Heartworms in Pets: What Every Pet Owner Should Know

As veterinarians, one of the most serious — and preventable — diseases we see in dogs and cats is heartworm disease. Sadly, there have been increases in the percentage of infected animals in recent years. Many pet owners are familiar with heartworms, but few understand how they spread, the severity of the damage they cause, and the importance of prevention.

What Are Heartworms?

Heartworms (Dirofilaria immitis) are parasitic worms that live in the heart,

lungs, and blood vessels of infected pets.

They can grow up to 12 inches long and cause severe, potentially fatal damage.

While dogs are the most commonly affected, cats, ferrets, foxes, coyotes, wolves, raccoons, opossums, seals, and sea lions can also be infected.

The Heartworm Life Cycle

Understanding the cycle helps explain why prevention is crucial:

1. Mosquito bite – A mosquito bites an infected animal and picks up microscopic larvae (called microfilariae).

2. Mosquito spreads larvae – When the mosquito bites another dog or cat, it transmits the larvae.

3. Larvae develop – Over 6–7 months, larvae mature into adult worms living in the heart and lungs.

4. Reproduction – Adult worms release more microfilariae into the bloodstream, ready to be picked up by more mosquitoes.

This cycle makes mosquitoes the essential carrier, which is why heartworms are a risk even for indoor pets.

Heartworm Disease in Dogs

Dogs are the natural host for heartworms, meaning worms can live and reproduce inside them.

• Mild infection signs: coughing, fatigue, weight loss.

• Severe infection signs: heart failure, lung disease, collapse, and sudden death.

Heartworm Disease in Cats

Cats are much more resistant to heartworms, but are still at risk. Even one or two worms can cause serious problems and even be fatal.

• Signs include coughing, vomiting, breathing difficulty, or sudden death.

• There is no safe treatment for cats — only prevention.

Prevention Is Key

Heartworm disease is easier to prevent than to treat.

• Monthly preventives (oral, topical, or injectable) kill larvae before they mature.

• Dogs should be tested annually, even while on prevention, to ensure protection.

• Prevention should be given year-round, especially in mosquito-heavy states like Florida.

• There is evidence of some heartworms becoming resistant to heartworm preventives, primarily in the Mississippi River Valley.

✅ Bottom Line: Heartworm disease is dangerous, widespread, and potentially fatal — but absolutely preventable with consistent protection.