Author(s): Mary Kamb, Sharon Roy

Infectious Agents

Ascaris lumbricoides (Ascaris or roundworm), Ancylostoma duodenale (hookworm), Necator americanus (hookworm), and Trichuris trichiura (whipworm) are helminths (parasitic worms) that infect the intestine. Due to the role of contaminated soil in their transmission, this group of nematode worms are known as soil-transmitted helminths (STH). Strongyloides stercoralis (threadworm) is sometimes included in the STH (see Sec. 5, Part 3, Ch. 21, Strongyloidiasis).

Transmission

STH are transmitted through ingestion of the tiny, infectious eggs of Ascaris, whipworm, and some hookworm, and through skin transmission for hookworm. People of all ages can become infected. Adult female worms produce thousands of eggs daily that are passed in feces and, if conditions allow, deposited in soil. Once in soil, infective larvae of Ascaris and whipworms develop in the fertile eggs and, if ingested by a human host, hatch and develop into adult worms over several months. Hookworm eggs are not infective—the eggs hatch and release larvae that must mature in soil before they become infective. Hookworm infection usually occurs when larvae penetrate the skin of people walking barefoot on contaminated soil; Ancylostoma duodenale also can be transmitted when larvae are ingested. Occasionally, human infection with Ascaris suum (pig roundworm) can occur due to ingestion of infectious eggs shed in pig feces.

Epidemiology

Globally, ≈2 billion people are infected with ≥1 STH, which together account for most parasitic disease burden worldwide. STH have widespread global distribution and are endemic in countries with tropical or subtropical climates and where sanitation is poor, human feces are used as fertilizer (“night soil”), or water supplies are contaminated. Although all travelers to endemic countries have some risk for STH infection, risk increases for long-term travelers and expatriates going to countries with poor general sanitation. Travelers can minimize risk by taking preventive measures.

Historically, STH infections were common in people living in US states where warm, moist climate and lack of sanitation enabled transmission; current prevalence of infection in those areas is unknown. Most reported infections in the United States are among immigrant and refugee populations. Since the introduction of predeparture treatment, stool testing for STH is unnecessary for most refugees. Because Ascaris, whipworm, and hookworm do not multiply in hosts (as opposed to threadworm), reinfection occurs only as a result of additional exposure to the infective-stage larvae.

Clinical Presentation

Most STH infections are asymptomatic, especially when few worms are present. With Ascaris, pulmonary symptoms (Löffler syndrome) associated with marked eosinophilia and fever occur in a few patients when larvae pass through the lungs. Heavy roundworm infection also can cause intestinal discomfort, impaired nutritional status, and obstruction. Hookworm infection can lead to anemia due to blood loss and chronic protein deficiency, particularly in children. Whipworm infection can cause chronic abdominal pain, blood loss, diarrhea, dysentery, and rectal prolapse. Travelers rarely develop these more severe manifestations, however, which generally are associated with high worm burdens in indigenous populations.

Diagnosis

Diagnosis is through detection of characteristic eggs using standard microscopy to examine fresh stool specimens. Stool concentration methods (e.g., Kato-Katz, McMaster, or FLOTAC techniques) can improve diagnostic yield. Collecting and testing 3 stool specimens on 3 separate days also improves detection because of variable shedding.

In returning travelers, parasitic eggs might not appear in stool for several months after exposure or symptom onset, because after infection female worms do not produce eggs for ≥40 days for Ascaris and ≥70 days for whipworm or hookworm. Serology to detect STH antibodies is not available in the United States. PCR testing is more sensitive and specific than microscopy, but tests are generally still unavailable commercially.

Co-infection with ≥1 STH or other parasitic worms common in some endemic areas can make diagnosis challenging. Request assistance with parasitological diagnosis through DPDx (www.cdc.gov/DPDx). Clinical consultations are available through the Parasitic Diseases Branch of the Centers for Disease Control and Prevention (404-718-4745; parasites@cdc.gov).

Treatment

Treatment of intestinal ascariasis consists of anthelminthic therapy, which effectively reduces morbidity but does not prevent reinfection. The drugs used most often to treat hookworm and Ascaris are albendazole and mebendazole, and for whipworm a combination of albendazole plus ivermectin. These drugs are safe for children but should be avoided or used with caution in pregnant or lactating people.

Prevention

No vaccines or drugs are available to prevent STH infection. Travelers can minimize infection risk by using preventive measures aimed at reducing ingestion or exposure to soil contaminated with human feces. Preventive measures include careful hand hygiene; washing, peeling, and cooking raw vegetables and fruit; and boiling or treating water (see Sec. 2, Ch. 8, Food & Water Precautions, and Sec. 2, Ch. 9, Water Disinfection). To avoid hookworm infection, travelers should avoid walking barefoot in areas where hookworm is common or where soil might be contaminated by human feces.

CDC website: Soil-transmitted helminths