Which stage of Trichuris trichiura is infective for humans?

• A. Proglottid
• B. Filariform larva
• C. Rhabditiform larva
• D. Embryonated egg

Answer: (C)

Infection with Trichuris trichiura starts when a person ingests eggs that have fully developed in the environment. These embryonated eggs are the infectious form because the environment has allowed the egg to mature enough to release larval stages once ingested. After swallowing an embryonated egg, the egg hatches in the small intestine and the larvae migrate to the cecum to mature into adult whipworms that embed in the mucosa.

This differs from other options: a proglottid is a segment of a tapeworm, not related to whipworm transmission. a filariform larva is the infective form for some nematodes like hookworms, but whipworm is not acquired by a free-living larva. a rhabditiform larva is an early larval stage in many nematodes and is not the infective form for Trichuris trichiura. The key point is that the infectious stage is the embryonated egg.