Cats who live where mosquitoes do, whether they live indoors or out, should [ideally] receive heartworm preventatives. That’s the premise of this post and I hope you’ll all take it to heart. 

While it’s true that cats are not the ideal host for heartworms (Dirofilaria immitis), they can become infected by heartworm carrying mosquitoes, nonetheless (as can humans, though usually only immunosuppressed ones). This is a relatively new finding. In fact, it’s not something we discussed in vet school while I was there fifteen years ago. 

Though we always knew it was possible, we hadn’t yet understood the disease process for cats, nor were we aware of the significant incidence of hearworm disease among them. Now we know that feline heartworm infection causes sudden death, vomiting, asthma and other chronic lung diseases in cats. And there is no treatment for the disease...only symptomatic treatment for the signs they may manifest.

Before ten or so years ago, many cats previously considered straighforward cardiac deaths, sudden acute respiratory distress patients, gastrointestinal disease suspects or feline asthma sufferers had to be recharacterized as potential heartworm cases. We began testing all cats with these signs (if we got there in time) and noted a high incidence. 

How high? 

Though it’s hard to say, some studies report as high as 20% exposure and 10% infection rates in heartworm endemic areas. Though the general practitioners among us reject this high an incidence, it’s clear our cats have a need for protection. Especially when the disease, once it does strike, can be deadly. 

But not always.

A recent study out of Italy found that 34 of 43 heartworm infected cats cleared the disease within 49 months. The others, usually the older ones, died within this four-year period. This is good news, better than previously expected. Survival is not only possible. It seems likely. But it’s still pretty scary. Especially since 23 of the 34 who cleared the infection showed signs of illness. And we still don’t know how to make them feel better––not always.

But we do know how to prevent the disease. We have lots of experience with dogs, after all. 

All it takes is a monthly oral or topical dose. I use Pfizer’s Revolution with tremendous success and, thus far, nary a side effect. One Pfizer insider even boasted of the product’s ability to prevent heartworm infection when applied every three months––though I didn’t take his word for it. I still recommend it monthly.

While I’m far more insistent about using preventatives in chronically exposed outdoor kitties, a recent rash of heartworm exposure-positive indoor-only patients has redoubled my efforts to bring feline heartworm awareness to cats both indoors and out.

It’s also a good idea to take this news outside of the South Florida and Eastern Texas, heartworm-rich enclaves. That’s because cats in “Heartworm Central” states seem to be faring better in their heartworm incidence relative to cats in parasite-easy areas (usually in northern latitudes). These latter cats are proving ill-protected and susceptible, just as these areas’ dogs are. 

Here's the incidence as of 2005 (note that more north central regions as far north as Canada are seeing more cases than they did even four years ago):

It’s the out-of-sight-out-of-mindedness that creates these non-prevention circumstances. That, along with the non-year-roundedness of the need for prevention and the newness of feline heartworm awareness. That’s my take, anyway. Some insect watchers even postulate that climate change is responsible for the impressive incidence of heartworm disease in cats as well as dogs dogs in places previously considered low heartworm risk areas. 

Furthermore, it's been my experience that the presence of pesky fleas has cut down on the risk of heartworms. That's probably because so many flea meds carry heartworm preventatives, too. For example, I can easily recommend Revolution or Advantage-multi for cats who have fleas and know that the heartworm prevention power is there as well. It’s easier to get my clients to accept prevention if it’s built in to a product they’re willing to use, anyway. 

But what about your indoor kitties? Though I can understand why you might not want to apply a poison to your lower-risk cats, my experience with the heartworm positive indoor-only variety means that all cats in mosquito-prone zones are at risk. 

Is it worth the risk of applying monthly meds? In the absence of harder evidence not yet available, we can only guess. But mine leans in favor of prevention. What else would you expect a veterinarian to say?