Helminths, Soil-Transmitted

Infectious Agents

Ascaris lumbricoides (roundworm), Ancylostoma duodenale (hookworm), Necator americanus (hookworm), and Trichuris trichiura (whipworm) are helminths (parasitic worms) that infect the intestine and are transmitted via contaminated soil.

Transmission

Eggs are passed in feces from an infected person. Infection with roundworm and whipworm occurs when eggs in soil have become infective and are ingested. Hookworm infection (see the Cutaneous Larva Migrans section) usually occurs when larvae penetrate the skin of people walking barefoot on contaminated soil. One kind of hookworm (Ancylostoma duodenale ) can also be transmitted when larvae are ingested.

Epidemiology

A large part of the world’s population is infected with 1 or more of these helminths, but the prevalence is highest in tropical and subtropical countries where water supplies and sanitation are poor. Travelers to these countries should be at low risk of infection if preventive measures are taken. Infections in the United States are typically seen in immigrant and refugee populations. Since these worms do not multiply in hosts, reinfection occurs only as a result of multiple contacts with the infective stages.

Clinical Presentation

Most infections are asymptomatic, especially when few worms are present. Pulmonary symptoms occur in a small percentage of patients when roundworm larvae pass through the lungs. Roundworm can also cause intestinal discomfort, obstruction, and impaired nutritional status. Hookworm infection can lead to anemia due to blood loss and chronic protein deficiency. Whipworm infection can cause chronic abdominal pain, diarrhea, blood loss, dysentery, and rectal prolapse. However, travelers are rarely at risk because these more severe manifestations are generally associated with high worm burdens seen in indigenous populations.

Diagnosis

The standard method for diagnosing soil-transmitted helminths is by identifying eggs in a stool specimen using a microscope.

Treatment

The drugs most commonly used are albendazole and mebendazole. Mebendazole is available in the United States only through compounding pharmacies.

Prevention

Food and water precautions (see Chapter 2, Food & Water Precautions). To avoid hookworm infection, travelers should not walk barefoot in areas where hookworm is common and where there may be human fecal contamination of the soil. In general, avoid ingesting soil that may be contaminated with human feces, including where human fecal matter or wastewater is used to fertilize crops.

CDC website: www.cdc.gov/parasites/sth

Bibliography

  1. 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.  [PMID:16679166]
  2. Brooker S, Bundy DAP. Soil-transmitted helminths (geohelminths). In: Cook GC, Zumla A, editors. Manson's Tropical Diseases. 22nd ed. London: Saunders; 2009. pp. 1515–48.
  3. Brooker S, Clements AC, Bundy DA. Global epidemiology, ecology and control of soil-transmitted helminth infections. Adv Parasitol. 2006;62:221–61.  [PMID:16647972]

Author

Christine Dubray