On the merits of heartworm prevention

Patty Khuly, DVM
Updated: February 18, 2016
Published: March 22, 2008
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One of my newest clients arrived at the office with reams of printouts in tow extolling the horrors of administering monthly poisons to protect our pets from heartworms. She also happened to live in the soggiest, boggiest part of Miami where mosquitoes are rumored to suck the life out of anything vaguely resembling a mammal.

Now, I’m not a big lover of poison therapies but, unfortunately, every “drug” I use is a toxin—it’s just a question of quantity. Sure, risk and reward factors into my use heavily. I’m not one to recommend a drug willy nilly. And pet parents are always granted leave to decline—though I’ll be sure to note on my medical records that they’ve done so…to cover my butt, of course.

(Last thing any vet wants is for their lax note-taking to land them in hot water when Humphrey gets heartworms over the next year. Nope. I’m not playing that game.)

I’m occasionally pooh-poohed by my veteran dog owners on what they perceive as the dubious merits of my heartworm prevention protocols. Based on their read of the literature, they’re often inclined to rely on the studies describing adverse reactions to ivermectin, milbemycin and selamectin (Heartgard, Interceptor and Revolution, respectively, the drugs we vets most commonly employ for parasite prevention).

And though these cases are by no means discountable out of hand, they represent just a tiny fraction of the millions of dogs who have received these meds over the past thirty years. More importantly, their cautionary numbers don’t measure up to the very real risk of heartworm disease—even among indoor pets.

Myth 1: My dog doesn’t need heartworm preventatives year-round because he lives where mosquitoes are rarely seen except in summer months.

If the ground doesn’t freeze solid where you live you likely need prevention year-round for 100% safety but it’s true that winter months offer a low, sometimes minuscule risk of infection.

Myth 2: My dog never goes outside so it’s pointless to give him these drugs.

Never? Doesn’t he need to go for a walk? Do you never see mosquitoes in your house? Ever? OK, then, for all of you living in condos in the Windy City where wee-wee pads reign supreme. You’re excused.

Myth 3: My cat doesn’t need heartworm preventatives because it’s rare in cats—and she doesn’t go outside anyway.

I’ve seen heartworm positives (antibody tests, which indicate exposure) in indoor cats. So far no antigen positives, but it’s a numbers game—eventually I’ll get one. In Florida, a at least, Heartworm’s not rare in cats.

Heartworm disease is still a real problem with serious consequences. Those heartworm preventative naysayers? They have a point. They're right to question the utility of ANY drug their pet consumes on a regular basis. But don't expect their vet to buck the establishment in ways that puts their hospital at risk, not when mosquitoes and their wily ways still manage to infect unsuspecting pets even in low-risk scenarios.

I know some of you won’t agree with me. So here’s a chance for you to ream me a new hole in my head through which your wise ideas might penetrate. Warning: My skull’s a bit thick on this issue so you’ll have to be very persuasive.