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3C) Correct. The size of these microfilariae is characteristic of D. immitis, the canine heartworm. However, because the prepatent period of D. immitis is six to seven months,2 this puppy was most likely born to a microfilaremic dam and acquired the microfilariae in utero.
Without treatment with macrocyclic lactones, microfilariae will survive in the circulation of a dog for several months. However, these microfilariae will not develop into adult heartworms unless they are picked up by a mosquito, molt into the infective third-stage larvae, and are then reintroduced to a dog when the mosquito subsequently feeds.