Parasites as aetiological agents in chronic diseases. Epidemiological associations and potential mechanisms of pathogenesis.

The aetiological role of parasitic infection has been well established, through epidemiological studies, for many chronic diseases prevalent in the tropics. Examples include Schistosoma mansoni infection leading to portal hypertension, Schistosoma haematobium infection leading to obstructive uropathy and squamous cell carcinoma of the bladder, Clonorchis sinensis leading to cholangiocarcinoma, and Taenia solium infection leading to epilepsy. Our understanding of the pathogenesis of these associations, however, continues to evolve, with new insights from many fertile avenues in chronic disease research. There are also tenuous associations such as schizophrenia with toxoplasmosis, and the findings from observational studies that link malignancy and epilepsy with a range of helminthic infections. Imprecise case definitions, and the lack of good animal models, are limitations to the evidence for a causal role of parasitic infection in these diseases.