Trichuris

Trichuris trichiura is a soil-transmitted nematode named for its shape: a thin, thread-like anterior end and a thicker posterior, giving the worm a whip-like appearance. The narrow front end threads into the mucosa of the cecum and colon while the wider back end projects into the lumen. Its eggs are described as barrel-shaped or football-shaped with a clear plug at each pole.

Transmission is fecal-oral. Eggs passed in stool must embryonate in warm, moist soil for a few weeks before becoming infective, so person-to-person spread does not occur directly. Ingestion of embryonated eggs on contaminated hands, vegetables, or soil leads to hatching in the small intestine; larvae then migrate to the large intestine and mature. Unlike Ascaris and hookworm, Trichuris has no pulmonary migration phase.

Light infections are often asymptomatic. Heavy worm burdens produce chronic colitis with abdominal pain, tenesmus, and bloody or mucoid diarrhea, and can cause iron deficiency anemia and growth impairment in children. Rectal prolapse with visible worms on the prolapsed mucosa is the classic presentation in heavily infected children. Diagnosis is made by finding the characteristic barrel-shaped eggs on stool microscopy, and treatment uses benzimidazole anthelmintics such as albendazole or mebendazole.

USMLE Step 1 parasitology asks you to match the organism to its distinguishing feature. For Trichuris, that means the barrel-shaped bipolar-plugged egg, the cecum and colon as the site of infection, and rectal prolapse as the buzzword — points that are frequently contrasted with Trichinella, which is acquired from undercooked pork and encysts in skeletal muscle rather than living in the gut.