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To quote Cats the musical, “A cat is not a dog.” This is a fact that is both obvious and often overlooked. Sometimes, cats and dogs are subject to the same illnesses, but that does not mean that they will have the same symptoms or can receive the same treatments. Heartworm disease is a prime example.
Heartworm is the common name for a blood-borne parasite, Dirofilaria immitis, which normally resides in the heart or attached large blood vessels of an infected animal (1). Mature female worms are 6-14 inches (15-36 centimetres) long and 1/8 inch (3 millimetres) wide, with mature males about half that size (1). They can infect dogs, cats, ferrets, and even, rarely, humans (2). Wild hosts include wolves, coyotes, foxes, and sea lions (2). The heartworm lifecycle begins when a mosquito bites an infected animal. The mosquito sucks up larvae circulating in the bloodstream, called microfilaria (2). The larvae develop into an infectious stage in the mosquito and then enter a new host animal when the mosquito bites it (2). The larvae migrate to the right side of the new host’s heart, where they will mature into adult worms over the next six months.
Heartworms can only be transmitted by mosquitos, not between individual animals, although having a significant population of infected hosts increases the likelihood of infection. Heartworms can live for 5-7 years in dogs and 2-4 years in cats (3), so the infection burden in an animal population can increase quickly over several mosquito seasons, and individuals can be infected multiple times. Heartworm infection is possible throughout the United States and in many other countries throughout the world. Thirty species of mosquito can carry the larvae (1). Infected wildlife or mosquitoes can travel great distances, and infected pets may be moved or adopted to new areas (1), so it is safest to assume that if there are mosquitoes where you live, there is a risk of heartworm.
Cats are not a very good host for heartworms. Canines are more hospitable to the parasites. Cats are infected 5-20% as often as dogs in the United State, depending upon the location (4), and the worms often fail to mature inside their bodies. Cats also are unlikely to have circulating microfilaria, and about a third of infections in cats have mature worms of the same sex and therefore produce no larvae at all (5). However, just because cats are an imperfect host does not mean that they cannot suffer severe or sometimes fatal symptoms.
Because cats are so much smaller than most dogs, they don’t have space for nearly as many of the parasites. They usually have only 1-3 mature worms, if they have any adults at all (2). Even a few worms is still considered a heavy infection, however, due to a cat’s size (3). And even a cat with only immature worms can still have significant disease. Although the term “heartworm disease” would suggest that the symptoms primarily affect the heart, in cats heartworms tend to cause respiratory problems. Heartworms, once mature, often take up residence in the pulmonary arteries that take blood from the heart to the lungs (6). There, worms can block the arteries, and dead and dying worms cause inflammation that compounds the problem (6).
Cats are subject to a complication known as heartworm-associated respiratory disease (HARD). The symptoms of HARD are rapid and labored breathing, coughing, and gagging. Unfortunately, these symptoms are also indicators of asthma or bronchitis, making it difficult to distinguish a heartworm infection from other forms of respiratory distress. Other symptoms of heartworm infection are also non-specific: vomiting, loss of appetite, and weight loss. In more severe cases, cats may have seizures, faint, struggle to walk, or accumulate fluid in their abdomens. Sometimes, the first sign that something is wrong is sudden death.
If heartworm is suspected in a cat, it is much harder to diagnose an infection than in a dog. It is typical to have to use multiple tests, often more than once (7). This is because the unusual nature of heartworm infection in cats makes many tests less reliable, including the antigen blood test which is the gold standard for dogs (7). The antigen test detects mature female worms, but will not detect an infection with only male or only immature worms (5). Because both of those types of infection are likely in cats, there is a high rate of false negative results. Still, 50-70% of cats are estimated to have at least one female worm, so the test is worth a try (7).
The antibody test is another blood test which detects the cat’s immune response to the parasite. While antigen tests can only detect an infection after 6-8 months, once the parasites are mature, the antibody test is effective as early as two months after infection (7). The antibody test can be highly sensitive, but it’s reliability decreases over time and may be affected by the use of certain medications to treat the infection (7). It can also still be positive after a cat has fought off an infection but the antibodies are still in their blood (7). As the antigen and antibody tests are both useful but have major pitfalls for cats, they are best used together.
If the results of the antigen and antibody blood tests are inconclusive, there are other options. The blood can be tested for circulating microfilaria, but less than 20% of cats will have microfilaria, and then only for 1-4 weeks, so the chances of finding them are low (1). X-rays and ultrasounds of the heart and its vessels can look for changes in the shape of the heart and pulmonary arteries (1). Sometimes an ultrasound will even find the mature worms themselves, although this is uncommon since cats have so few worms (1).
When a heartworm infection can be diagnosed, treatment is its own challenge. The medication used to treat heartworm in dogs, melarsomine dihydrochloride, has been shown to be toxic to cats (8, 9). Cats that appear to be doing well or have no symptoms at all may be allowed to continue fighting the infection on their own while being continually monitored and routinely re-tested (7). A course of steroids is a common treatment for cats that need more support.
If a cat is in need of acute care, a veterinarian will step in with treatments for shock and symptoms, such as oxygen, intravenous fluids and steroids, or bronchodilator drugs to open up their airways (7). The goal is to stabilize the cat until they can eliminate the infection on their own. As measures of last resort when an otherwise stable cat seems to have trouble doing this, the drug ivermectin can be given to kill adult worms, or worms may even be surgically removed (7).
Obviously, heartworm disease in cats is a complicated and difficult matter. The best treatment is prevention. Heartworm preventatives are available as monthly oral or topical medications, or an injection given every six months by your veterinarian (2). In the United States, heartworm medication is available by prescription only, but on the bright side, most heartworm medications also prevent intestinal parasites, and some are effective against external parasites like fleas and ticks (2). The American Heartworm Society recommends using year-round prevention even in places with cold winters, just in case (2). Indoor cats should be treated as well. Mosquitoes have a knack for making their way inside. About a third of cats with heartworm infections are indoor-only (1). You can’t underestimate the diabolical sneakiness of mosquitoes.
For more information about heartworms, visit the American Heartworm Society at https://www.heartwormsociety.org/ or any of the resources below.
Works Cited
- Barnette, C. and Ward, E. (n.d.). Heartworm disease in cats. VCA Animal Hospitals. https://vcahospitals.com/know-your-pet/heartworm-disease-in-cats
- American Heartworm Society. (n.d.). Heartworm in cats. https://www.heartwormsociety.org/heartworms-in-cats
- U.S. Food and Drug Administration. (2019, August 22). Keep the worms out of your pet’s heart! The facts about heartworm disease. link here
- Ryan, W.G., et al. (1995). Prevalence of feline heartworm disease – a global review. American Heartworm Society. Proceedings of the heartworm symposium 1995, 79-86. https://www.cabdirect.org/cabdirect/abstract/19960805975
- Stannard, R. (n.d.). The facts about feline heartworm disease. Today’s Veterinary Practice. https://todaysveterinarypractice.com/the-facts-about-feline-heartworm-disease/
- Cornell Feline Health Center. (n.d.). Heartworm in cats. Cornell University College of Veterinary Medicine. link here
- Jones, S., et al. (2014, October). Current feline guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in cats. American Heartworm Society. PDF
- McLeroy, L.W. (1998). Evaluation of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats with intravenously transplanted adult heartworms. MS Thesis, 95. University of Georgia, Athens, GA.
- Goodman, D.A. (1996). Evaluation of a single dose of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats. MS Thesis, 95. University of Georgia, Athens, GA.
Published June 21st, 2020
Updated November 4th, 2020
