Causes and Characteristics of Helminth Infections
Emphasis is placed on the four most common STH infections and the three most common schistosome infections. Together, these infections account for most of the global helminth disease burden.
Soil-transmitted Helminths
The four most common STHs are roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura), and the anthropophilic hookworms (Necator americanus and Ancylostoma duodenale). Recent estimates suggest that A. lumbricoides infects 1.221 billion people, T. trichiura 795 million, and hookworms 740 million (de Silva and others 2003) (table 24.1). The greatest numbers of STH infections occur in the Americas, China and East Asia, and Sub-Saharan Africa. Strongyloides stercoralis is also a common STH in some of these regions, although detailed information on the prevalence of strongyloidiasis is lacking because of the difficulties in diagnosing human infection. The life cycles of Ascaris, Trichuris, and hookworm follow a general pattern. The adult parasite stages inhabit the gastrointestinal tract (Ascaris and hookworm in the small intestine; Trichuris in the colon), reproduce sexually, and produce eggs, which are passed in human feces and deposited in the external environment.
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STH infections rarely cause death. Instead, the burden of disease is related less to mortality than to the chronic and insidious effects on the hosts' health and nutritional status (Stephenson, Latham, and Ottesen 2000; Stoltzfus and others 1997). Hookworms have long been recognized as an important cause of intestinal blood loss leading to iron deficiency and protein malnutrition. The iron deficiency anemia that accompanies moderate and heavy hookworm burdens is sometimes referred to as hookworm disease (Hotez and others 2004). When host iron stores are depleted, the extent of iron deficiency anemia is linearly related to the intensity of hookworm infection (Stoltzfus and others 1997). Because of their underlying poor iron status, children, women of reproductive age, and pregnant women are frequently the ones most susceptible to developing hookworm anemia (Brooker, Bethony, and Hotez 2004). Iron deficiency anemia during pregnancy has been linked to adverse maternal-fetal consequences, including prematurity, low birth-weight, and impaired lactation (WHO 2002).
Chronic STH infections resulting from Ascaris, Trichuris, and hookworm can dramatically affect physical and mental development in children (WHO 2002). Studies have also shown that the growth and physical fitness deficits caused by chronic STH infections are sometimes reversible following treatment with anthelmintic drugs (Stephenson, Latham, and Ottesen 2000). The effects on growth are most pronounced in children with the heaviest infections, but light infections may also contribute to growth deficits if the nutritional status of the community is poor (Stephenson, Latham, and Ottesen 2000).
Schistosomiasis
Five major species of parasitic trematodes of the family Schistosomatidae—Schistosoma haematobium, S. intercalatum, S. japonicum, S. mansoni, and S. mekongi—infect humans. These parasites have a complex, indirect life cycle involving an intermediate snail host. Disease is caused primarily by schistosome eggs, which are deposited by adult worms in the blood vessels surrounding the bladder or intestines. Urinary schistosomiasis, in which the bladder is affected, is caused by infection with S. haematobium, which occurs mainly in Africa. Intestinal schistosomiasis results from infection with S. mansoni, which occurs in the Middle East, South America, and Africa, and from infection with S. japonicum, which occurs in parts of China and the Philippines (Ross and others 2002). Two other schistosome species are known to cause intestinal schistosomiasis in restricted geographical areas: S. intercalatum, found in Central Africa, and S. mekongi, found in Cambodia and the Lao People's Democratic Republic. Schistosomiasis is estimated to affect 187 million people worldwide (table 24.1).
A serious acute illness accompanied by fever and lymphadenopathy, known as Katayama Syndrome, can result from heavy schistosome infections. Chronic disease is mostly due to perforation of blood vessels and entrapment of eggs by host tissues. The host's reaction to entrapped eggs results in granuloma formation. S. haematobium causes bladder wall pathology, leading to ulcer formation, hematuria, and dysuria. Granulomatous changes and ulcers of the bladder wall and ureter can lead to bladder obstruction, dilatation, secondary urinary tract infections and subsequent bladder calcification, renal failure, lesions of the female and male genital tracts, and hydronephrosis. S. haematobium is also associated with increased risk of bladder cancer. The morbidity commonly associated with S. mansoni infection includes lesions of the liver, portal vein, and spleen, leading to periportal fibrosis, portal hypertension, hepatosplenomegaly, splenomegaly, and ascites. Schistosomiasis also causes chronic growth faltering and can contribute to anemia (Ross and others 2002).
