Causal Agents:Nematodes in the genus, Trichostrongylus.  Although primarily parasites of animals, several species of Trichostrongylus have been known to infect humans, including T. orientalis, T. colubriformis, and T. axei.

Life Cycle:

Eggs are passed in the stool of the definitive host (usually a herbivorous mammal) , and under favorable conditions (moisture, warmth, shade), larvae hatch within several days.  The released rhabditiform larvae grow in the soil or on vegetationl , and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective .  Infection of the human host occurs upon ingestion of these filariform larvae .  The larvae reach the small intestine, where they reside and mature into adults.  Adult worms inhabit the digestive tract of their definitive hosts and may occur as incidental infections in humans .

Geographic Distribution:Worldwide, but more common where livestock is raised.

Clinical Features:Most infections are asymptomatic.  Heavy infections can cause gastrointestinal problems (abdominal pain, diarrhea, anorexia), headache, fatigue, anemia and eosinophilia.

Laboratory diagnosis:Microscopic identification of eggs in feces is evidence of infection.  Because eggs may be difficult to find in light infections, a concentration or flotation procedure is recommended.  Patients may have co-infections with hookworm, so care must be taken to differentiate the two.

Diagnostic findings

Microscopy Morphologic comparison with other intestinal parasites

Treatment:Pyrantel pamoate is the drug of choice; alternatives include mebendazole and albendazole.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

Microscopy

Eggs of Trichostrongylus spp. are thin-shelled, colorless and measure 75-95 µm in length by 40-50 µm in width.  Eggs taper at one end and the inner membrane may be wrinkled.  Eggs of Trichostrongylus spp. are similar to hookworm eggs, but the eggs of the latter smaller at 60-75 µm long by 35-40 µm wide and have a shorter length:width ratio.  Eggs are shed in feces.

A B

A, B:  Eggs of Trichostrongylus sp. in an unstained wet mount of stool.

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C, D: Eggs of Trichostrongylus sp. in an unstained wet mount of stool.  Images courtesy of the Indiana State Department of Health.