Are all heartworm infections equal? The short answer is: no.
The two main species that get heartworm infections that I learned about back in vet school was dogs and….sea lions! It wasn’t until toward the end of my time in vet school that the field was discovering that cats in fact get heartworm too. The worms don’t like living inside a cat as much as a dog (or a sea lion for that matter), but heartworm infection has been identified often enough in cats to bring about concern.
When a dog has heartworms, he or she has multiple worms. They look like spaghetti and, true to name, live in the heart. Technically, they live in an artery that sends blood from the heart to the lungs, but pulmonary-artery-worm just doesn’t roll off the tongue. Hence, we call them heartworms.
The heartworm test that veterinarians typically offer detects a molecule that female worms excrete into the blood. The more female worms, the stronger the positive on the test. Dogs have mixed populations of male and female worms, so we always know if they are there or not.
Cats are tough. I mean really, is anything with cats easy? They often get an infection of a single worm. Well, the odds are 50/50 that it will be a male or female worm. If “it’s a girl!” then the traditional test we use in dogs will detect it just fine. If the worm is male, however, the standard test will not detect it. Males do not excrete anything detectable. There are other tests we can do by sending blood to the lab, but they are not very specific either.
The thing is, if a dog has heartworms, we treat it. The treatment is expensive and horrible (crate rest for 3 months is never fun!!) but many dogs do well. Cats, however, have no treatment. In studies, they’ve tried treating cats with heartworm, but both the worm and the cat died. Not good.
So if cats have no treatment, the best thing is prevention prevention prevention! I only test a cat if he or she is exhibiting clinical signs. Most cats with heartworm will present with a cough. Guess what – asthma or other heart disease in cats can present with a cough too. So, the diagnosis unfortunately isn’t clear cut. Thankfully, heartworm is less common in cats than dogs. Turns out a cat’s system isn’t quite as accommodating as a dog’s (or a sea lion for that matter).
Bottom line – use monthly preventative year round on both dogs and cats! If you live in Alaska, then OK, maybe you can skip it in the winter. Around here, we’ll have an arctic blast, then a 60 degree day. Better safe than sorry.