How Does a Dog Get Heartworms?

Heartworm is transmitted only through mosquito bites — it cannot spread from dog to dog directly.

Here's how it works:

1. A mosquito bites an infected dog and picks up microscopic heartworm larvae called microfilariae.

2. Over 10–14 days, those larvae develop into an infectious stage inside the mosquito.

3. When that mosquito bites your dog, it deposits the infectious larvae through the bite wound.

4. The larvae slowly migrate through your dog's tissues, eventually reaching the heart and pulmonary arteries over the next several months.

5. Adult worms can begin producing new microfilariae 6–9 months after the initial infection — continuing the cycle.

This is why prevention must be consistent. Monthly preventives work by killing any larvae your dog picked up in the previous 30 days — before they can ever reach the heart. Miss a dose, and there's a window for larvae to slip through.

Heartworms have been diagnosed in all 50 states. The highest risk is in the South and Southeast — Mississippi, Louisiana, Texas, Arkansas, Tennessee, and Florida top the list — but no region is truly safe. The American Heartworm Society recommends year-round prevention for every dog, everywhere.