Feline Heartworm Disease
A disease that is sufficiently common, potentially fatal, difficult to detect, impossible to cure, and easy to prevent.
Feline heartworm disease can be a significant danger to your pet cat(s). While the incidence is lower than canine heartworm disease, feline infections often go undiagnosed.
Heartworm infections in cats are sometimes caused by only a single worm. The most common heartworm tests used in cats are antigen tests. Specifically, the test is checking for the presence of a substance (antigen) made by female heartworms present in the cat’s bloodstream. That means, at best, only half of single worm infections would show up on traditional feline heartworm blood tests (since it is only testing for the feline worm’s presence). Actually, there’s an even lower detection rate since a single worm frequently doesn’t shed enough protein to be picked up by the hearworm tests available. In other words, it takes an infection of several female worms to test positive which rarely happens in cats – usually there are only 1 or 2 worms total, some male, some female.
Veterinarians also rely on using an antibody test (versus the antigen test referred to previously) which indicate exposure to heartworms. The antibody test is testing to see if the cat has made an immune response after being exposed to hearworm larvae. Many cats will test positive, demonstrating heartworm larvae were injected into the cat by a mosquito bite. However, many of these “positive” cats will never get sick because their immune system kills the worm or the parasite ends up in an anatomic location that does not cause harm.
A study looking at the relative risk of indoor cats and outdoor cats found that indoor cats were actually more likely to contract heartworm disease than outdoor cats. 25% of heartworm infections occur in exclusively indoor cats! That finding is certainly counterintuitive. An indoor only animal should have less exposure to mosquitoes, so there must be something else at play. It has been proposed that there may be some “inoculation” benefit to regular exposure to heartworms in cats that get more mosquito bites. Perhaps the outdoor cats build up a natural immunity while an indoor cat remains immune-naïve until bitten by an infected mosquito. More research is needed before detailed conclusions can be drawn.
Cat owners are significantly underutilizing heartworm prevention products. In fact, the American Heartworm Society reports that only around 5% of cat owners administer preventives regularly. Symptoms of heartworm in cats can vary from asthma-like signs to sudden death. The most common form is a syndrome called heartworm associated respiratory disease, or H.A.R.D. In the past, many cats were improperly diagnosed with asthma or chronic bronchitis. Veterinarians often wondered why some cats with asthma would suddenly do better after years of requiring treatment. Now we know that many of these cases were really heartworm infections in the lung. Cats with heartworm associated respiratory disease may experience shortness of breath, wheezing, coughing, weight loss, or collapse.
In addition to typical H.A.R.D. symptoms, degrading dead worms can incite a severe inflammatory process and migrating worms can block blood flow to the heart or brain. An infected cat can literally go from showing no symptoms to being dead within an hour.
Unlike in dogs, there is no medical way to rid a cat of heartworms once they are infected. At that point, the best we can do is manage symptoms and hope for the best. That makes prevention just as, or even more important in cats than in dogs. We have a disease that is sufficiently common, potentially fatal, difficult to detect, impossible to cure, and easy to prevent.
Feline heartworm preventatives also prevent intestinal parasites in cats. Cats are a significant source of roundworm and hookworm infection in people. Microscopic worm eggs can be passed in cat feces and live in the soil for up to two years. The CDC estimates that approximately 10,000 Americans are affected by these parasites every year. Consequently, parasitologists recommend cat owners deworm their pets at least four times a year to help protect human health. Luckily, feline heartworm preventatives also serve as monthly intestinal dewormers.
If your cat is not on a heartworm prevention program, please ask your veterinarian for a recommendation.