In common with all other animals, human beings are susceptible to a range of parasitic organisms. The most important and commonest of these have been with man for countless years and have become so well adapted that in most cases man is their major if not only host. Although parasitic infections occur throughout the world, it is in the wet tropics and subtropics where they are found at their greatest prevalence and intensity. Most developing countries are also located in these areas and the consequent poverty, poor hygiene, and inadequate sanitation augment the favorable environmental conditions to enhance proliferation of these organisms. Only those that are known to interfere with host nutritional status will be discussed in this article.

Parasitic infections of the gastrointestinal tract are among the commonest diseases in the world (Table 1) and in most developing countries there has been little improvement in prevalence rates for many years. Indeed in some cases, e.g., schistosomiasis, local prevalence has been increasing with expanding irrigation schemes. Their association with poverty ensures that these diseases occur in areas where poor child growth and malnutrition are common and where there are persistent health problems. While there is no doubt that severe infections of any parasite can result in severe illness or even death of the host, such cases are rare even in areas of high prevalence and the norm is for low to moderate parasite numbers, which result in few, if any overt clinical symptoms. Nevertheless, by causing subtle reductions in appetite, digestion and absorption; by increasing chronic inflammation, and by inducing

Table 1 Estimated world prevalence of parasites important to human nutrition

Approx. prevalence (millions)

Helminth parasites

Ascaris lumbricoides



Necator americanus and


Ancyclostoma duodenale


Trichuris trichiura (whipworm)


Schistosoma haematobium,


S. japonicum, and

S. mansoni

Strongyloides stercoralis and


S. fulleborni

Protozoal parasites

Giardia intestinalis

200 symptomatic cases, total

much higher

Entamoeba histolytica

400 but may be much higher

Cryptosporidium spp.


Data from Crompton, DWT (1999) How much human helminthiasis is there in the world? Journal of Parasitology 85: 397-403; Olsen BE, Olson ME, and Wallis PM (2002) Giardia: The Cosmopolitan Parasite. Wallingford, UK: CABI; Haque R et al. (2003) Current concepts: amebiasis. New England Journal of Medicine 348: 1565-1573.

nutrient loss, particularly of iron and protein, it is believed that such low-level but long-term infections contribute to the persistently poor nutritional state of many, especially children, in the developing world.

The most important parasites of man are from two main groups: the helminth worms and protozoans. Although several hundred different species have been described, the vast majority of infections are caused by relatively few.

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