Parasitic diseases and psychiatric illness.

Distinguishing parasitic diseases from other infections and tropical medical disorders based on microbiological classification is a matter of convenience. Organic brain syndromes are associated with both protozoan and helminthic infections; side-effects of drugs commonly used to treat parasitoses may impair mood and cause anxiety, agitation or psychosis. Emotional states may in turn affect the experience of medical illness. Psychiatrically significant features of medical illness are determined both by pathophysiology and by the personal and social context in which they occur. Many factors affect mental health in the tropics where the synergy of infection, emotional strengths, vulnerabilities, social supports and stressors is critical. This review discusses parasitic diseases of psychiatric interest by virtue of their effects on thinking, mood and behaviour; and it distinguishes issues that apply mainly to indigenous populations and visitors to endemic areas. In some paradoxical instances the psychiatric influence of parasitic diseases does not require infection; the review concludes by considering the prime example, delusions of parasitosis, which is a primary psychiatric disorder.