A puppy infected with Dirofilaria immitis the day it is born will not test positive for heartworm microfilariae until it is ---- old.

A puppy infected with Dirofilaria immitis the day it is born will not test positive for heartworm microfilariae until it is ---- old.

• A. 3 to 4 months
• B. 12 months
• C. 6 to 7 months
• D. 1 month

Answer: (C)

Microfilariae appear in the bloodstream only after the heartworms reach reproductive maturity and begin shedding microfilariae. If infection happens at birth, the worms must develop to adulthood first, which takes about six to seven months. Only after this maturation period will circulating microfilariae be detectable, so the puppy won’t test positive for microfilariae until around six to seven months old. Earlier ages are too soon for maturation and microfilariae production, while waiting until a year is beyond the typical window for initial microfilariae appearance.

Microfilariae appear in the bloodstream only after the heartworms reach reproductive maturity and begin shedding microfilariae. If infection happens at birth, the worms must develop to adulthood first, which takes about six to seven months. Only after this maturation period will circulating microfilariae be detectable, so the puppy won’t test positive for microfilariae until around six to seven months old. Earlier ages are too soon for maturation and microfilariae production, while waiting until a year is beyond the typical window for initial microfilariae appearance.