Chapter 3Infectious Diseases Related To Travel
Ascaris lumbricoides (roundworm), Ancylostoma duodenale (hookworm), Necator americanus (hookworm), and Trichuris trichiura (whipworm) cause infection in humans. Two other helminths cause infections in humans Enterobius vermicularis (pinworm) and parasites of the genus Schistosoma; these infections are discussed specifically in the Pinworm (Enterobiasis, Oxyuriasis, Threadworm) and Schistosomiasis sections later in this chapter.
MODE OF TRANSMISSION
Adult female worms in the intestine of infected people produce eggs that are excreted in the stool. Defecation on the ground and use of stool to fertilize crops allow eggs to reach soil—the necessary environment for the next phase of their development. Infection with Ascaris and Trichuris occurs when eggs in soil have become infective and are ingested.
Hookworm eggs are not infective; they release larvae in soil that can penetrate skin. Hookworm infection primarily occurs when skin comes in contact with contaminated soil (by walking barefoot, for example) but can also occur through the ingestion of larvae. Ascaris larvae, after they have hatched from eggs in the small intestine, and hookworm larvae, after they have penetrated the skin, migrate through the lungs before they become adult worms in the intestine.
Helminths are widespread; the prevalence is highest in tropical, developing countries. In 2002, an estimated 1.5 billion, 1.3 billion, and 1.1 billion people were infected with Ascaris, hookworm, and Trichuris, respectively. The risk of infection in travelers is low. Since adult worms do not multiply in the body, travelers who are diagnosed with intestinal worms generally have few worms. Sporadic exposure to infection is less likely to produce symptomatic disease. Because eggs must pass through a developmental phase in soil before becoming infective or releasing infective larvae, soil-transmitted helminth infections are not transmitted person to person.
Most infections are asymptomatic, especially when few worms are present. Heavy infections with intestinal helminths almost never occur in travelers.
In developing countries, moderate to heavy Ascaris infections can impair the nutritional status of children. The most serious complication is intestinal obstruction, usually of the small intestine. Pulmonary symptoms occur in a small percentage of patients when Ascaris larvae pass through the lungs. These symptoms include cough, fever, and chest discomfort. Hookworm infection can lead to anemia and protein deficiency due to blood loss. Trichuris infection can cause blood loss as well as dysentery and rectal prolapse. However, travelers are almost never at risk for these more severe manifestations of intestinal helminths.
Diagnosis is made by identifying the eggs of soil-transmitted helminths in the microscopic examination of a stool specimen. Adult Ascaris worms may occasionally be coughed up or found in stool or vomit.
Soil-transmitted helminth infections are usually treated with albendazole or mebendazole, drugs that are effective and well tolerated.
PREVENTIVE MEASURES FOR TRAVELERS
No vaccine is available. Drugs are not used to prevent infection. In addition to usual food precautions (see Chapter 2, Food and Water Precautions), travelers should avoid walking barefoot on soil that may be contaminated with sewage, where human feces may have been used as fertilizer, or where people may have defecated.
- Bethony J, Brooker S, Albonico M, Geiger SM, Loukas A, Diemert D, et al. Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm. Lancet. 2006 May 6;367(9521):1521–32.
- Brooker S, Bethony J, Hotez PJ. Human hookworm infection in the 21st century. Adv Parasitol. 2004;58:197–288.
- Brooker S, Clements AC, Bundy DA. Global epidemiology, ecology and control of soil-transmitted helminth infections. Adv Parasitol. 2006;62:221–61.
- Cooper E. Trichuriasis. In: Guerrant RL, Walker DH, Weller PF, editors. Tropical Infectious Diseases: Principles, Pathogens & Practice. 2nd ed. Philadelphia: Elsevier; 2006. p. 1252–56.
- Crompton DW. Ascaris and ascariasis. Adv Parasitol. 2001;48:285–375.
- Gilles HM. Soil-transmitted helminths (geohelminths). In: Cook GC, Zumla A, editors. Manson’s Tropical Diseases. 21st ed. London: Saunders; 2003. p. 1527–36.
- Hotez PJ. Hookworm infections. In: Guerrant RL, Walker DH, Weller PF, editors. Tropical Infectious Diseases: Principles, Pathogens & Practice. 2nd ed. Philadelphia: Elsevier; 2006. p. 1265–73.
- Maguire JH. Intestinal nematodes (roundworms). In: Mandell GL, Bennett JE, Dolin R, editors. Principles and Practice of Infectious Diseases. 6th ed. Philadelphia: Elsevier; 2005. p. 3260–6.
- Seltzer E, Barry M, Crompton DWT. Ascariasis. In: Guerrant RL, Walker DH, Weller PF, editors. Tropical Infectious Diseases: Principles, Pathogens & Practice. 2nd ed. Philadelphia: Elsevier; 2006. p. 1257–64.
- WHO Expert Committee. Prevention and control of schistosomiasis and soil-transmitted helminthiasis. World Health Organ Tech Rep Ser. 2002;912:i–vi, 1–57, back cover.
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