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.

Diagnosis: The Two-Test Rule

AHS guidelines are explicit: every antigen-positive dog needs a concurrent microfilariae test (modified Knott or direct smear). This matters clinically, not just academically. Microfilaremic dogs require glucocorticoid pretreatment before macrocyclic lactone administration to blunt the reaction from rapid microfilarial die-off. And microfilaremic dogs are actively transmitting — that has public health implications in the exam context.

Heat treatment of the antigen sample is used when you have discordant results — specifically a microfilaria-positive dog with a negative antigen test. Antigen-antibody complexes can block detection; heating the sample at 103°F for 10 minutes dissociates the complexes and unmasks blocked antigen. This is not a routine screening step.

AHS Classification and Staging

The AHS assigns class based on clinical signs and diagnostic findings. This directly determines pre-treatment workup and approach.

The AHS Treatment Protocol

The three-injection melarsomine protocol kills approximately 98% of adult worms. The two-injection protocol only gets about 90%. The exam tests the exact timing.

Doxycycline targets Wolbachia — the obligate intracellular endosymbiont that heartworms need to survive and reproduce. Eliminating Wolbachia does two things: it weakens the worms before melarsomine and it reduces the inflammatory response driven by Wolbachia surface proteins (WSPs) released during worm death. That second point is why it reduces pulmonary thromboembolism severity. Doxycycline is not directly adulticidal.

Caval Syndrome: Recognize It Fast

Caval syndrome is the exam’s high-stakes scenario. A heavy worm burden causes retrograde migration of adult worms into the right atrium, right ventricle, and vena cava. The worms physically obstruct the tricuspid valve, causing severe tricuspid regurgitation, decreased cardiac output, and intravascular hemolysis as RBCs are mechanically damaged passing through the worm mass.

The clinical picture: acute collapse, hemoglobinuria (dark red-brown urine), pale mucous membranes, right-sided systolic murmur, jugular venous distension with visible pulsations, hepatomegaly. Echocardiography shows the classic parallel hyperechoic lines (equals sign appearance) in the right atrium and right ventricle.

Management is surgical extraction via jugular venotomy — stabilize with IV fluids and blood transfusion if needed, then extract worms using alligator forceps or a retrieval device under echocardiographic or fluoroscopic guidance. Melarsomine is absolutely contraindicated in caval syndrome. A massive worm die-off from adulticidal treatment would be fatal. Without surgical intervention, mortality approaches 100% within 24–72 hours.

Small breeds — Chihuahuas in particular — are overrepresented in caval syndrome. The same absolute worm burden causes a much greater relative obstruction in a small right atrium. The NAVLE will present a small breed with acute collapse; do not be distracted by the signalment.