Introduction to Helminth Infections

Biology of Helminths

Helminths are multicellular organisms that range from less than 1 cm to more than 10 m in length. They are covered by a cuticle or tegument that protects them from digestion and environmental stresses. Reproductive organs take up a large part of the body regardless of whether the sexes are separate or the species is hermaphroditic, as is the case with cestodes and nonschistosomal trematodes. Neuromuscular, digestive, excretory, and secretory systems typically are smaller and less complex, in keeping with the parasitic state.

The life cycle of all worms includes an egg, one or more larval stages, and the adult. Transmission to humans occurs by ingestion of helminth eggs or larvae, penetration of intact skin by larvae, or inoculation of larvae by biting insects. Depending on the species, humans are the only host; the intermediate host, in which asexual reproduction takes place; or, when there are one or two intermediate hosts, the definitive host in which sexual reproduction occurs.

Most helminths are unable to complete their life cycle within the human host, and development of eggs or larvae on soil, in water, within a plant, arthropod, or other animal intermediate host is necessary. Hence the geographic distribution of these parasites reflects the environmental conditions necessary for development of eggs or larvae or for survival of intermediate hosts and vectors. The only way for the intensity of infection in a person to increase is by further exposure to the infective stage; in the absence of continued exposure the infection lasts only as long as the life span of the adult worm.

In contrast, a few species, most notably Strongyloides stercoralis , are able to reproduce and multiply in numbers within the definitive human host. In the case of Strongyloides , infectious larvae can be passed directly from one person to another, and transmission is possible in all geographic areas. Infection can persist for the life span of the host, and in the setting of immunosuppression, accelerated autoinfection can lead to overwhelming numbers of organisms even after a distant and light exposure.

Epidemiology

The prevalence of helminth disease is highest in warm, developing areas, where climate, environment, and an abundance of vectors favor completion of the life cycle and where poverty leads to increased exposure to parasites because of poor sanitation, lack of clean water, and inadequate housing. Human activity can facilitate transmission, as seen in the huge numbers of new cases of schistosomiasis and foodborne trematode infections resulting from water resource development projects for hydroelectric power, irrigation, and aquaculture. Conversely, in some endemic areas, large-scale control programs have led to interruption or dramatically decreased transmission of dracunculiasis (guinea worm disease), filariasis, onchocerciasis, and other parasitic worms. Helminth infections are less common in temperate and industrialized areas, where they have been imported after travel or residence in tropical areas or acquired locally from domestic or wild animals via improperly prepared meat, fish, or vegetables, or from close personal contact, as in the case of pinworm infections.

Helminths produce large numbers of eggs or larvae and have a high reproductive capacity, which can lead to an extremely high prevalence of human infection when conditions are conducive to transmission, such as in rural areas in the tropics. Helminths are not uniformly distributed in human populations but are overdispersed, with most infected individuals harboring low worm burdens and only a small number harboring heavy infections. The basis for aggregation of helminths in human populations may be related to the intrinsic biology of the parasites and density-dependent constraints on parasites, such as competition for nutrients, parasite-induced pathology, and host factors, including genetic susceptibility to infection, immunity, nutrition, and behavioral factors.