Which factor decreases the sensitivity of the occult heartworm test?

• A. No microfilariae present
• B. Female worms only
• C. Male worms only
• D. No clinical signs

Answer: (C)

The key idea is that this occult heartworm test detects antigen produced mainly by adult female heartworms. When only male worms are present, there are no adult females producing that antigen, so the test is likely negative even if worms are in the body. That directly lowers the test’s sensitivity because a real infection can be missed due to the absence of female worms. In contrast, whether microfilariae are detectable in the blood or whether the animal shows clinical signs doesn’t determine this antigen-based test’s ability to detect infection, since those factors don’t reflect the presence of female worm antigen. Therefore, having only male worms is the situation that most reduces sensitivity.