Strongyloides stercoralis: A Parasitic Worm That Can Kill

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Strongyloides stercoralis has a life cycle conducive to long-term infestation. Image courtesy of the CDC

On March 21, 2013, the New England Journal of Medicine published a report of a 65-year-old immigrant from Vietnam who died from complications of an overwhelming parasitic infection. The parasite was a nematode – or roundworm – called Strongyloides stercoralis.

What is Strongyloides stercoralis?

Strongyloides stercoralis is a tiny nematode that lives in the intestines of humans and a number of other animals. Female worms, only 2.5mm long, move through the lining of the small intestine and produce eggs, which hatch within hours. The larvae pass out into the outside world in the stool, and can survive for some time as free-living nematodes, even reproducing through several generations. Sooner or later, however, infective parasitic larvae develop, which have only to come in contact with the skin of a potential host to continue their parasitic life cycle.

People acquire Strongyloides stercoralis when they come in contact with soil or fecal matter that contains the infective larvae. About half a millimeter long, these larvae can penetrate bare skin and move through the body quickly, arriving in the intestine within a couple of days. All parasitic larvae mature to female worms and reproduce without the contribution of a male. (There are males in the free-living soil stages.)

Strongyloides stercoralis Autoinfection

The life cycle of S. stercoralis is not straightforward. Just as the stages in the soil can be free-living or infective, larvae newly emerged from eggs in the intestine can pass out in the stool or develop quickly into infective stage larvae. The infective larvae can migrate through tissue, return to the intestine, and mature to adult female worms. This means that the parasite can reproduce and multiply within a single host, an ability that is rather unusual in parasitic worms.

The autoinfective ability of S. stercoralis means that, although the individual worms only live a few years, infection can persist for decades. Individuals who acquired the parasite in their youth can still be infected some 65 years later. Most infections are asymptomatic: they do not make the host ill.

A parasite that can both persist in a host for years without doing any harm, and reproduce in the host in such overwhelming numbers that death rapidly follows seems like a puzzling contradiction. The answer to this puzzle lies in the immune system. In a healthy individual with a normal immune system, the rate of autoinfection – the number of infective larvae that can remain in the body and mature to adults – is held in check. Should the immune system falter, however, S. stercoralis can multiply in large enough numbers to overwhelm.

A Textbook Case of Disseminated Strongyloidiasis

The man whose case appeared in the New England Journal of Medicine was taking glucocorticoids for another condition. Drugs of this type suppress the immune response and are known to aggravate the autoinfective cycle of S. stercoralis into overwhelming disseminated infection, often ending with the death of the unfortunate host.

Migrating larvae do damage to organs and tissues they travel through, and can carry bacteria with them from the intestine. When thousands of larvae are involved, the consequences can be severe. The patient in question suffered bacteremia (bacteria in the bloodstream), presumably due to intestinal bacteria carried by the larvae, and had difficulty breathing. Laboratory tests revealed adult worms, eggs, and larvae in his lungs, and in his stool.

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