Heartworms: Now Showing in New Venues

Heartworm disease has always been thought of as a canine affliction, but newer studies have shown us that this is also a disease of cats. If you live on one of the six most populated continents and have a pet cat or dog, you should familiarize yourself with this very common (and life-threatening) infection.

Heartworms mature and behave differently in cats as opposed to dogs, even though it is the same species of parasite carried by the same types of mosquito. The maximum mature worm size in cats is much smaller; on the other hand, their heart is very small compared to that of a dog (thus much more easily obstructed). Heartworms can’t live as long inside a cat’s body, since they evolved to survive perfectly inside the dog. That creates an even more treacherous situation since the one thing more damaging than live heartworms is the corpses of dead heartworms. In contrast with the dog, there is no treatment for feline heartworm infection. Sudden death is not an uncommon outcome for the unlucky feline facing this disease.

Dog or cat, it takes about 6 months for a mosquito bite to mature into the adult parasite which resides inside the heart. Yes, it’s a real worm that you could see with the naked eye (except for the fact that the chambers of the heart are a pretty private place). In dogs, that’s when the visible signs of disease begin. You might not be able to notice easily at one or two worms, but newcomers are always welcome inside the heart until it’s so cramped that they’re wrecking the place. A simple blood test can easily detect the presence of these fully developed worms.

Full-grown worms are rare in the cat. Their immune system is pretty hardy, so the maturation rate of this parasite (i.e., babies successfully migrating from the bite site to their permanent home in the heart) is only about 10% when compared to a dog. However, the damage and destruction begin far sooner in the feline lung, where baby heartworms wreak havoc as they meander toward the heart, waging war against that very combative immune system. The resulting inflammation causes difficulty breathing, coughing (quite unusual in the domestic cat), and/or persistent vomiting. Obvious changes in behavior begin just 90 days after the tainted mosquito bite and may go undiagnosed for quite some time. Since the standard blood test is only able to detect full-grown worms, only one in ten heartworm-infested cats will test positive on it.

Even very severe heartworm damage would thus be impossible to diagnose in cats if not for the development of a blood test which can sniff out immature heartworms. It’s nowhere near as reliable, but in some cases we can arrive at a relatively confident diagnosis using the two blood assays in conjunction with each other.

Heartworm prevention in cats is pretty much identical to the dog: easy to administer and virtually flawless in performance. So why risk it? Ask at your next vet visit how simple it is to protect cats and dogs alike.

Dr M.S. Regan