Strongyloidiasis is an infection with a parasitic roundworm known as Strongyloides stercoralis. It occurs in tropical and subtropical areas, as well as in the southern part of the United States. S. stercoralis has a complex life cycle involving infection through the skin by the larvae which travel to the intestine, where they reproduce causing chronic infection in the original host. They can also pass onto new hosts and cause further infections.
In persons with compromised immunity, such as organ transplant recipients, strongyloidiasis can cause a hyperinfection involving the intestines and the rest of the body. The condition can also be difficult to diagnose, since the symptoms are varied and non-specific. However, the parasitic worms can be eliminated by drug treatment. All patients at risk of hyperinfection should be treated and those about to undergo an organ transplant ought to be screened for the infection.
S. stercoralis has a more complicated life cycle than other parasitic worms, which means it can set up a high burden of persistent infection in a human host, especially one who has weakened immunity. Where the burden of parasites is low, the individual may have no, or merely intermittent, symptoms.
The parasites enter the skin and pass through the blood and lungs to the intestines. Therefore, those with a significant burden of parasites will develop symptoms relating to these areas. Skin symptoms include dermatitis and irritation, while the lung stage may involve dry cough, wheezing, fever, shortness of breath, and maybe coughing up blood. Parasites in the intestine will cause bloating, swelling, flatulence, indigestion, and diarrhea.
Hyperinfection—sometimes called disseminated stronglyoidiasis—may cause blood poisoning, peritonitis (inflammation of the lining of the abdominal cavity), neurological complications, and liver problems. The symptoms of strongyloidiasis are easily confused with other medical conditions, such as irritable bowel syndrome. Diagnosis depends upon identifying the S. stercoralis larvae within either a stool or a duodenal fluid sample.
Transmission of stronglyoidiasis starts when larvae in contaminated soil penetrate the skin of the human host and are transported through blood to the lungs. Here they first penetrate the alveoli, the tiny air sacs through which gases are exchanged between the lung surface and the blood. From here, the larvae travel up to the throat area and are swallowed, reaching the small intestine.
In the small intestine, the larvae become female adult worms. These lay eggs, by parthenogenesis—that is, without involvement of a male worm. The resulting larvae may pass through the stool, returning to the environment to repeat the cycle and infect other hosts. They can also cause so-called autoinfection in which the larvae continue to develop and penetrate the mucosal surface of the intestine, or the skin of the anal area. They then repeat the previous infection cycle—skin, lungs, and intestine—thereby massively increasing the parasitic burden on the host. Alternatively, the larvae may spread throughout the body, causing the complications described above.
S. stercoralis infection is found in humid tropical and subtropical areas where the larvae can survive in the soil. Cases have also been found in temperate areas, including the southeastern part of the United States. The infection is more frequently found in rural areas, institutional settings, and among immigrants from the developing world. Those with reduced immunity, including patients with leukemia, organ transplant recipients, or those receiving steroids, seem to be at higher risk of strongyloidiasis. However, HIV/AIDS does not seem to be a risk factor, despite the patient's immunocompromised status. In the Caribbean and Japan, an association between strongyloidiasis and human T-cell leukemia has been found.
WORDS TO KNOW
AUTOINFECTION: Re-infection of the body by a disease organism already in the body, such as eggs left by a parasitic worm, is autoinfection.
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
HYPERINFECTION: A hyperinfection is an infection that is caused by very high number of disease causing microorganisms. The infection results from an abnormality in the immune system that allows the infecting cells to grow and divide more easily than would normally be the case.
IMMUNOCOMPROMISED: A reduction of the ability of the immune system to recognize and respond to the presence of foreign material.
ROUNDWORM: Also known as nematodes; a type of helminth characterized by long, cylindrical bodies. Roundworm infections are diseases of the digestive tract and other organ systems that are caused by roundworms. Roundworm infections are widespread throughout the world, and humans acquire most types of roundworm infection from contaminated food or by touching the mouth with unwashed hands that have come into contact with the parasite larva. The severity of infection varies considerably from person to person. Children are more likely to have heavy infestations and are also more likely to suffer from malabsorption and malnutrition than adults.
S. stercoralis infection can be treated successfully by ivermectin. People being assessed for an organ transplant ought to be screened for infection and treated before the operation is performed. Travelers should avoid contamination with soil in areas of the world where S. stercoralis is endemic.
Although infection with S. sterocoralis may not cause any symptoms, the nature of the parasite's life cycle means that some patients may be at risk of life-threatening complications. The path the larvae take through the body mean that there are many and varied symptoms, which cause confusion with other conditions. Careful diagnosis of the condition is essential, so the parasite can be eradicated before the infection spreads throughout the body in those whose immunity is compromised.
Peters, Wallace, and Geoffrey Pasvol. Tropical Medicine and Parasitology. 5th ed. London: Mosby, 2002.
Centers for Disease Control and Prevention. “Parasites and Health: Strongyloidiasis.” September 26, 2005. <http://www.dpd.cdc.gov/dpdx/HTML/Strongyloidiasis.htm> (accessed May 1, 2007).