Canine Heartworm Disease: NAVLE Study Guide

Canine heartworm disease shows up on the NAVLE in multiple forms — the straightforward antigen-positive dog needing staged treatment, the acute caval syndrome emergency, and the microfilaremia distinction question. The AHS protocol is tested heavily. Know it cold.

The Lifecycle: What the Exam Actually Tests

Dirofilaria immitis is transmitted by mosquitoes. The mosquito ingests microfilariae (L1) during a blood meal, develops them to infective L3 larvae, and deposits L3 at the bite site. From there: L3 → L4 → young adult via subcutaneous migration → pulmonary arteries. The prepatent period is 6–7 months from infection to detectable antigen. This is why annual testing still matters even in dogs on consistent prevention — a missed dose or early infection may not be detectable for months.

Only female worms produce detectable antigen. That detail is a classic trap question. A dog with all-male worm burden tests antigen-negative. That is an occult infection.