The nematode Trichuris trichiura, the whipworm or threadworm, is a very common parasite that occurs worldwide but is most abundant in warm, moist climates. It still exists in the southern United States, but it has declined there and in other developed countries in recent decades with improved sanitation; it is now found mostly in poor tropical countries. Adult worms range up to 2 inches in length, so whipworms were probably seen by ancient observers, but they were first clearly recognized by an early Portuguese writer on tropical medicine, Aleixo de Abreu, in 1623. Several scientists described the species in the mid-eighteenth century. Archaeological evidence shows that the worm infected people in the Americas prior to the voyage of Columbus.

Trichuris attaches itself to the wall of the large intestine and passes its eggs in the host’s feces. Eggs require 10 to 14 days in the soil to mature or “embryonate.” Embryonation is most successful in warm, moist soils in shady places. Like Ascaris, which has a similar range, Trichuris infects people who have swallowed embryonated eggs in soil or in contaminated food or water. Unlke Ascaris, however, the whipworm does not require an elaborate period of larval migration in the host. The eggs hatch in the, small intestine, where the larvae spend some time before moving to their home in the cecum.

Trichuriasis rarely causes much harm unless there is a heavy worm load. Severe infections can cause abdominal discomfort, bloody or mucoid diarrhea, weight loss, weakness, and anemia. Masses of worms can cause appendicitis, and prolapse of the rectum can occur in children harboring large numbers of worms.