The importance of heartworm prevention in cats (yes, cats)

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Everyone has that one pet who lives on well beyond their years. For me, that was my cat Fuzzy Man. He was legendary—not just for his unmatched personality and unique talent for causing mischief, but also for his storied veterinary history. He survived hepatic lipidosis, a urinary tract obstruction, and believe it or not, heartworm disease.

In general, cats are approximately 20% to 30% less likely than dogs to develop heartworm disease after being bitten by an infected mosquito, and their worm burden is generally significantly less.^1,2 In my experience as a clinician, it is not uncommon for many clients to reject recommendations on heartworm prevention because their dog doesn’t spend much (or any) time outside. As hard as it can be to persuade some dog owners, it can often be even harder to persuade cat owners, regardless of whether the cat has any outdoor access.

In Fuzzy Man’s case, he was an indoor-only cat, not counting the handful of times he would slip out the front door for a few seconds before being whisked back inside. Even though I live in Florida, I only used flea prevention as needed at the time and had no idea what heartworms even were. That is why it is of utmost importance to ensure pet parents are aware of these potential dangers that may have never even occurred to them.

When Fuzzy Man had his first senior lab work panel done in 2007 at 7 years old, his heartworm test came back positive. His situation exemplifies the importance of preventive care: Had he not received routine lab work, we might have never known about his heartworm status. Treatment began by starting a tapering dose of prednisolone and performing routine chest x-rays every few months while we monitored for clinical signs at home. Eventually, we started a 1-month course of doxycycline, as well as monthly heartworm prevention. As is the case with many cats that develop heartworm disease, Fuzzy Man never showed any of the associated respiratory clinical signs, such as coughing, increased respiratory effort, and/or tachypnea.

In October 2008, I was in my fourth month of vet school in another state when I received a frantic call from my brother in Florida, who was now Fuzzy Man’s caretaker. Apparently, Fuzzy Man had vomited several times, was very lethargic, and was not very responsive when my brother got home from work. Fuzzy Man was rushed to an emergency clinic. I spoke with the vet to make sure they knew his heartworm status. After running lab work, the vet suspected bartonellosis due to mild anemia. The vet was not sure that Fuzzy Man’s clinical signs were due to heartworm disease because he was not coughing, and his chest x-rays looked clear. He was stabilized overnight and sent to his primary vet for continued care the next day. When his condition declined again, the primary vet advised to either take him to a different emergency clinic with more advanced diagnostic capabilities or consider letting him go. My brother and I would have happily donated any one of our vital organs if Fuzzy Man needed it, so off to another emergency clinic he went.

Upon speaking with the vet at the new emergency clinic, I explained Fuzzy Man’s medical history, current diagnostics, and that the previous vet was skeptical about attributing this to heartworm disease. The response I got was, “Well, cats hardly ever follow a textbook. Your cat looks like he’s in shock, and heartworm die-off would be pretty high on my list, even if he doesn’t have any of the other clinical signs.”

In the pathophysiology of the disease, the breakdown of the heartworms will typically incite pulmonary inflammation and thromboembolism. These, in turn, can often lead to fatal acute lung injury.^3,4 So, of course, I was warned that despite every effort made, there was a significant chance Fuzzy Man may not make it.

In addition to supportive care, Fuzzy Man was started on aspirin and was monitored intensively. During the next few days, he started to perk up, regained his appetite and energy, and even tried to sneak out of his kennel once or twice. He was discharged from the clinic after 1 week, and we were instructed how to gradually taper him off the aspirin. He continued to be symptom free at home, and 6 months later, his heartworm test was negative.

Neither he nor any of my other cats ever missed a dose of heartworm prevention ever again.

Because heartworm disease has been found in all 50 states and is considered an endemic in all 48 contiguous states,^5-7 it is strongly recommended to implement year-round heartworm prevention for cats to protect against heartworm disease. Additionally, treatment options for heartworm-infected cats are limited compared with dogs because the frequently used adulticide melarsomine can be toxic to cats.^8,9

Needless to say, Fuzzy Man was definitely one of the lucky ones. Many cats that get heartworm disease do not live to tell the tale. In practice, I routinely use his story to inform both cat and dog owners about the very real dangers of heartworm disease. There’s always a little twinge of satisfaction when convincing an owner who never thought heartworm prevention was important because their dog “doesn’t spend a lot of time outside.” Plus, it helps to know that Fuzzy Man’s legacy has extended well past his life span.

To learn more about Fuzzy Man's story, check out the interview below with the author, Ellen Bishai, VMD, PhD.

References

1. McCall JW, Dzimianski MT, McTier TL, et al. Biology of experimental heartworm infections in cats. In: Soll MD, ed. Proceedings of the American Heartworm Symposium ’92. American Heartworm Society; 1992:71-79.
2. Dillon AR, Blagburn B, Tillson DM, et al. Immature heartworm infection produces pulmonary parenchymal, airway, and vascular disease in cats. J Vet Intern Med. 2007;21:608-609.
3. Atkins CE, DeFrancesco TC, Coats JR, Sidley JA, Keene BW. Heartworm infection in cats: 50 cases (1985-1997). J Am Vet Med Assoc. 2000;217(3):355-358. doi:10.2460/javma.2000.217.355
4. McCall JW, Genchi C, Kramer LH, et al. Heartworm disease in animals and humans. In Rollinson D, Hay SI, eds. Advances in Parasitology. Academic Press; 2008:193-285.
5. Bowman DD, Atkins CE. Heartworm biology, treatment, and control. Vet Clin North Am Small Anim Pract. 2009;39(6):1127-1158. doi:10.1016/j.cvsm.2009.06.003
6. Kozek WJ, Vazquez AE, Gonzalez C Jr, et al. Prevalence of canine filariae in Puerto Rico and the Caribbean. In: Proceedings of the Heartworm Symposium ’95. American Heartworm Society; 1995.
7. Ludlam KW, Jachowski LA Jr, Otto GF. Potential vectors of Dirofilaria immitis. J Am Vet Med Assoc. 1970;157(10):1354-1359.
8. Goodman DA. Evaluation of a single dose of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats. Master’s thesis. University of Georgia; 1996.
9. McLeroy LW. Evaluation of melarsomine dihydrochloride for adulticidal activity against Dirofilaria immitis in cats with intravenously transplanted adult heartworms. Master’s thesis. University of Georgia; 1998.