Thursday 29 July 2010

Strongyloides stercoralis

Helminth; Nematode (Parasite)

Disease; Strongyloidiasis

Geographic Distribution;
Worldwide distribution but more prevalent in warm climates and where the groundwater is high. Endemic in the tropics and sub-tropics as well as the South-eastern United States, particularly Kentucky and Tennessee. The geographic range is found to overlap that of the hookworm.

Life Cycle;
Strongyloides has a somewhat more complex life cycle. Infection is usually acquired by skin penetration of the infective filariform larvae. The larvae are long and slender up to 630 µm in length by 16 µm in width. After penetration the larvae are carried by the circulatory system to the lungs where they migrate into the alveoli. (other routes may exist) Migrating up the respiratory tree they reach the trachea and pharynx where they are swallowed to then travel to the duodenum and upper jejunum. After about two weeks of development, the female begins producing eggs. Thin shelled oval eggs are roughly 50 to 59 µm in length by 30 to 34 µm in width, making them a bit smaller than hookworm eggs. Eggs hatch to release non-infective rhabditiform larvae that pass out of the intestinal tract via feces. Once in the soil they develop into free living male and female worms where they eventually produce infective filariform larvae.

Strongyloides stercoralis larvae (fecal concentrat X400)
(click on photo to enlarge for better viewing)

Autoinfection can occur with Strongyloides. Rhabditiform larvae within the intestine can develop into the infective filariform larvae as they pass through the bowel. These larvae can then reinfect the host by once again penetrating the bowel, traveling through the circulatory system to the lungs where they once again cycle through to reach the intestine or be finally passed out of the body in faeces.

Strongyloides stercoralis larvae in fecal concentrate (X250)
(click on photo to enlarge for better viewing)

Strongyloides stercoralis larvae (Iron Hematoxylin Stain X250)
(click on photo to enlarge for better viewing)

Rhabditiform larvae that pass in the stool usually measure up to 380 by about 20 µm wide. There is usually a readily visible genital primordial that occurs about two-thirds of the way down from the anterior end. Strongyloides and Hookworm larvae appear similar however on closer examination it becomes obvious that the buccal cavity of S.stercoralis is quite short while the mouth of the Hookworm rhabditiform larvae is about three times as long.

Strongyloides stercoralis larvae (anterior end) showing short buccal cavity (X1000)
(click on photo to enlarge for better viewing)

Strongyloides stercoralis showing genital primordium (Iron Hematoxylin X 1000)
(click on photo to enlarge for better viewing)

In the human the infective cycle can have three areas of involvement.
  • Skin (cutaneous) - usually little reaction by the host except for possible pruritis & erythemea with larger infecting numbers.
  • Pulmonary (lungs) - may be asymptomatic or present with pneumonia-like symptoms. Symptoms may include coughing, wheezing, shortness of breath, fever and pulmonary infiltrates. (Loeffler’s Syndrome). Sputum samples can be examined directly for the presence of larvae.
  • Bowel (intestinal mucosa) - symptoms may mimic a peptic ulcer with accompanying abdominal pain. May also mimic Crohn’s disease.
In chronic disease cases, the host and parasite reach an equilibrium - both co-exist with little or no detriment. Undiagnosed disease can persist in travelers some 30 to 40 years after having returned from endemic areas. Infection may not be evident until some event upsets the equilibrium, allowing the parasite to flourish. Immunosuppresive therapy may predispose an individual to this hyper-infective syndrome.

In addition to tissue damage caused by the migrating larvae, Strongyloides infection may result in bacterial sepsis caused by the extra-intestinal dissemination of gut flora. Peritonitis, brain damage and respiratory failure may also be consequences of infection.

Confirmation of infection is accomplished by recovery of adult worms, larvae or eggs in the stool or sputum. Stools may vary day to day with the presence of Strongyloides often making it a challenge to diagnose even in concentrates. Duodenal aspirates and Entero-Test capsules provide alternative sampling techniques which may or may not be effective.
If only filariform larvae are recovered, Strongyloides larvae may be confused with Hookworm larvae. One distinguishing difference between the two larvae is that the tail of Strongyloides is notched while the hookworm is sharply pointed.

Strongyloides stercoralis egg containing larvae (fecal concentrate X400)
(click on photo to enlarge for better viewing)

As above but Iron Hematoxylin Stain (X400)
(click on photo to enlarge for better viewing)

In heavy infections eggs may be recovered from the stool as well as both rhabditiform larvae and filariform larvae. Adult worms may also be present.

Female Strongyloides stercoralis (Iron Hematoxylin Stain X250)
(click on photo to enlarge for better viewing)
Repeated doses of thiabendazole may be required to cure a Strongyloides infection. Cure rates range from 55 to 100 percent. Patients should be screened for Strongyloides infection prior to receiving immunosuppressive drugs.

Epidemiology and Prevention;
Hygene is all important in preventing Strongyloides infection. Avoiding contact with contaminated soil, surface water and feces will break the infective cycle.

New- February 23, 2012
To read about Strongyloides stercoralis isolated from a sputum sample and to see additional photographs of the worm and egg check out my 2012 post on Strongyloides stercoralis (Sputum)