Common Intestinal Roundworms

General Considerations

Medically important worms belonging to the phylum Nematoda (roundworms) parasitize the gastrointestinal tract of humans. It is estimated that 60% of the world population is infected with at least one intestinal helminth. The worms are commonly acquired through ingestion of contaminated food (especially raw or undercooked vegetables), through skin contact with contaminated soil, or, in some instances, from direct contact with infected persons or their fomites.

Individuals most likely to be infected with intestinal nematodes will frequently give a history of one of the following: (1) travel or residence in the developing world, (2) emigration from the developing world, (3) residence in a rural farming community, (4) ingestion of organically grown vegetables, (5) the use of untreated wastewater for agriculture, or (6) pica. In low-resource tropical countries, environmental sanitation may be substandard or absent. However, unsanitary conditions may be found in isolated rural areas and in farming communities in high-resource countries as well.

Fresh vegetables can serve as a major source of food-borne parasitic disease. A study performed by in Ghana showed that vegetables (cabbage, sweet bell pepper, carrot, lettuce, tomato, and onion) sold at open-aired markets in Accra were more heavily contaminated than those sold at supermarkets. Lettuce (61%) and cabbage were the most contaminated, and the least was tomato (18%). The most prevalent parasites were Strongyloides stercoralis larvae (43%) and Cryptosporidium parvum oocysts (16%). Helminth eggs detected included those of hookworm nematodes ( Ancylostoma duodenale, Necator americanus ), Trichuris trichiura , Enterobius vermicularis , and the trematode Fasciolopsis buski . Other pathogens contaminating the vegetables included Entamoeba histolytica , Giardia lamblia , Cyclospora cayetanensis , Entamoeba coli , and Isospora belli .

In rural El Salvador, the use of solar urine-diverting desiccating latrines was found to be associated with a lower risk of intestinal helminths relative to use of pit latrines, or the absence of any type of latrine, presumably because the solid waste dries out more quickly in such a system, and eggs require a certain amount of time in moist soil to embryonate. In Mexico, intestinal helminth prevalence rises in children from low-income families and unemployed or less educated mothers. In Alexandria, Egypt, the risk of vegetable-transmitted parasites including Ascaris lumbricoides was found to be reduced by soaking leafy vegetables in water with an added substance such as vinegar, lemon juice, or salt; however, only 9.6% of households did this.

In general, the prevalence of intestinal parasites is higher in rural regions. For example, in a study of Bangladeshi men seeking work abroad, geohelminth infection rates were found to be 5.2% in those from urban areas and 27.6% in those residing in rural areas. However, in a study performed in India that looked at intestinal parasite rates in slum, rural, and urban populations, the highest prevalence (19%) was found in those residing in slums. Thus, insufficient access to sanitation of solid human waste is a key factor in transmission of these pathogens.

Pica, or geophagy, is not rare: in one study in Kenya, 77% of children ate soil daily, presumably triggered by iron-deficiency anemia. Intestinal roundworms transmitted by contact with contaminated soil are known collectively as “geohelminths” or “soil-transmitted helminths” (STHs).

Etiology

Species of geohelminths causing infections in humans include A. lumbricoides (“common roundworm”), Ascaris suum (“pig roundworm”), Ancylostoma duodenale and Necator americanus (“human hookworms”), T. trichiura (“whipworm”), S. stercoralis , and E. vermicularis (“threadworm” or “pinworm”). Human intestinal infection with the marine roundworm Anisakis will also be considered in this chapter.

Clinical Features

• 1.

  Patients with intestinal nematodes will frequently be asymptomatic.

• 2.

  Returned travelers should be reassured that, with the exception of Strongyloides , these parasites cannot complete their life cycle within the human host, and even without treatment they will die of old age, often within a year of infection.

• 3.

  Vague abdominal complaints are sometimes the only symptoms reported with light to moderate infections.

• 4.

  Chronic, heavy infections are associated with growth stunting in children.

• 5.

  Occasionally, asymptomatic patients will pass a recognizable worm, and this will be the first sign that they have acquired an intestinal parasite.

• 6.

  At other times, the diagnosis is suspected or made on finding eosinophilia on a routine white blood cell differential analysis or on finding ova or parasites in a stool examination done for screening purposes.

In general, the severity of signs and symptoms will be related to the intensity of the infection (worm burden). Young children, possibly because of unsanitary habits (a tendency to put dirty fingers and objects into their mouths, play outdoors without shoes, etc.) and possibly because of immature host defense mechanisms, tend to acquire heavier parasite loads than adults living in the same area.

These infections are remarkably prevalent. Ascariasis is commonly associated with diarrhea in the developing world. In a study performed in rural south Mozambique, A. lumbricoides was the second most common pathogen isolated from stool, following diarrheogenic E. coli (9.3% and 22.6%, respectively). Almost one billion people are infected with hookworm. The hallmark of human hookworm infection is iron-deficiency anemia. For salient features of infection with intestinal nematodes, see Table 45.1 .

Prevention

General food safety practices should dramatically reduce the incidence of ascariasis and trichuriasis, which are spread via contaminated foods (see Chapter 8 ). Wearing shoes should also afford some protection against hookworm infection and strongyloidiasis, which are spread via skin contact with infested soil. There now exists a vaccine for canine hookworm; an antihookworm human vaccine is under clinical investigation.

Diagnosis