Which stage of Trichuris trichiura is infective for humans?

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

Answer: (B)

The key idea is that Trichuris trichiura is acquired by ingesting eggs that have become embryonated in the environment. After being shed in feces, the eggs need warm, moist soil to mature into embryonated, infective forms. When a person swallows these embryonated eggs, the eggs hatch and larvae develop inside the large intestine, ultimately producing the adult worms in the cecum and colon. So the initial infectious form is the embryonated ovum. This differs from organisms where infection comes from filariform larvae or from tapeworm proglottids, and from others where noninfective larval stages (like rhabditiform larvae) are involved.