Which best describes the point at which heartworm larvae would be classified as microfilariae?

• A. When they are released into the host's bloodstream by adult female worms
• B. When they first enter the mosquito vector during a blood meal
• C. When they migrate to the host's heart and mature into adult worms
• D. When they develop into infective third-stage larvae (L3) inside the mosquito

Answer: (A)

Verified step by step guidance

1. Understand the life cycle of heartworms (Dirofilaria immitis), focusing on the stages of larvae development and their location within the host and vector.
2. Identify that microfilariae are the early larval stage of heartworms that circulate in the bloodstream of the definitive host (usually a dog).
3. Recognize that adult female heartworms produce and release microfilariae into the host's bloodstream, which can then be taken up by a mosquito during a blood meal.
4. Note that when larvae first enter the mosquito, they are microfilariae, but they develop further inside the mosquito into infective third-stage larvae (L3), which are not microfilariae.
5. Conclude that the defining characteristic of microfilariae is their presence in the host's bloodstream immediately after being released by adult female worms, before they enter the mosquito vector.