Abstract
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.
Highlights
Schistosomiasis is caused by digenic trematodes of the genus Schistosoma with Schistosoma mansoni and Schistosoma haematobium causing the majority of human infections
Exposure to water infested with the infective schistosome cercariae is the main risk factor for schistosomiasis there is considerable variation in infection intensity between people with similar exposures and schistosomiasis cases aggregate in families, some of this variation has been attributed to the genetics of the human immune response [2,3,4]
In the present review of the genetics of human susceptibility to schistosomiasis we focus on loci associated with egg/worm burden and hepatic fibrosis
Summary
Schistosomiasis is caused by digenic trematodes of the genus Schistosoma with Schistosoma mansoni and Schistosoma haematobium causing the majority of human infections. In the present review of the genetics of human susceptibility to schistosomiasis we focus on loci associated with egg/worm burden and hepatic fibrosis. Schistosomal Fecal Egg Count and Worm Burden Studies of the human genetics of susceptibility to schistosomiasis have focussed on two classes of phenotype; infection associated phenotypes and pathology related phenotypes. For example 22 out of 119 Kenyan school children had developed high S. mansoni egg burden (>100 eggs per gram of feces) 12 months after treatment, whilst 70 children still had low (
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