What condition is indicated by microfilaria found in fresh blood smears of a dog?

• A. Rabies
• B. Canine distemper
• C. Heartworms
• D. Coccidiosis

Answer: (C)

Microfilariae in a dog's fresh blood smear point to heartworm infection. These are the circulating larval forms of the heartworm (Dirofilaria immitis) produced by adult worms living in the pulmonary arteries and right side of the heart. Finding them in blood is a classic clue that heartworms are present. The other options don’t fit because rabies and canine distemper are viral diseases with no microfilariae in the bloodstream, and coccidiosis is caused by intestinal protozoa. Keep in mind that not all heartworm infections will show microfilariae, but when they’re seen, it’s a strong indicator of heartworms.