Heartworm Part I: Basic Biology and Pathology of the Dreaded Beastie

Patty Khuly, DVM
Updated: November 07, 2013
Published: October 02, 2006
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Heartworm is a tricky thing. This long white worm vets proudly display on their exam room shelves (nestled within those lurid plastic models of dog hearts) is a wily beastie. It not only has the ability to wreak havoc on your dog’s heart and your cat’s lungs, it can have a similar effect on your finances should your pet become infected. Prevention, indeed, is the mainstay of medical management but this approach, too, can get expensive over a pet’s lifetime.

I find that most people don’t truly understand the ins and outs of the parasite’s activity, beyond the very basics of infection by its mosquito taxi service. So here’s a primer:

It first gets into your pet through the bite of a mosquito during its microscopic larval phase. The injected young-uns then swim along in the pet’s bloodstream, enjoying their life in the twisting water slide that is the circulatory system. Eventually they grow big enough so they can no longer slip through the vessels—at which point their time on Splash Mountain is over.

Once large enough, juvenile worms get stuck in the biggest part of the vasculature, the chambers of the heart. In the case of cats, the vessels of the lungs are preferred. There they have to grow up to do adult things—like find a mate. Boy worms wait for a girl worm (and vice versa) to grow up and join him so they can have worm sex and make worm babies (microfilariae—these are what we look for under the microscope to help diagnose heartworm disease in the dog).

Newborn babies swim through the long blood slide, saying hello to mom and dad every time they come back to the heart or lungs. Until perhaps one day, happily making like Flipper in the warm fluid, they are unceremoniously slurped up by a hungry mosquito. Within the mosquito bowels, they receive a special message allowing for their maturation to the infective larval phase.

If the larvae are lucky, the mosquito finds another dog to suck on and the beastie is free again to roam the playground of its youth, no longer confined to the insect’s bloody backside. So maybe they, too, can grow up to be a big worm and have big worm sex.

I realize this is gross—yet accurate and descriptive. It explains how dogs and cats get this thing into their body, and why it’s not transmissible without the efforts of a mosquito (which acts as nothing more than a glorified limo service providing a hormonal rite of passage into larvae-hood).

Other animals might also receive a dose of heartworm larvae through the sting of a mosquito. Like most humans, however, these recipients have antibodies which counteract the creatures. Cats’ antibodies are not as effective as most, though, and sometimes (rarely, it seems) the larvae have a chance to make it to adulthood. Although uncommon, when cats get heartworm the things grow in their lungs. This makes the disease is much more deadly to kitties. Because lungs are very sensitive organs, it also makes the disease incurable. Killing the worms in the lungs often means killing the cat.

Dogs` heartworms are well adapted to their hearts, doing minimal damage unless left for long periods of time (months or years). When we give a drug to kill the worms (Immiticide), the worms typically die slowly in the heart. Heartworm treatment drugs of years past were arsenic based and had lots of deadly side effects. Today’s drugs are much more gentle on both dog and worm. It terminates their relationship less violently.

When we give heartworm medication on a monthly basis, the goal is to give a tiny dose of a worm killer (Ivermectin, Milbemycin, or Selamectin) to bump off the larvae invading via mosquito on a regular basis. It has to be given monthly so the larvae never get a chance to grow up and colonize the heart.

Now that you understand the basics we can tackle some of the more fun issues involved in the politics of heartworm prevention. This I’ll leave for part deux of my heartworm series. Time for lunch!