Q: My vet recommended heartworm testing and prevention chews for my pets, but I’m worried about side effects and cost. Are these really needed?
A: For most dogs, the answer is yes. Heartworm disease is a common and life-threatening cardiovascular disease caused by a worm parasite — Dirofilaria immitis, which is spread by more than two dozen mosquito species. Vets in the southeastern United States and the Mississippi River Valley diagnose hundreds of cases a year, and most shelter dogs in these regions are infected. There are also isolated pockets of disease in other states, and case numbers have been rising over the last decade. And while the worm is best able to survive and reproduce in dogs and wild canids (such as wolves and coyotes), it can also cause disease in cats and occasionally humans. Here’s what you need to know.
What do heartworms look like?
Adult heartworms are translucent and resemble thin spaghetti; female worms reach 10 to 15 inches long. Although their name implies that the worms live in the heart, they prefer to be in the pulmonary arteries (the large vessels that carry blood from the heart to the lungs, where blood is oxygenated before being delivered to the rest of the body). But “pulmonary artery worm” doesn’t roll off the tongue quite as easily.
Most infested dogs have about 10 to 20 adult worms in their arteries, but this number can range from one to several hundred. Dogs that live in a region with a lot of infected mosquitos typically have more worms than dogs in other parts of the country. Adult worms can live for five to seven years in dogs — if the infested dog lives that long. Cats typically have low numbers of immature worms that only live for a few months, although in some cats, worms reach maturity and can live two to four years.
Are heartworms spread between animals?
Heartworms can only be spread by mosquitos. After mating in a dog’s pulmonary arteries, female worms release microscopic larvae into the blood, where they can stay for up to two years. Mosquitos ingest these larvae when they feed. The larvae mature inside the mosquito for a few weeks, then infest a new animal when the mosquito feeds again. Over the next six to seven months, the larvae make their way through tissues to the pulmonary arteries, where they mature into adults.
Why are heartworms so life-threatening?
Heartworms cause inflammation and blockage of the pulmonary arteries, but many infested dogs initially appear outwardly healthy. Illness typically develops gradually, with the earliest signs being a cough, then exercise intolerance. As worms die and bits of them break off and lodge in the lungs, respiratory distress and sudden death can occur. Exercise exacerbates this problem. Sometimes a clump of worms in the pulmonary arteries moves backward (against the flow of blood) into the right side of the heart. This can cause heart failure, which leads to weakness and fluid accumulation in the abdomen. The presence of the worms can also cause anemia and kidney problems.
The death of even a few immature worms in cats leads to acute respiratory disease, so the condition is often referred to as HARD (heartworm-associated respiratory disease). Signs of illness in cats include coughing, rapid breathing and, occasionally, vomiting. Because cats don’t move around as much, around one-third of infested cats appear healthy, but they can collapse and die without warning.
When humans get infected — which is uncommon — juvenile worms wander to abnormal places, especially the lungs, but even the eye and the testicles. When the body tries to wall off these worms, a small mass of tissue (a granuloma) forms around the worm, which usually doesn’t make humans sick or need treatment. However, if this granuloma is discovered by chance on a chest X-ray or ultrasound, it can look like cancer or other serious diseases such as tuberculosis. Because there’s no simple blood test for heartworm disease in people (and, because it’s rare, doctors might not think of it), this can lead to all sorts of emotionally traumatic and invasive testing, such as a lung biopsy, before doctors identify the cause.
How is heartworm disease diagnosed?
Blood tests can detect a substance shed by female worms (the heartworm antigen test) or the microfilaria themselves (the Knott’s test). Sometimes, one of these tests is negative but the other is positive, so they’re often done together. Unfortunately, if there are only a few worms, only male worms or only juvenile worms, both tests can come back negative. These situations are especially common in cats, so vets also sometimes use a blood test that detects the immune response to the worm in cats. However, even this test can come back with a false negative. For some animals the only way to find worms is with an ultrasound of the heart and pulmonary arteries, generally done by a veterinary cardiologist.
Can heartworm disease be treated?
Your vet can kill the worms using a series of injections. But when worms die and break up in the bloodstream, they can lodge in small vessels in the lungs, leading to breathing problems or sudden death. To reduce this risk, vets also prescribe steroids and strict rest. Pets with severe infestations may need surgery to physically remove the worms from the heart and blood vessels, which are accessed through the jugular vein.
Early treatment is key to a good outcome, so it’s recommended that vets do heartworm blood tests on all dogs annually, even if they appear outwardly healthy.
How do you prevent heartworm disease?
Marisa Ames, a veterinary cardiologist at the University of California at Davis and president of the American Heartworm Society, emphasized that heartworm disease is preventable. There are multiple chewable or spot-on medications approved by the Food and Drug Administration that you can give your pet monthly (spot-on medications are a liquid that you squeeze onto your pet’s skin after parting their hair), or there’s a once-yearly injection.
“These medications are nearly 100 percent effective when given as directed,” Ames said. “Heartworm disease causes severe damage to the blood vessels of the lungs and treating heartworm disease is a lengthy process. An ounce of prevention is worth a pound of cure.”
Because transmission of heartworms can occur indoors or outdoors and throughout the year — even in cooler climates — the American Heartworm Society recommends year-round heartworm prevention for dogs and cats, starting at 8 weeks of age. Many medications for pets not only prevent heartworm, but also other parasites, including fleas and ticks. If you’re already giving your pet a parasite preventive, check the label to see if it also protects against heartworm, or talk to your vet about it. Dogs over 7 months of age (the time it takes for adult worms to develop) should be tested before starting prevention drugs.
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