What Do Heartworms Look Like?
QUICK ANSWER
Heartworms (Dirofilaria immitis) are long, thin, spaghetti-like parasitic worms that live in the heart, lungs, and associated blood vessels of infected dogs. Adult worms can grow up to 12 inches long. They're transmitted exclusively through mosquito bites, and without treatment, heartworm disease is progressive and can be fatal.
You can't see heartworms from the outside. By the time a dog shows symptoms, the disease has usually been developing internally for months. That's what makes heartworm prevention so important and what makes the disease so dangerous when it goes undetected.
What do heartworms actually look like?
Adult heartworms are white to off-white, thin, and can reach up to 12 inches in length. They look like strands of cooked spaghetti. In a heavily infected dog, dozens to hundreds of worms can be present, tangled together in the heart chambers and pulmonary arteries. The damage they cause comes from their physical presence (obstructing blood flow) and the inflammatory reaction they trigger in the cardiovascular system.
How do dogs get heartworms?
The only way a dog contracts heartworm is through a mosquito bite. According to the American Heartworm Society, an infected mosquito deposits microscopic heartworm larvae (microfilariae) onto the dog's skin during a bite. The larvae enter through the bite wound, migrate through the dog's body over several months, and eventually reach the heart and lungs where they mature into adults. The entire process from bite to adult worm takes about 6 to 7 months. A dog cannot catch heartworm directly from another dog.
What are the symptoms?
In the early stages, there are often no visible symptoms, which is why annual testing is so important. As the disease progresses, signs include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. In advanced cases, dogs may develop a swollen belly (from fluid accumulation due to heart failure), difficulty breathing, and pale gums. By this stage, the damage to the heart and lungs may be permanent even with treatment.
Is heartworm preventable?
Completely. Monthly heartworm preventatives (available as chewable tablets, topical treatments, or injectable formulations) are highly effective at killing heartworm larvae before they reach the heart. The American Heartworm Society recommends year-round prevention regardless of where you live, since mosquitoes can be active in mild winters and indoor environments. Prevention costs a fraction of treatment, which involves a series of injections, months of exercise restriction, and carries risks of its own. An annual heartworm test ensures your dog is negative before continuing prevention.
Heartworm disease is serious, progressive, and potentially fatal, but it's also one of the most preventable diseases in dogs. A monthly preventative and an annual test are all it takes. If you're not already on a prevention program, talk to your vet about getting started.
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