hand 103 (16.3%) of cohorts who were initially negative at the base line became S.mansoni positive after 18 months and 213(34.1%) remained negative for S.mansoni. The force of re-infection after six months was significant {(P=0.0001), (OR 0.47) CI at 95% (0.31–0.71)}. Nevertheless the force of reinfection was not significant after 18 months {(P=0.766), (OR 0.95) CI at 95% (0.68–1.34)}.The geometric mean eggs excretion of the 80 cohorts who were S.mansoni positive at 18 months was 151.967.This did not reach the geometric mean egg excreted by the same cohorts at baseline which was 285.05. The egg excretion was reduced by 46.8%. Similarly there was marked decrease in clinical symptoms amongst the cohorts. Our study suggests evidence of long-term benefit of praziquantel in Kigungu and that a yearly administration of praziquantel to the community could be a regiment for mass drug administration (MAD) for this community to control schistosomiasis morbidity.

CD23-bound IgE augments and dominates recall responses through human naive B cells

Human peripheral blood BCRμ(+) B cells express high levels of CD23 and circulate preloaded with IgE. The Ag specificity of CD23-bound IgE presumably differs from the BCR and likely reflects the Ag-specific mix of free serum IgE. CD23-bound IgE is thought to enhance B cell Ag presentation to T cells raising the question of how a B cell might respond when presented with a broad mix of Ags and CD23-bound IgE specificities. We recently reported that an increase in CD23(+) B cells is associated with the development of resistance to schistosomiasis, highlighting the potential importance of CD23-bound IgE in mediating immunity. We sought to determine the relationship between BCR and CD23-bound IgE-mediated B cell activation in the context of schistosomiasis. We found that crude schistosome Ags downregulate basal B cell activation levels in individuals hyperexposed to infectious worms. Schistosome-specific IgE from resistant, occupationally exposed Kenyans recovered responses of B cells to schistosome Ag. Furthermore, cross-linking of CD23 overrode intracellular signals mediated via the BCR, illustrating its critical and dominating role in B cell activation. These results suggest that CD23-bound IgE augments and dominates recall responses through naive B cells.

Human schistosomiasis mansoni: immune responses during acute and chronic phases of the infection

Schistosoma mansoni infection may occur either as an acute infection in individuals who have recently visited an endemic area, with no previous contact with the parasite, or as a lasting chronic disease, if not interrupted by specific chemotherapy. The acute phase is characterized by symptoms such as fever, cough, diarrhea, anorexia, and arthralgias in combination with leukocytosis and eosinophilia, and a high cellular immune response to schistosome antigens especially those from the parasite's eggs. In the chronic phase, most patients living in endemic areas are asymptomatic, and their immune responses to egg antigens are modulated. A few develop periportal fibrosis of the liver, which may result in the hepatosplenic form of the disease. The humoral response (IgG, IgM and IgE) in acute patients to egg and worm antigens does not differ from the chronic phase. However, a high level of IgG and IgM antibodies to KLH were detected in acute patients. Acute patients express a considerably higher in vitro cellular responsiveness than do chronic patients, especially to egg antigens. They present a mixed profile of Th1 and Th2 cytokines. Ultrasound examinations of endemic population reveal a high heterogeneity between the patients as regards the presence and intensity of periportal fibrosis. Most patients are asymptomatic and their immune responses to schistosoma egg antigens (SEA) are modulated. In contrast, a high percentage of patients with incipient fibrosis (early stage of hepatosplenic) responded strongly to SEA. Patients with advanced hepatosplenic disease were likely to be non-responders to SEA. Most of the chronic patients presented a Th2 profile with low production of interferon-gamma (IFN-gamma). The intensity of infection favors the production of interleukin (IL)-10. After adjusting for age, sex, and intensity of infection, a strong correlation was observed between the production of IL-13 and the degree of fibrosis. Chronic asymptomatic patients and those with incipient fibrosis expressed very high levels of heterogeneity of their antibody responses. IgG response to soluble worm antigen preparation (SWAP) was distinct and significantly higher in hepatosplenic patients than in those asymptomatic or with incipient fibrosis. Levels of IgG4 to SEA were significantly higher in sera from patients with incipient fibrosis as compared to uninfected and hepatosplenic groups. Polyclonal idiotypic antibodies and their fragments F(ab')2, directly stimulate in culture T cells of schistosomiasis patients in presence of IL-1. Polyclonal idiotypic antibodies are able to modulate in vitro granuloma formation around SEA-polyacrylamide. The importance of idiotypes for protection or pathology in schistosomiasis is still not clear.

Can schistosomiasis really be consigned to history without a vaccine?

Recently, considerable enthusiasm has been expressed for expanding and combining control efforts for neglected tropical diseases (NTDs). While these efforts are laudable, the drugs in question require repeated mass administration for indefinite periods of time, and their use to achieve eradication is fraught with challenges. Mass drug administration is unlikely to be effective in isolation, and should not proceed without concurrent control methods, such as vaccines. Schistosomiasis is one of the most important NTDs, and one whose effective control is unlikely in the absence of improved sanitation and a vaccine. Recent advances in biotechnologies have enhanced antigen discovery and new molecules that show promise as recombinant vaccines are being reported. Funding bodies supporting research into the control of schistosomiasis should invest not only in mass drug administration but also in the development of new control strategies, including the development of vaccines.

Individual and village-level study of water contact patterns and Schistosoma japonicum infection in mountainous rural China

OBJECTIVE

To describe the exposure patterns related to schistosomiasis transmission in 10 villages in rural Xichang County, Sichuan, China.

METHODS

Individual and village-level study of water contact exposure and Schistosoma japonicum reinfection; after initial infection survey and treatment, reinfection was determined 2 years later for 1604 individuals, of whom 578 also participated in a cross-sectional survey to assess their water contact behaviours.

RESULTS

The highest intensity of reinfection was observed in farmers aged 20-29 years, with no difference between sexes. While water contact measured as m(2)-minutes of contact was not associated with reinfection, an exposure metric computed by spatially weighting water contact by cercarial risk was correlated with both infection status and intensity. Village-level indicators based on snail density, number of infected snails, mouse bioassay data, and averaged individual-level exposures were associated with village reinfection rates.

CONCLUSION

Age-acquired immunity may be present in this population, but the study lacked sufficient power to discern differences in the exposure infection relationship with age.

Associations between specific antibody responses and resistance to reinfection in a Senegalese population recently exposed to Schistosoma mansoni

We examined associations between schistosome-specific antibody responses and reinfection in Senegalese individuals recently exposed to Schistosoma mansoni. The effects of treatment, age, intensity of infection and duration of exposure on schistosome-specific antibody responses were also investigated by comparing immune responses in individuals exposed for less than 3 years with responses in people exposed for more than 8 years. All individuals were bled before treatment as well as 6 and 12 weeks after. We used a statistical model that included interaction terms between time, age, infection intensity and duration of exposure. The overall patterns of most specific antibody responses by age were similar to those previously published for S. mansoni, Schistosoma japonicum and Schistosoma haematobium infections in different endemic areas. In general, a boost in specific antibody responses against adult worm antigen (SWA) was observed at 6 weeks after treatment whereas the majority of isotype responses against egg antigen (SEA) were not affected by treatment. Our analysis showed that the effect of treatment on schistosome-specific antibody responses is influenced by age, infection intensity and duration of exposure. We found no evidence that treatment matures the specific antibody response of children recently infected with S. mansoni. Our results indicate that the build-up of potentially protective immunoglobulin E (IgE) responses was associated with duration of exposure, or, in other words, experience of infection. Interestingly, in recently exposed individuals there was a significant association between IgA responses to SWA and resistance to reinfection. Resistance to reinfection and production of IgA-SWA was associated with adulthood independently of exposure patterns, suggesting that susceptibility to S. mansoni and the development of protective immune responses is age-dependent.

Human schistosomiasis

Schistosomiasis or bilharzia is a tropical disease caused by worms of the genus Schistosoma. The transmission cycle requires contamination of surface water by excreta, specific freshwater snails as intermediate hosts, and human water contact. The main disease-causing species are S haematobium, S mansoni, and S japonicum. According to WHO, 200 million people are infected worldwide, leading to the loss of 1.53 million disability-adjusted life years, although these figures need revision. Schistosomiasis is characterised by focal epidemiology and overdispersed population distribution, with higher infection rates in children than in adults. Complex immune mechanisms lead to the slow acquisition of immune resistance, though innate factors also play a part. Acute schistosomiasis, a feverish syndrome, is mostly seen in travellers after primary infection. Chronic schistosomal disease affects mainly individuals with long-standing infections in poor rural areas. Immunopathological reactions against schistosome eggs trapped in the tissues lead to inflammatory and obstructive disease in the urinary system (S haematobium) or intestinal disease, hepatosplenic inflammation, and liver fibrosis (S mansoni, S japonicum). The diagnostic standard is microscopic demonstration of eggs in the excreta. Praziquantel is the drug treatment of choice. Vaccines are not yet available. Great advances have been made in the control of the disease through population-based chemotherapy but these required political commitment and strong health systems.

Human schistosomiasis haematobium: effective diagnosis of active infection using a pair of monoclonal antibodies against soluble egg antigen

The present study was designed to prepare monoclonal antibodies (MAbs) against Schistosoma haematobium soluble egg antigen (SEA) with immunodiagnostic potential for urinary schistosomiasis. From a panel of MAbs, a pair of IgG1 MAbs (2D/11C and 10B/2C) specific for S. haematobium SEA was selected. Both MAbs recognized one band with a 42-kDa molecular weight by western blots. The pair of MAbs was employed in sandwich ELISA for the detection of circulating schistosome antigen (CSA), one as antigen-capturing antibody and the other as peroxidase-conjugated antigen-detecting antibody. The lower detection limit of the assay was 1 ng/ml of S. haematobium SEA. The assay was performed on sera of 65 S. haematobium-infected patients, 25 patients infected with other parasites (Fasciola hepatica, Echinococcus granulosus), and 20 noninfected individuals. CSA was demonstrated in 89% of the S. haematobium-infected group. However, CSA was negative in the sera of healthy individuals and patients infected with other parasites, giving an overall specificity of 100% for the CSA assay. A positive correlation (r=0.37, p<0.01) was detected between the number of S. haematobium eggs excreted in 10 ml urine and the CSA level detected in the sera of S. haematobium-infected patients. Our data show that the use of anti-S. haematobium MAbs for the detection of CSA provides a sensitive and specific method for the immunodiagnosis of active S. haematobium-infected patients. Moreover, CSA assay using this anti-S. haematobium MAb/ELISA system was proven to correlate with intensity of infection and hence morbidity assessment.

Immunity to schistosomiasis in animals: an update

The present paper reviews the available literature on the development of immunity to animal Schistosoma infections. The majority of the studies on animal schistosomiasis were performed in cattle and pigs and only Schistosoma mattheei, S. bovis and S. japonicum received particular attention, mainly because of their recognized veterinary significance or zoonotic aspect. Although it is an accepted fact that acquired resistance to Schistosoma is of major importance in the regulation of infection intensity in the field, almost nothing is yet known of either the nature of the antigens or of the immune mechanisms involved. The recent studies on immunity development focus in particular on the occurrence of maternal to foetal transfer of immunological substances related to animal Schistosoma infections and possible effects of these transfers on the immunity development of the foetus/newborn. Since congenital infections for Schistosoma species other than S. japonicum are extremely rare, the most plausible route for foetal contact is the transplacental or postnatal transfer of immunological substances. Prenatal transfers of specific antibodies and antigens via placental lesions and postnatal transfers via the colostrum were observed in cattle and pigs, and subsequent modifications of the immune response of the newborn were observed. Placental lesions induced by Schistosoma eggs could allow other pathogens to cross the placenta.

Human schistosomiasis mansoni: intensity of infection differentially affects the production of interleukin-10, interferon-gamma and interleukin-13 by soluble egg antigen or adult worm antigen stimulated cultures

The effect of the intensity of infection (eggs per gram faeces, epg) on the production of interferon-gamma (INF-gamma), interleukin (IL)-10 and IL-13 by peripheral blood mononuclear cells (PBMCs) from individuals living in a Schistosoma mansoni-endemic area was evaluated. In vitro stimulation of PBMCs with soluble egg antigen (SEA) resulted in significantly higher secretion levels of IFN-gamma in egg-negative individuals compared with those with an intensity of infection of more than 100 epg. In contrast, the egg-positive group produced significantly higher amounts of IL-10. Levels of IL-13 did not differ significantly between egg-positive and egg-negative groups. These findings suggest that IL-10 is an important cytokine in the control of the T helper cell (Th) type 1 responses during human S. mansoni infection, shifting the immune response from Th0 in egg-negative individuals from an endemic area to a Th2 polarization in chronic infected individuals.

Juvenile rhesus monkeys have more colonic granulomas than adults after primary infection with Schistosoma mansoni

Adults and children have differences in their susceptibility to schistosomiasis. Whether this age-dependent innate susceptibility influences parasite-caused granulomogenesis is difficult to assess in humans. Therefore, we exposed juvenile and adult female rhesus monkeys to primary infection with Schistosoma mansoni. Hepatic and intestinal granuloma formation was observed in both pre-pubescent and adult monkeys. Two distinct stages of granulomas were discerned, the exudative and the productive stage. In the intestine, more granulomas were generated in the colon than in the ileum. In contrast to the adult animals, the juvenile rhesus monkeys had higher numbers of colonic granulomas, these higher numbers being predominantly of the more advanced productive stage. Juvenile animals had a statistically non-significant increased worm burden. These results suggest that juvenile rhesus monkeys have a significantly more intense and advanced colonic response towards entrapped S. mansoni eggs after primary schistosome infections and, thereby, are more susceptible to parasite infection.

Are the cellular immune responses of children and adults with Schistosoma mansoni infection intrinsically different? Cytokines produced ex vivo in response to antigens and mitogens

In recently exposed communities, intensity of schistosomiasis infection increases as children age and then drops again in adulthood, indicating that host maturity is an important aspect of resistance to schistosomiasis. We investigated whether the cellular immune response to the parasite was correlated with age in subjects with similar daily patterns of exposure, current intensities of infection and number of years of exposure. The cellular immune response of subjects with either 'low' (under 200 eggs per gram (EPG)) or 'high' (over 400 EPG) intensities of infection was investigated, in a recently established focus where subjects had similar histories of exposure and number of years of experience with Schistosoma mansoni. Subject's whole blood was cultured with adult worm antigen (AWA), a mixture of phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), or left unstimulated, and culture supernatants were tested for IL-4, IL-5, IL-10 and IFN-gamma. Children and adults tended to respond differently to schistosome antigen. The most statistically significant illustration of this was the negative correlation between age and IL-5 produced by samples from people with low intensities of infection cultured with AWA (P < 0.003, P < 0.05 after Bonferroni correction). IL-10 produced by samples cultured with PHA and LPS was also notably lower in children than in adults, although not formally significant after Bonferroni correction. This indicates that it is possible for age, independently of intensity of infection or experience with the parasite, to influence the immune response to schistosomiasis.

Comparison between schistosomiasis transmission modelings considering acquired immunity and age-structured contact pattern with infested water

In order to analyze the effects of acquired immunity and the contact pattern with infested water on the overall transmission of schistosomiasis, a semi-stochastic model is proposed. The model's assumptions are the simplest possible to enhance the differences between two hypotheses. With respect to the human host, it is assumed the mounting of an immune response after elapsing a fixed period of time L from the first infection, which is partially effective and never lost. With respect to the contact pattern with infested water, it is assumed a decreasing age-related function. Both models are compared to a purely random model, which is taken as the basic model.

The relationship between age, sex, egg-count and specific antibody responses against Schistosoma mansoni antigens in a Ugandan fishing community

In schistosomiasis endemic areas, antibody isotype responses against Schistosoma mansoni antigens vary with host age, sex and duration or intensity of infection, and are associated with susceptibility or resistance to infection. Identifying the quality and quantity of these responses is important to our understanding of the host-parasite relationship; however, the various host and parasite factors have a strong tendency to confound each other. We investigated the relationships and interactions between age, sex, faecal egg-counts and specific antibody isotype (IgA, IgG1, IgG2, IgG3, IgG4, IgE, IgM) responses to S. mansoni worm (SWA) and egg (SEA) antigens, amongst 380 individuals aged 5-59 from a fishing community from Uganda. This community was characterized by high levels of exposure, and high infection intensities, with higher infection intensities in males than in females. Multivariate anova was conducted with interaction terms between the three categorized explanatory variables. Most anti-SWA responses increased with age, whereas anti-SEA responses tended to decline with age, especially after puberty. IgG1-SWA, IgG4-SWA, IgG4-SEA, IgE-SWA responses increased with egg count, whereas IgG2-SEA decreased with egg count. IgG1-SWA, IgG4-SWA, IgE-SWA and IgG4-SEA responses were independently higher in males, whereas IgG2-SEA responses were independently higher in females. The significant effects of sex on isotype responses to adult worm antigens may be partly because of different levels of cumulative exposure. IgG4-SEA and IgG4-SWA were both strongly correlated with egg count. Patterns of IgE-SWA responses were qualitatively different to IgG4 responses, suggesting independent pathways of regulation.

Schistosomiasis mansoni in areas of low transmission: epidemiological characterization of Venezuelan foci

Severe schistosomiasis is a rare event in Venezuela nowadays, after a successful national campaign by the Schistosomiasis Control Program. Unfortunately, this program has practically disappeared, and snail surveillance in field is not a priority, anymore. Thus, schistosomiasis has become a neglected disease in this country. However, surveys in different populations from the endemic area have shown particular epidemiological features described herein. In five communities we evaluated 2,175 persons and searched for the presence of Biomphalaria glabrata snails. Some markers were used for classifying schistosomiasis foci: mean age of the persons with Schistosoma mansoni eggs in the stools, serological tests, presence of B. glabrata snails, and intensity of infection. Places without B. glabrata snails and with few schistosomiasis cases were defined as "past transmission sites"; a site with abundant snails but few cases was defined as "potential risk"; "new transmission" foci were characterized by the presence of infected snails and young people passing eggs in the stools. A "re-emergent" focus has shared these last features, showing in addition a place where schistosomiasis had been reported before. Recent evidences of active transmission with the increasing dispersion of B. glabrata snails, point out the necessity for the re-establishment of the Schistosomiasis Control Program in Venezuela.

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.

Human water contacts patterns in Schistosoma mansoni epidemic foci in northern Senegal change according to age, sex and place of residence, but are not related to intensity of infection

In an epidemic focus in northern Senegal, adults had lower intensities of infection than adolescents, a phenomenon that could not be attributed to immunity acquired over the previous 10-15 years of exposure to the parasite because all age groups had had the same number of years' experience of the worm. This article considers whether this pattern could have been because of higher levels of exposure to the parasite in younger age groups. Personal contact with infected water was recorded using a questionnaire in Schistosoma mansoni foci not more than 3 years old and in another, 10-year-old focus. Many aspects of contact (e.g. frequency, duration or time of day of contact) may contribute to the number of encounters with infective cercariae (true exposure), so various assumptions regarding the relationship between water contact and true exposure were tested resulting in a range of exposure indices. People reported a mean of 4.4 separate contacts, and spent a median of 57 min per day in water. Patterns of water contact differed depending on the exposure index used, e.g. considering duration, males spent a longer time in water than females (P < 0.001). But using frequency, females had more contacts with water than males in most villages (P < 0.001). Generally, exposure levels dropped as people become aged (P < 0.001) and residents of the older focus were more exposed than residents of other foci (P < 0.002). Intensity of (re)infection was not related to exposure either alone or in models incorporating age, sex and/or village irrespective of the index used. There is therefore evidence that age, sex and place of residence determine exposure but none to suggest that exposure had an influence on the relationship between these factors and intensity of infection. We propose therefore that in this population other factors have principal importance in determining intensity of infection.

Chemotherapy-induced, age-related changes in antischistosome antibody responses

Humoral responses directed against Schistosoma mansoni soluble egg antigen were studied in Zimbabwean children before and after treatment with either praziquantel (PZQ) or oxamniquine (OXAM). Treated children showed a significant increase in the proportion producing IgE and IgG3 and in mean levels of IgE, IgM, IgG3 six weeks post-treatment. At 18 weeks post-treatment, the proportion of treated children producing IgA, IgE, and IgG3 increased while the proportion producing IgG1 and IgG4 decreased. Mean levels of IgA, IgE, and IgG3 were higher than pre-treatment levels while levels of IgG1, IgG4 and IgM were lower. Statistical analyses showed that the magnitude of change in levels of IgE, IgM and IgG3 at 6 weeks post-treatment and of IgE, IgG3 and IgG4 at 18 weeks post-treatment was significantly greater in treated compared to untreated children, and there were no significant differences in immune responses between children treated with praziquantel and those treated with oxamniquine. The magnitude of change in IgE at 6 and 18 weeks, IgM at 6 weeks and IgG3 at 18 weeks post-treatment were significantly associated with age in treated but not in untreated children, with the change being greater in younger children. This suggests that treatment induced a change in the age-antibody relationship for these isotypes, and that the age-antibody relationship is not robust to chemotherapy. Pre-treatment infection levels were significantly associated (positive correlation) with the magnitude of change for IgE and IgG3 at 18 weeks post-treatment. Taken together, these results indicate that the age-antibody relationship observed in these children is due, at least in part, to cumulative host experience of parasite antigens and not host age alone.

Differential production in vitro of antigen specific IgG1, IgG3 and IgA: a study in Schistosoma haematobium infected individuals

This study has evaluated the individual control of isotype production and the influence of external signals that can be experimentally provided in vitro, in antibody responses to two different recombinant Schistosoma antigens (Sh28GST and TPx-1). Peripheral blood mononuclear cells or enriched B cell fractions obtained from S. haematobium infected Senegalese adults were induced to terminal differentiation in vitro. The production of antibody to either antigen was donor-dependent and for each donor it was antigen-dependent. Differentiation to IgG1 and IgG3 production, and possibly IgA, specific to these conserved parasite antigens could be regulated differentially in vitro. Exogenous IL-2 and IL-10 or IL-10 and TGF-beta led to the production of specific IgG3 or IgG1 and/or IgA, respectively. This is the first report on such experimentally induced differential regulation of antigen-specific IgG1 and IgG3. This may have implications in designing protocols for protein based-vaccinations aiming at eliciting antibody responses of certain protective-type isotypes.

Immune correlate study on human Schistosoma japonicum in a well-defined population in Leyte, Philippines: I. Assessment of 'resistance' versus 'susceptibility' to S. japonicum infection

This study describes the categorical classification of 155 individuals living in an endemic village in Macanip, Leyte, Philippines as 'resistant' or 'susceptible' to Schistosoma japonicum infection using available exposure, infection and reinfection data collected from a 3-year water contact (WC) study. Epidemiological parameters including age, sex, and infection intensities in relation to observed reinfection patterns are also described. This classification was used in subsequent immunological studies described in two accompanying papers to identify protective immune mechanisms among resistant individuals induced by defined candidate vaccine molecules for S. japonicum. The study suggests that individuals who were most vulnerable to rapid reinfection were children belonging to the 5-14 age group. A drop in incidence at age group 15-19 and decreased intensity of infection starting at this age group and older (15+) suggests development of immunity. Controlling for the effect of the other variables, a multivariate analysis showed significant association for sex, in that females were more likely to be resistant. This implies that other than acquired immunity to infection, some age-dependent host factors may also play an important role in the overall changes of reinfection patterns seen in schistosomiasis japonica in this population.

Evaluation of the patterns of Schistosoma mansoni infection and re-infection in Senegal, from faecal egg counts and serum concentrations of circulating anodic antigen

Infection and re-infection patterns were evaluated in a recent Schistosoma mansoni focus in northern Senegal, by determining concentrations of serum circulating anodic antigen (CAA), as a measure of worm burden, and counting eggs in faeces before, 6 or 12 weeks and 1 year after praziquantel treatment in two subsequent cohorts (cohort A and B). No differences in egg counts and CAA concentrations or their relationship were found between the cohorts, which were examined 2 years apart. Within both cohorts, CAA concentrations showed the same, typical, age-related patterns as egg counts, with a peak in children and a strong decline in adults. These trends were apparent both before and 1 year after treatment. The results indicate that an age-related resistance to infection and to re-infection has been firmly established, at a steady level, in the recent S. mansoni focus investigated, with no indication of a gradual development of immunity or anti-fecundity immunity over a period of 2 years. Both shortly and 1 year after treatment, the decrease in egg counts was stronger than that in CAA concentrations, indicating that that there had been a reduction in worm fecundity after treatment. The possibility that praziquantel may induce anti-fecundity immunity has important implications for the use and interpretation of the results of (egg-count-based) re-infection studies designed to follow the development of naturally acquired immunity.

Resistance to reinfection with Schistosoma mansoni in occupationally exposed adults and effect of HIV-1 co-infection on susceptibility to schistosomiasis: a longitudinal study

BACKGROUND

Previous studies have reported age-dependent development of resistance to reinfection by schistosomes and identified immunological correlates of this resistance. However, whether resistance exists that is independent of age effects has been questioned. We did a longitudinal investigation of reinfection by Schistosoma mansoni in an adult population with high occupational exposure.

METHODS

We monitored a cohort of 96 male car washers working along the shores of Lake Victoria, Kenya during 349.7 person-years for frequency of water contact and infection with S mansoni. Patients were treated with praziquantel upon study entry and after reinfection with S mansoni. Bivariate analyses and a multivariate proportional hazards model were used to assess the effects of water contact, previous infections, and HIV-1 on S mansoni reinfection rates.

FINDINGS

13 car washers did not get reinfected or only became reinfected after an extended time (91 weeks). 47 initially had a short time to reinfection (15 weeks) but on subsequent treatments showed increased time to reinfection (29-38 weeks). 36 consistently displayed short times to reinfection (<15 weeks) despite multiple reinfection and treatment cycles. Decreased CD4 T-cell counts in HIV-1-positive individuals corresponded to increased susceptibility to S mansoni reinfection.

INTERPRETATION

Adults similarly exposed to schistosomiasis are either resistant to reinfection; susceptible, but develop resistance to reinfection after multiple treatments; or remain susceptible to reinfection. Thus, immunological resistance to reinfection with S mansoni exists or can develop independent of age effects. The consequence of HIV-1 co-infection suggests that CD4 T cells contribute to this resistance.

Differences in prevalence, intensity of infection and parasite-specific antibody levels do not predict different age-infection profiles

Acquired immunity is believed to influence the age-infection profile of Schistosoma infections. We compared antibody responses against Schistosoma mansoni adult worm antigen (AWA) and soluble egg antigen (SEA) in 164 residents of two communities with different levels of infection. IgG, IgA, IgM, IgE, and IgG subclass 1 to 4 antibodies were determined by ELISA. Seventy-five of the subjects were from Harbu, an area with a prevalence of 39% and an intensity of infection of 116 eggs per gram of stool (EPG), whereas 89 subjects were from Bati, with a prevalence of 66% and intensity of infection of 256 EPG. In both communities the prevalence and the intensity of infection were highest in the age group 10-14 years, although both were significantly higher in Bati than in Harbu. Mean levels of AWA-specific IgA, IgM, IgG, IgG1 and IgG2, and of SEA-specific IgG, IgM, IgG2 and IgG3 were significantly higher in Bati than in Harbu. However, mean levels of IgE against worm and egg antigens were significantly higher in Harbu than in Bati. Significant differences were detected in the levels of IgA, IgE, IgG, IgM, IgG1 and IgG2 against AWA, and in IgE, IgM, IgG2 and IgG3 against SEA according to the place of residence. The levels of anti-AWA IgG, IgG1 and IgG2 and anti-SEA IgG, IgG1 and IgG4 were significantly associated with the intensity of infection. Anti-AWA IgM levels were associated with age, whereas sex and age had interacting effects on the levels of AWA-specific IgG1 and SEA-specific IgG and IgM. Antibody responses exhibited different age-related patterns in the two communities. This may indicate that differences in history of exposure influence the evolution of immune responses. However, the study did not support the view that differences in antibody levels between communities subject to different levels of infection result in a systematic deviation in age-infection profile (the "peak shift").

Does the neuropeptide somatostatin have therapeutic potential against schistosomiasis?

The neuropeptide hormone somatostatin is used to treat bleeding oesophageal varices and to reduce portal pressure, and can prevent progression to severe fibrosis in chronic liver disease. We believe that somatostatin can also have therapeutic potential against schistosomiasis, based on recent observations that severe morbidity symptoms are associated with low levels of host somatostatin in patients infected with Schistosoma mansoni. The administration of exogenous doses of this neuropeptide could therefore alleviate the pathology caused by schistosomiasis.

Gender-dependent specific immune response during chronic human Schistosomiasis haematobia

The cellular and humoral acquired immune responses to Schistosoma haematobium 28 kD gluthathione S-Transferase (Sh28GST) antigen were evaluated in a Senegalese population chronically infected with S. haematobium parasite. We show a gender-dependent immune response in adult individuals presenting similar intensities of infection. Indeed, the specific IgA response and production of TGF-beta and IL-10 were found significantly higher in females compared to males. In addition, we showed that this profile was combined with a weak production of Th1-related cytokines (TNFalpha and IFNgamma) and was associated with an absence of proliferation to the antigen. A significantly higher Nuclear Matrix Protein 41/7 secretion, an apoptosis marker, was specifically observed in mononuclear blood cell cultures of females suggesting that a specific cell death process was engaged in a gender-dependent manner. This specific profile could be associated with the so-called T helper type-3 (Th3) immune response specifically promoting the production of IgA and would be developed upon the down-regulation of the specific Type-1 response by a probable cell death mechanism. This gender-dependent immune regulation, which may be under the influence of nonimmunological factors like sexual hormones, may be related to the chronicity of the infection.

Human T- and B-cell responses to Schistosoma mansoni recombinant glyceraldehyde 3-phosphate dehydrogenase correlate with resistance to reinfection with S. mansoni or Schistosoma haematobium after chemotherapy

Recently we reported that human T- and B-cell recognition of a 42-kDa protein (p42) in soluble extracts of adult Schistosoma mansoni worms correlates with resistance to reinfection with S. mansoni or S. haematobium. Amino acid microsequencing of p42 revealed that it consists predominantly of schistosome glyceraldehyde 3-phosphate dehydrogenase (SG3PDH). We have expressed SG3PDH in Escherichia coli and purified the recombinant protein in a soluble and enzymatically active form. Recombinant SG3PDH (rSG3PDH) reacted with human monospecific antibodies to p42. Lymphoproliferation and production of interleukin-4 and gamma interferon (IFN-gamma) after in vitro stimulation with rSG3PDH and serum isotype responses to rSG3PDH were examined in individuals with extremes of resistance and susceptibility to reinfection after treatment of previous S. mansoni or S. haematobium infection. Lymphoproliferation and IFN-gamma production in response to rSG3PDH and the presence of serum immunoglobulin G1 (IgG1), IgG3, and IgA antibodies to rSG3PDH generally characterized individuals who are resistant to reinfection after chemotherapy. The data indicate that T- and B-cell immune reactivity to rSG3PDH correlates with resistance to reinfection, confirming previous studies identifying SG3PDH as a target of protective immunity in humans, and suggest that SG3PDH should be investigated as a possible vaccine for human schistosomiasis.

The epidemiology of a recent focus of mixed Schistosoma haematobium and Schistosoma mansoni infections around the 'Lac de Guiers' in the Senegal River Basin, Senegal

A village with mixed Schistosoma mansoni and S. haematobium infections (probably in a early endemic phase) was identified around the Lac de Guiers in the Senegal River Basin. In documenting the epidemiology of both schistosomes, we focused on prevalence and intensity of infection, transmission patterns and the impact of treatment. S. mansoni prevalences (near 100%) and egg counts (overall geometric mean eggs per gram of faeces (epg) of 589 were high in all age groups, with 35% of individuals excreting > 1000 epg, and showing a slow decline in egg output only after the age of 30 years. The overall prevalence (28%) and egg counts (2% > 50 eggs/10 ml) of S. haematobium were low, with mean counts of 6.3 eggs/10 ml. Maximal mean S. mansoni egg counts were found in 5-9 year-old boys and in 15-19 year-old girls; S. haematobium maximal counts in 1-4 year-old boys and in girls aged 5-9. Extremely high Biomphalaria pfeifferi infection ratios were recorded over the whole year. Following a single treatment, re-infection was rapid with prevalences and mean egg counts of both Schistosoma species reaching pretreatment levels within 7 months.

Schistosomiasis control based on repeated chemotherapy in a rural village of the sugar-cane zone in northeast Brazil

A schedule of repeated chemotherapy with oxamniquine, consisting of biannual treatment of school-aged (7-13 years) children and annual treatment of all other age groups, was used in a representative rural village from a highly endemic area of schistosomiasis in Pernambuco. Significant reductions in infection were obtained only after two cycles of treatment, as the overall prevalence decreased from 72.6% to 41.7% and the geometric mean egg counts per gram of faeces among positives fell from 188.4 to 76. In a school-aged cohort (n = 29) three treatments at six-month intervals were necessary to significantly reduce the proportion of positives (from 75.9% to 51.7%). In a cohort of children under 7 years of age (n = 20) the proportion of positives actually increased (from 30% to 45%) despite two annual treatments. Water contact was intense and host snail density was relatively high. As there is no short-term perspective of improved sanitation, auxiliary measures such as focal mollusciciding are needed for an adequate control of schistosomiasis in this and alike areas.

Age-related worm load and worm fecundity patterns in human populations, as indicated by schistosome circulating antigens

Recently, our group determined the relationship between serum CAA levels and fecal egg counts in two foci with very intense Schistosoma mansoni transmission: Maniema (Zaire), an area endemic for S. mansoni since several decades, and Ndombo (Senegal), where transmission has only been established since a few years. The objective was to study and compare age-related worm load and worm fecundity patterns in these two different endemic settings. Here, we will summarize the most important findings and conclusions of this study.

The stabilizing effects of the acquired immunity on the schistosomiasis transmission modeling--the sensitivity analysis

A mathematical model is proposed to analyze the effects of acquired immunity on the transmission of schistosomiasis in the human host. From this model the prevalence curve dependent on four parameters can be obtained. These parameters were estimated fitting the data by the maximum likelihood method. The model showed a good retrieving capacity of real data from two endemic areas of schistosomiasis: Touros, Brazil (Schistosoma mansoni) and Misungwi, Tanzania (S. haematobium). Also, the average worm burden per person and the dispersion of parasite per person in the community can be obtained from the model. In this paper, the stabilizing effects of the acquired immunity assumption in the model are assessed in terms of the epidemiological variables as follows. Regarded to the prevalence curve, we calculate the confidence interval, and related to the average worm burden and the worm dispersion in the community, the sensitivity analysis (the range of the variation) of both variables with respect to their parameters is performed.