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Heartworm 103 - Those Pesky Cats

Cats are not just small dogs, and feline heartworm disease is a prime example. The first two articles of this series dealt with how heartworms cause disease and preventative/diagnostic strategies primarily in relation to dogs. Today's article will outline specifically how heartworms effect cats; in many ways they are a more frustrating and elusive problem in cats, made all the more dangerous because many owners don't realize feline heartworm is a serious concern.
Accidental Guests: Heartworm Disease in Cats
Cats are not the natural host for heartworms. Unfortunately, no one seems to have explained this to the mosquitoes that spread infection or the heartworm larvae that cause it. Consequently, these uninvited and unadapted parasites really, really tick off the cats immune system. The first consequence of this is positive - cats seem much better than dogs at killing off larvae before they develop into adults. Studies suggest only about one-third to one-half of naturally infected cats develop adult worms, though it's tough to nail down accurate percentages due to the difficulties in testing that we'll go into later. 
Hey, human. Indoor cats get
heartworms too.
The second consequence of cats' super-miffed immune system is a negative one, however. Killing off the larvae within the lungs results in significant inflammation. Vessels and lung tissue gets thickened, making blood flow and air exchange less efficient and resulting in the classic signs of respiratory disease - coughing, rapid or labored breathing, and general malaise. This is now referred to as heartworm-associated respiratory disease (HARD), and it is a major reason why I've started recommending all cats receive monthly preventatives. Depending on the studies you look at, perhaps as many as 15% of cats in some areas are exposed to heartworms and thus at risk for HARD - even if they never develop adult worms. And if you think your kitty is free and clear because it's indoor only, think again - in one published series of cases, 27% were indoor-only cats.
What about the ones who do develop adult worms, though? Whereas dogs may have dozens of adults, cats rarely have more than two or three - again, their immune system is pretty good at whacking those larvae. This means many cats with adults have such a low burden they may show none of the cardiovascular problems typical in dogs. The caveat to this is that when those adults die off after a year or three, the dead worms are big enough to block off major lung vessels - and between that and the cat's once-again huge inflammatory response it can be enough to quickly kill them. Indeed, most veterinary texts list "sudden death" as a major "sign" of feline heartworm infection. That's not the way I like to diagnose things.
Diagnosing Heartworms in Cats... really, really frustrating.
In the second article we talked about how the main test used to diagnose heartworm in dogs looks for a protein produced by adult female worms. This test works equally well in cats, if there is an adult female worm. In one study on naturally infected shelter cats over 50% had only a single worm, and none had more than four. This means cats are fairly likely to have male-only infections, and thus come up negative on this test - even though a single male could potentially kill them. 
Cats aren't good at tests.
Even more importantly, a lot more cats will develop the serious respiratory disease discussed above (HARD) without ever developing adult worms. On the bright side, there areantibody tests available that check for exposure to heartworm larvae. These would be great for diagnosing HARD or even male-only adult infections...except studies suggest they aren't that sensitive and miss many exposed cats, maybe more than 50% of them. 
Not surprisingly, I don't spend much time screening outwardly-healthy cats for heartworm. I'll run both tests when I suspect a cat has HARD or adult worms...but if they're negative, I don't cross heartworm off the list. Chest x-rays or ultrasound of the heart can be useful, but diagnosing feline heartworm can be elusive even when you're sure that's the problem.
Treating Feline Heartworm
Here's a quick topic. Remember how one dead adult may be enough to gunk up the lungs and cause sudden death? We can kill the worms easily enough, but we're just as likely to kill the cat. Yeah...we don't treat feline heartworm. "Treatment" is pretty much relegated to using anti-inflammatory drugs to control the signs and hoping for the best when the worms do die off. It's pretty frustrating when you find yourself crossing your fingers and wishing a freeloading parasite a long, healthy life.
Heartworm Preventatives for Cats
Most veterinarians used to be wishy-washy at best about recommending monthly preventatives for cats because adult infections were relatively uncommon, and serious complications (like death) even moreso. But the respiratory complications - which can potentially be life-threatening themselves - are looking more and more common. Complicate this by the fact we don't have a great diagnostic method  (meaning we likely are underestimating prevalence) and hopefully you're starting to understand why I've started recommending all cats stay on a monthly preventative. 
As of this writing, there are two topical products and two oral products on the market, and all are effective for heartworm prevention. Both topicals are great products, covering fleas and some common intestinal parasites while sparing you the adventure of trying to get a pill into a cat. Neither oral product covers fleas, but Interceptor covers several of the more common intestinal parasites. It's a good alternative in areas where fleas aren't a concern as it's a fair bit cheaper than the topicals.
I've covered a lot of ground in these three articles, and in a good bit of depth. If you're not 100% convinced that heartworm is a serious risk that warrants monthly preventatives, I'm happy to address any lingering questions or concerns in the comments section below!