Genome search for additional human loci controlling infection levels by Schistosoma mansoni

The Influence of Genetic and Environmental Factors and Their Interactions on Immune Response to Helminth Infections

Helminth infection currently affect over 2 billion people worldwide, with those with the most pathologies and morbidities, living in regions with unequal and disproportionate access to effective healthcare solutions. Host genetics and environmental factors play critical roles in modulating and regulating immune responses following exposure to various pathogens and insults. However, the interplay of environment and genetic factors in influencing who gets infected and the establishment, persistence, and clearance of helminth parasites remains unclear. Inbred strains of mice have long been used to investigate the role of host genetic factors on pathogenesis and resistance to helminth infection in a laboratory setting. This review will discuss the use of ecological and environmental mouse models to study helminth infections and how this could be used in combination with host genetic variation to explore the relative contribution of these factors in influencing immune response to helminth infections. Improved understanding of interactions between genetics and the environment to helminth immune responses would be important for efforts to identify and develop new prophylactic and therapeutic options for the management of helminth infections and their pathogenesis.

Immunological and Biochemical Interplay between Cytokines, Oxidative Stress and Schistosomiasis

The host–parasite schistosome relationship relies heavily on the interplay between the strategies imposed by the schistosome worm and the defense mechanisms the host uses to counter the line of attack of the parasite. The ultimate goal of the schistosome parasite entails five important steps: evade elimination tactics, survive within the human host, develop into adult forms, propagate in large numbers, and transmit from one host to the next. The aim of the parasitized host on the other hand is either to cure or limit infection. Therefore, it is a battle between two conflicting aspirations. From the host’s standpoint, infection accompanies a plethora of immunological consequences; some are set in place to defend the host, while most end up promoting chronic disease, which ultimately crosses paths with oxidative stress and cancer. Understanding these networks provides attractive opportunities for anti-schistosome therapeutic development. Hence, this review discusses the mechanisms by which schistosomes modulate the human immune response with ultimate links to oxidative stress and genetic instability.

The Genetics of Human Schistosomiasis Infection Intensity and Liver Disease: A Review

Schistosomiasis remains the fourth most prevalent parasitic disease affecting over 200 million people worldwide. Control efforts have focussed on the disruption of the life cycle targeting the parasite, vector and human host. Parasite burdens are highly skewed, and the majority of eggs are shed into the environment by a minority of the infected population. Most morbidity results from hepatic fibrosis leading to portal hypertension and is not well-correlated with worm burden. Genetics as well as environmental factors may play a role in these skewed distributions and understanding the genetic risk factors for intensity of infection and morbidity may help improve control measures. In this review, we focus on how genetic factors may influence parasite load, hepatic fibrosis and portal hypertension. We found 28 studies on the genetics of human infection and 20 studies on the genetics of pathology in humans. S. mansoni and S. haematobium infection intensity have been showed to be controlled by a major quantitative trait locus SM1, on chromosome 5q31-q33 containing several genes involved in the Th2 immune response, and three other loci of smaller effect on chromosomes 1, 6, and 7. The most common pathology associated with schistosomiasis is hepatic and portal vein fibroses and the SM2 quantitative trait locus on chromosome six has been linked to intensity of fibrosis. Although there has been an emphasis on Th2 cytokines in candidate gene studies, we found that four of the five QTL regions contain Th17 pathway genes that have been included in schistosomiasis studies: IL17B and IL12B in SM1, IL17A and IL17F in 6p21-q2, IL6R in 1p21-q23 and IL22RA2 in SM2. The Th17 pathway is known to be involved in response to schistosome infection and hepatic fibrosis but variants in this pathway have not been tested for any effect on the regulation of these phenotypes. These should be priorities for future studies.

Candidate gene family-based and case-control studies of susceptibility to high Schistosoma mansoni worm burden in African children: a protocol

Background: Approximately 25% of the risk of Schistosoma mansoni is associated with host genetic variation. We will test 24 candidate genes, mainly in the T _h2 and T _h17 pathways, for association with S. mansoni infection intensity in four African countries, using family based and case-control approaches. Methods: Children aged 5-15 years will be recruited in S. mansoni endemic areas of Ivory Coast, Cameroon, Uganda and the Democratic Republic of Congo (DRC). We will use family based (study 1) and case-control (study 2) designs. Study 1 will take place in Ivory Coast, Cameroon, Uganda and the DRC. We aim to recruit 100 high worm burden families from each country except Uganda, where a previous study recruited at least 40 families. For phenotyping, cases will be defined as the 20% of children in each community with heaviest worm burdens as measured by the circulating cathodic antigen (CCA) assay. Study 2 will take place in Uganda. We will recruit 500 children in a highly endemic community. For phenotyping, cases will be defined as the 20% of children with heaviest worm burdens as measured by the CAA assay, while controls will be the 20% of infected children with the lightest worm burdens. Deoxyribonucleic acid (DNA) will be genotyped on the Illumina H3Africa SNP (single nucleotide polymorphisms) chip and genotypes will be converted to sets of haplotypes that span the gene region for analysis. We have selected 24 genes for genotyping that are mainly in the Th2 and Th17 pathways and that have variants that have been demonstrated to be or could be associated with Schistosoma infection intensity.   Analysis: In the family-based design, we will identify SNP haplotypes disproportionately transmitted to children with high worm burden. Case-control analysis will detect overrepresentation of haplotypes in extreme phenotypes with correction for relatedness by using whole genome principal components.

Candidate gene family-based and case-control studies of susceptibility to high Schistosoma mansoni worm burden in African children: a protocol

Background: Approximately 25% of the risk of Schistosoma mansoni is associated with host genetic variation. We will test 24 candidate genes, mainly in the T _h2 and T _h17 pathways, for association with S. mansoni infection intensity in four African countries, using family based and case-control approaches. Methods: Children aged 5-15 years will be recruited in S. mansoni endemic areas of Ivory Coast, Cameroon, Uganda and the Democratic Republic of Congo (DRC). We will use family based (study 1) and case-control (study 2) designs. Study 1 will take place in Ivory Coast, Cameroon, Uganda and the DRC. We aim to recruit 100 high worm burden families from each country except Uganda, where a previous study recruited at least 40 families. For phenotyping, cases will be defined as the 20% of children in each community with heaviest worm burdens as measured by the circulating cathodic antigen (CCA) assay. Study 2 will take place in Uganda. We will recruit 500 children in a highly endemic community. For phenotyping, cases will be defined as the 20% of children with heaviest worm burdens as measured by the CAA assay, while controls will be the 20% of infected children with the lightest worm burdens. Deoxyribonucleic acid (DNA) will be genotyped on the Illumina H3Africa SNP (single nucleotide polymorphisms) chip and genotypes will be converted to sets of haplotypes that span the gene region for analysis. We have selected 24 genes for genotyping that are mainly in the Th2 and Th17 pathways and that have variants that have been demonstrated to be or could be associated with Schistosoma infection intensity.   Analysis: In the family-based design, we will identify SNP haplotypes disproportionately transmitted to children with high worm burden. Case-control analysis will detect overrepresentation of haplotypes in extreme phenotypes with correction for relatedness by using whole genome principal components.

Host genetics and parasitic infections

Parasites still impose a high death and disability burden on human populations, and are therefore likely to act as selective factors for genetic adaptations. Genetic epidemiological investigation of parasitic diseases is aimed at disentangling the mechanisms underlying immunity and pathogenesis by looking for associations or linkages between loci and susceptibility phenotypes. Until recently, most studies used a candidate gene approach and were relatively underpowered, with few attempts at replicating findings in different populations. However, in the last 5 years, genome-wide and/or multicentre studies have been conducted for severe malaria, visceral leishmaniasis, and cardiac Chagas disease, providing some novel important insights. Furthermore, studies of helminth infections have repeatedly shown the involvement of common loci in regulating susceptibility to distinct diseases such as schistosomiasis, ascariasis, trichuriasis, and onchocherciasis. As more studies are conducted, evidence is increasing that at least some of the identified susceptibility loci are shared not only among parasitic diseases but also with immunological disorders such as allergy or autoimmune disease, suggesting that parasites may have played a role in driving the evolution of the immune system.

Host genetics and population structure effects on parasitic disease

Host genetic factors exert significant influences on differential susceptibility to many infectious diseases. In addition, population structure of both host and parasite may influence disease distribution patterns. In this study, we assess the effects of population structure on infectious disease in two populations in which host genetic factors influencing susceptibility to parasitic disease have been extensively studied. The first population is the Jirel population of eastern Nepal that has been the subject of research on the determinants of differential susceptibility to soil-transmitted helminth infections. The second group is a Brazilian population residing in an area endemic for Trypanosoma cruzi infection that has been assessed for genetic influences on differential disease progression in Chagas disease. For measures of Ascaris worm burden, within-population host genetic effects are generally more important than host population structure factors in determining patterns of infectious disease. No significant influences of population structure on measures associated with progression of cardiac disease in individuals who were seropositive for T. cruzi infection were found.

IRAK-2 regulates IL-1-mediated pathogenic Th17 cell development in helminthic infection

Infection with the trematode parasite Schistosoma mansoni results in distinct heterogeneity of disease severity both in humans and in mice. In the experimental mouse model, severe disease is characterized by pronounced hepatic egg-induced granulomatous inflammation mediated by CD4 Th17 cells, whereas mild disease is associated with reduced hepatic inflammation in a Th2-skewed cytokine environment. Even though the host’s genetic background significantly impacts the clinical outcome of schistosomiasis, specific gene(s) that contribute to disease severity remain elusive. We investigated the schistosome infection in wild-derived mice, which possess a more diverse gene pool than classically inbred mouse strains and thus makes them more likely to reveal novel mechanisms of immune regulation. We now show that inbred wild-derived MOLF mice develop severe hepatic inflammation with high levels of IL-17. Congenic mice with a MOLF locus in chromosome 6, designated Why1, revealed high pathology and enabled the identification of Irak2 as the pathogenic gene. Although IRAK-2 is classically associated with TLR signaling, adoptive transfer of CD4 T cells revealed that IRAK-2 mediates pathology in a CD4 T cell specific manner by promoting Th17 cell development through enhancement of IL-1β-induced activation of transcription factors RORγt and BATF. The use of wild-derived mice unravels IRAK-2 as a novel regulator of IL-1-induced pathogenic Th17 cells in schistosomiasis, which likely has wide-ranging implications for other chronic inflammatory and autoimmune diseases.

Schistosomes are trematode helminths that cause widespread disease in vertebrates and are responsible for over 200 million human infections worldwide. The species Schistosoma mansoni causes a hepatic granulomatous inflammatory and fibrosing reaction against tissue trapped parasite eggs that varies greatly in humans and among mouse strains, implying that the host’s genetic background plays a critical role in determining disease severity. Although exacerbated hepatic inflammation is known to be associated with an increase in CD4 Th17 cells, specific genes conducive to high pathology are unknown. In this study we used genetically diverse inbred wild-derived mice and found that their natural severe immunopathology and high IL-17 levels are regulated by the interleukin-1 (IL-1) receptor-associated kinase-like 2 (IRAK-2). We demonstrate that T cell intrinsic IRAK-2 affects disease severity by enhancing the development of Th17 cells, which results from an increased sensitivity to IL-1β induced activation of the lineage-specific transcription factors RORγt and BATF. Our findings thus identify IRAK-2 as a single regulator of pathogenic Th17 cell development in murine schistosomiasis and reveal a novel mechanism that is likely to operate in other chronic inflammatory and autoimmune diseases.

Genetic control of severe egg-induced immunopathology and IL-17 production in murine schistosomiasis

Infection with the trematode parasite Schistosoma mansoni results in a distinct heterogeneity of disease severity, both in humans and in an experimental mouse model. Severe disease is characterized by pronounced hepatic egg-induced granulomatous inflammation in a proinflammatory cytokine environment, whereas mild disease corresponds with reduced hepatic inflammation in a Th2 skewed cytokine environment. This marked heterogeneity indicates that genetic differences play a significant role in disease development, yet little is known about the genetic basis of dissimilar immunopathology. To investigate the role of genetic susceptibility in murine schistosomiasis, quantitative trait loci analysis was performed on F(2) progeny derived from SJL/J and C57BL/6 mice, which develop severe and mild pathology, respectively. In this study, we show that severe liver pathology in F(2) mice 7 wk after infection significantly correlated with an increase in the production of the proinflammatory cytokines IL-17, IFN-gamma, and TNF-alpha by schistosome egg Ag-stimulated mesenteric lymph node cells. Quantitative trait loci analysis identified several genetic intervals controlling immunopathology as well as IL-17 and IFN-gamma production. Egg granuloma size exhibited significant linkage to two loci, D4Mit203 and D17Mit82, both of which were inherited in a BL/6 dominant manner. Furthermore, a significant reduction of hepatic granulomatous inflammation and IL-17 production in interval-specific congenic mice demonstrated that the two identified genetic loci have a decisive effect on the development of immunopathology in murine schistosomiasis.

Genetic epidemiology of human schistosomiasis in Brazil

Human schistosomiasis presents the classic, complex disease phenotype, with marked variation in the intensity of infection, the immune response to infection, and the development of schistosome-related pathology. Determining the role of host genetics in schistosomiasis is complicated by the numerous parasite and environmental factors involved in transmission. However, as a result of the increased availability of sequence data, novel statistical methods, and new methods of study design, the last decade has seen significant advances in identifying the role of host genetics in schistosome infection around the world. Many of these advances have taken place in Brazil. Epidemiological studies in Brazil have shown that the intensity of infection (worm burden) is a heritable phenotype (41%). Human genome scans have identified a locus responsible for controlling Schistosoma mansoni infection intensity on chromosome 5q31-q33. There is also evidence for genetic control of pathology due to S. mansoni, with linkage reported to a region containing the gene for the interferon-gamma receptor 1 subunit. Numerous association studies have also provided evidence for major histocompatibility complex control of pathology in schistosomiasis. Recent candidate gene studies suggest a role of other immune response genes in controlling helminth infection and pathology. We chronicle the many advances made in understanding the role of host genetics in S. mansoni infection that have taken place in Brazil by phenotype studied: infection intensity, immune response, and disease development. Results from Brazilian studies are compared with studies of S. mansoni and other schistosome species elsewhere in the world.

Coinfection with the intestinal nematode Heligmosomoides polygyrus markedly reduces hepatic egg-induced immunopathology and proinflammatory cytokines in mouse models of severe schistosomiasis

Infection with the trematode helminth Schistosoma mansoni results in a parasite egg-induced, CD4 T-cell-mediated, hepatointestinal granulomatous and fibrosing inflammation that varies greatly in severity, with a higher frequency of milder forms typically occurring in regions where the disease is endemic. One possible explanation for this is that in these regions the degree of inflammation is lessened by widespread concurrent infection with gastrointestinal nematodes. We tested this hypothesis by establishing a murine coinfection model in which mice were infected with the intestinal nematode parasite Heligmosomoides polygyrus prior to infection with S. mansoni. In CBA mice that naturally display a severe form of schistosomiasis, preinfection with H. polygyrus resulted in a marked reduction in schistosome egg-induced hepatic immunopathology, which was associated with significant decreases in the levels of interleukin-17 (IL-17), gamma interferon, tumor necrosis factor alpha, IL-23, IL-6, and IL-1beta and with increases in the levels of IL-4, IL-5, IL-10, and transforming growth factor beta in mesenteric lymph node cells, purified CD4 T cells, and isolated liver granuloma cells. There also were increases in liver Ym1 and forkhead box P3 transcription factor expression. In another model of high-pathology schistosomiasis induced in C57BL/6 mice by immunization with schistosome egg antigens in complete Freund's adjuvant, coinfection with the nematodes also resulted in a marked inhibition of hepatic immunopathology accompanied by similar shifts in cytokine production. These findings demonstrate that intestinal nematodes prevent Th1- and Th17-cell-mediated inflammation by promoting a strong Th2-polarized environment associated with increases in the levels of alternatively activated macrophages and T regulatory cells, which result in significant amelioration of schistosome-induced immunopathology.

Genetic haplotypes of Th-2 immune signalling link allergy to enhanced protection to parasitic worms

Parasitic worm infection, allergy and asthma involve increased IgE production, eosinophil activity, mucus secretion and smooth muscle reactivity, effected through Th-2 immune signalling. These pathological features of allergic disorder, common in developed countries, appear to be protective features in resistance to parasitic worm infections prevalent in many developing countries. We investigated how genetic variation in the Th-2 signalling transduction molecule STAT6 relates to these clinical disorders, using immune phenotyping by serum IgE levels and haplotyping nine STAT6 genetic variants in a rural Chinese population, where Ascaris infection is prevalent, and an urban UK population where Ascaris is largely unknown but asthma and allergy are prevalent. We show for the first time that STAT6 haplotypes relate clearly to IgE levels, allergy and worm burden. The haplotypes segregated into two groups: those with raised IgE/low worm burden tended to have increased risk of allergic disorder, whereas low IgE/high worm burden tended to have a reduced risk of allergies. By estimating the mean worm burden for each haplotype in China and the relative risk of asthma for the matching haplotype in the UK, we draw a cross-population comparison and show a negative correlation between worm burden and expected risk of asthma. These data imply that the origin of common up-regulating variants of Th-2 signalling, involving STAT6, promotes asthma and allergy in developed countries, whereas in developing countries it protects against parasitic worm infections. Selective evolutionary mechanisms, driven by parasitic worm infection, may underlie the genetic contribution to risk of allergy and asthma in humans.

Carriage of DRB1*13 is associated with increased posttreatment IgE levels against Schistosoma mansoni antigens and lower long-term reinfection levels

Praziquantel treatment for Schistosoma mansoni infection enhances Th2 responsiveness against parasite Ags, but also increases the variance in Ab isotype levels. This effect may arise partly from genetic heterogeneity. In this study, associations between HLA polymorphisms at three loci (HLA-DQB1, HLA-DQA1, and HLA-DRB1) and posttreatment Ig responses to S. mansoni Ags were assessed in 199 individuals aged 7-50 years from Uganda. Blood samples were assayed for IgG1, IgG4, and IgE levels against soluble worm Ag (SWA), soluble egg Ag, tegument Ag, and a recombinant tegumental Ag (rSm 22.6) 7 wk after treatment. Multivariate ANOVA analysis initially revealed associations between carriage of DRB1*13 and increased levels of IgG1, IgG4, and IgE against SWA, tegument Ag, and rSM22.6. Subsequent analysis of covariance, which controlled for correlations between isotype levels and also included pretreatment IL-4, IL-5, and IL-13 responsiveness against SWA as covariates, revealed an independent association only between DRB1*13 and a factor score summarizing IgE levels to worm-derived Ags, which was strongest in adults. A post hoc age- and sex-stratified analysis revealed lower reinfection intensities at 1 year, 22 mo, and 6 years after the first round of treatment among carriers of DRB1*13. These results indicate that genetic background has a prominent influence on the posttreatment Th2 immune response to S. mansoni Ags, as well as a downstream association with long-term reinfection levels.

Familial aggregation of human infection with Schistosoma japonicum in the Poyang Lake region, China

Despite the success of extensive control measures that have been implemented in China for over 50 years, the number of individuals infected with Schistosoma japonicum remains high in the existing endemic areas. A variance components analysis was undertaken to estimate the heritable and environmental components that contribute to S. japonicum infection in the Poyang Lake region of Jiangxi Province, PR China. The total target population was 3148 from four separate administrative villages. Two thousand seven hundred and five of these comprised 400 families ranging in size from 3 to 188. After adjustments were made for gender, water contact and past history of having had schistosomiasis, the heritable component was estimated to account for as much as 58% of the phenotype variation under the polygenic model. Household was not shown to be an important environmental factor. Incorporating village effects indicated that the results were valid for the total population. We conclude that genetic heritability in this region is high and plays an important role in determining risk of infection with S. japonicum.

The inherent complexities of gene-environment interactions

The article outlines the complexities of gene-environment interactions in the determination of human disease, especially as they relate to aging, and stresses the importance of continuing such studies, in spite of their inherent difficulties. First, a capsule review of the literature pertaining to studies of gene-environment interactions is presented, and designs and methodologies used to detect these interactions are briefly discussed. Second, research questions and problems that can be addressed as outcomes of gene-environment interaction studies are exemplified. Third, a number of illustrative examples of gene-environment interactions are presented. Fourth, various types of gene-environment interactions are briefly discussed. Fifth, concluding remarks are offered, and possibilities of studying gene-environment interaction within social and biological research on aging are outlined.

Epidemiology and morbidity of Schistosoma mansoni infection in a fishing community along Lake Albert in Uganda

Schistosoma mansoni infection, associated morbidity and symptoms were studied in Piida fishing community at Butiaba, along Lake Albert, Uganda, from November 1996 to January 1997. The study revealed that S. mansoni is highly endemic with an overall prevalence of 72%, a mean intensity of 419.4 eggs per gram (epg) faeces (geometric mean for positives only), with 37.8% of males and 33.0% of females excreting over 1000 epg. Prevalence and intensity peaked in the 10-14 year old age group and decreased with increasing age. Females were less heavily infected than males. Differences were also shown between tribes. Diarrhoea and abdominal pain were commonly reported in Piida. However, no clear-cut correlation between intensity of S. mansoni infection and these conditions could be demonstrated, indicating that retrospective questionnaires concerning S. mansoni related-symptomatology are of limited value. Organomegaly, as assessed by ultrasonography, was frequent and hepatomegaly was associated with heavy S. mansoni infection. No correlation was demonstrated between splenomegaly and infection. This study emphasizes that schistosomiasis mansoni is a major public health problem in Piida fishing community and presumably also in many similar fishing communities. These observations call for immediate intervention and can help in planning long-term strategies for sustainable morbidity control.

Enhanced egg-induced immunopathology correlates with high IFN-gamma in murine schistosomiasis: identification of two epistatic genetic intervals

The genetic basis of dissimilar immunopathology development among mouse strains infected with Schistosoma mansoni is not known. We performed a multipoint parametric linkage analysis on a cohort of F(2) mice, offspring of brother-sister mating between (high pathology CBA x low pathology BL/6)F(1) mice, to examine whether the observed differences in the type of immune response or the extent of hepatic immunopathology are linked to any particular genomic intervals. The F(2) mice exhibited cytokine responses and immunopathologies that revealed a statistically significant correlation between prominent egg Ag-stimulated IFN-gamma production by mesenteric lymph node cells and hepatic egg granuloma size. Increased IFN-gamma production showed suggestive linkage to a dominant CBA locus on chromosome 1 and a recessive CBA locus on chromosome 5; significantly, there was an epistatic interaction between the two IFN-gamma loci. An additional locus with suggestive linkage to granuloma formation and a CBA-recessive mode of inheritance was mapped to centromeric chromosome 13. Our analysis identified the first three genetic regions that appear to influence the immunopathology in murine schistosomiasis; however, further congenic dissection studies will furnish a more precise understanding of the genetic control of this disease.

An asthma-associated genetic variant of STAT6 predicts low burden of ascaris worm infestation

Th-2 immune mechanisms are involved in the pathology of asthma and in the protective immune response to parasitic worms. Common upregulating genetic variants of Th-2 immune signalling are risk factors for asthma, and we tested whether they may confer a counteradvantage in protecting against parasitic worms. We examined the intensity of infection by the parasitic worm, Ascaris lumbricoides, by microsopic counting of ascaris eggs in the stool of 614 schoolchildren from an area of endemic ascaris infection in China. We investigated the relationship between the intensity of ascaris infection and common, asthma-associated genetic variants of Th-2 and Th-1 immune signalling. Ascaris egg counts per gram of stool (epg), mean 1068 epg, ranged from barely detectable (<240 epg) to heavy (approximately 9600 epg) in a skewed distribution. Logistic regression, after exploratory discriminant analysis, showed a major association between a common genetic variant of the 3'-UTR regulatory elements of the signal transducer and transactivating factor (STAT6) (P=0.0002) and egg counts, at the 77 th centile. Linear regression after log transformation of egg counts confirmed a highly significant association with this STAT6 variant (P=0.001). Thus, a common, asthma-associated, genetic variant of the pivotal transduction and transactivating factor for Th-2 immune signalling, STAT6, predicts increased resistance to ascaris worm infection. The evolution of enhanced resistance to parasitic worm infection, through human genetic variation in Th-2 immune signalling, may represent one origin for asthma.

Genetics of susceptibility to human helminth infection

Recent studies have shown that host genetics is an important determinant of the intensity of infection and morbidity due to human helminths. Epidemiological studies of a number of parasite species have shown that the intensity of infection (worm burden) is a heritable phenotype. The proportion of variance in human worm burden explained by genetic effects varies from 0.21 to 0.44. Human genome scans have identified a locus responsible for controlling Schistosoma mansoni infection intensity on chromosome 5q31-q33, and loci controlling Ascaris lumbricoides intensity on chromosomes 1 and 13, although the genes involved have not yet been identified. There is also evidence for genetic control of pathology due to S. mansoni, and linkage has been reported to a region containing the gene for the interferon-gamma receptor 1 subunit. There is some evidence for genetic control of filarial infection, though little information on filarial disease. Association studies have provided evidence for major histocompatibility complex control of pathology in schistosomiasis and onchocerciasis. Recent candidate gene studies suggest a role of other immune response genes in controlling helminth infection and pathology, but require replication. Identification of the genetic loci involved may be important in the understanding of helminth epidemiology and the mechanisms of resistance and pathology.

Five-year impact of repeated praziquantel treatment on subclinical morbidity due to Schistosoma japonicum in China

We report the 5-year impact (1996-2001) of repeated praziquantel chemotherapy on subclinical morbidity related to Schistosoma japonicum infection. We repeated stool examinations and hepatosplenic ultrasonography in a cohort of 120 individuals living on an island with endemic infection in Dongting Lake, China. Prevalence of schistosome infection fell by 43% and intensity (geometric mean eggs per gram) declined by 80% over the 5 years. However, transmission persisted at a dangerously high rate of 13% per year for re-infection or new infection in the cohort. The prevalence of left-lobe enlargement and dilated portal vein fell significantly (P < 0.01) to about half initial levels although a few patients progressed during the study period. At study endpoint, infection was nearly twice as common if the portal vein was dilated (23% versus 13%, respectively), but this association was not statistically significant (P > 0.05). However, endpoint infection was even more strongly associated with left-lobe enlargement (57% versus 15%, P < 0.01). The proportions of subjects with improved parenchymal and periportal fibrosis were much higher than the proportions of subjects that progressed (P < 0.05). Reduction of prevalence and intensity of infection, and improvement of subclinical morbidity, were benefits of repeated treatments. Further research is needed to understand why some patients developed fibrosis despite substantial reductions in egg counts and to evaluate the functional importance of residual subclinical morbidity after chemotherapy-based control in the lake and marshland area of China.