Question

Which one of the following is the infective stage of Ascaris lumbricoides?

• A. Unsegmented egg
• B. Egg with first stage larva
• C. Egg with second stage larva
• D. Free third stage larva

Answer 1

Answer: (C)

Egg with second stage larva

The first larval stage is formed by segmentation in gastrula and is called as noninfectious rhabditoid larva (the first larva). Moulting of rhabditoid larva within egg shell forms second stage infectious rhabditoid larva or juvenile. Contaminated food or water transmits the infectious juvenile to host's small intestine wherein dissolution of egg shell by digestive juices set the second stage infectious juveniles free. From intestine, these are carried to liver via portal circulation and to heart via pulmonary circulation. Two successive moultings in lung alveoli release the third and fourth larval stage. Thus, the correct answer is option C.