Strongyloides stercoralis

Strongyloides stercoralis is an important cause of “super infections” which can rapidly lead to death. These “super infections” are precipitated or enhanced by the host’s loss of immunocompetence whether by disease or steroid use.

Disease / Pathology

Strongyloides stercoralis initially penetrate the skin of the host as the larvae form and gain entry to the cutaneous blood vessels. Mild irritation may occur at the site of entry. These larvae will then migrate to the lungs via to the pulmonary capillaries. One they reach the pulmonary capillaries they break out into the alveoli and ascend the trachea to be swallowed and proceed down to the small intestine. In the small intestine they penetrate the epithelium of the glands, molt twice and reach their maturity within 2 weeks.

It is in the lungs that signs of pneumonitis may first appear. The cough is dry.

Peptic ulcer like symptoms appear once the intestinal phase of the infection begins. There may be cumulative damage to the intestinal wall from the movements of the females within the tissues, the constant deposition and hatching of eggs, and escape of first-stage rhabditoid larvae. These rhabditoid larvae are the diagnostic stage generally looked for in an ova & parasite examination. The eggs of Strongyloides are seldom encountered in an ova & parasite examination. Larvae can occasionally be detected in the sputum, or other body fluids.

Eosinophilia can be present in either the specimen (sputum and feces) or complete blood cell count. In addition, patients may present with red itchy rashes on various parts of the body but most commonly on the trunk. This is a reaction to the larvae migrating through the epidermis layers and may be an indicator of a super infection.

Alterations in the immune system or steroid treatments may precipitate super infections in which the organism actively migrates throughout the body infecting multiple organs with dire consequences. Complications and death are directly due to the organ system most affected or due to a parasitic induced bacterial sepsis.

Location in the Host

Strongyloides stercoralis is found in the upper small intestine mucosal epithelium. In “super infections” the filariform larvae of the worms may migrate to all organs of the body.

Geographic Distribution

Strongyloides stercoralis is primarily a nematode found in warm climates but has also been found in temperate and cold regions. The islands of the Caribbean such as Puerto Rico have been found to have the nematode.

Life Cycle

Image courtesy of CDC

Morphology & Diagnosis

A Strongyloides stercoralis diagnosis is made by the detection of the active rhabditiform larvae in the stool. The eggs of the nematode are rarely seen in a feces examination. The larvae are many times confused with those of hookworm or Trichostrongylus.

The larvae can be distinguished by their tails. The filariform larvae of Strongyloides have a notch while those of hookworm filariform larvae have a pointed tail.

The rhabditiform stages are similar as well and can be distinguished by Strongyloides’s short buccal cavity when compared to the hookworm’s long buccal cavity.

Though the eggs are rarely seen in the feces for Strongyloides, it is important to note that when they are seen they are virtually indistinguishable from those of hookworm. Therefore a diagnosis cannot be made solely on the presence of one egg.

Larvae can be present in the sputum and as a result can first be detected by the presence of serpentine patterns of growth on the artificial media cultured. Larvae can also be detected by an astute technologist in the Gram’s smear of a sputum sample, albeit a rare finding.


Strongyloides stercoralis rhabditiform larvae in a Trichrome stained concentrated fecal specimen for ova & parasite examination. They are more commonly seen in the formalin concentrate. This is an high oil immersion magnification and is best used to look at the buccal cavity to confirm the identification as they can be mistaken for Hookworm – Photo by W. Vientos
Strongyloides stercoralis rhabditiform larvae in a formalin concentrate of feces stained with Lugol’s Iodine. This image was taken under low power magnification – Photo by W. Vientos
Strongyloiodes stercoralis rhabditiform larvae photo taken on high power magnification under immersion oil. Note the characteristic buccal cavity – Photo taken by W. Vientos.
Comparisons of Strogyloides stercoralis rhabditiform larvae
and Hookworm rhabditiform larvae – Illustration by W. Vientos