ALL Pets Need Heartworm Preventive ALL The Time

Yesterday we discussed a pet owner’s misconception that his pet, a dog in this instance, wasn’t at risk for heartworm disease because he was a mostly-indoor pet.

Everything we will say today will apply equally to your canine and feline friends.

We’ve known for nearly a century that dogs get heartworms. We’ve known for decades that cats get heartworms. Much more recently it became known that indoor cats get heartworms at about the same rate that outdoor cats do. Researchers wanted to know why, and their collaboration with entomologists showed that the mosquito that carries the heartworm parasite best also likes the indoor environment.

So, your pet doesn’t have to go outdoors at all to become infected with Dirofilaria immitis, the heartworm parasite. Mosquitoes are happy to bring them indoors to your pet.

When pets are described as “mostly indoor pets,” owners often forget that dogs go outdoors to use the bathroom four or more times each day. A single encounter with a single mosquito is enough to infect your pet with heartworms.

Another common misconception is that longhaired pets are immune or at lower risk of heartworm infestation. We need to be reminded that the belly surface of most pets is bald, and the ears have short hair, also. Not that any of this means anything to a mosquito, who can easily fit between hairs on your pets body to reach the skin to feed.

And in feeding, infect your pet with heartworms.

All pets, dogs and cats, need to be on heartworm preventive. And they need to get their heartworm preventive twelve months of the year without fail.

See you Monday, Dr. Randolph.


  1. My Daughter’s dog is on the second round of heart worm treatment. It is very expensive. She adopted a dog from the Mississippi area. She lives in Delaware. Two vets told her to give preventative treatment even though the dog still has the baby heart worms. Any advice?

    • This article will give you more understanding of Dirofilaria immitis microfilaria. They aren’t actually “baby” heartworms, in that they must go into another host and undergo several larval stages before they can infect another dog and become adult heartworms. None of which applies to your question or its answer, but I thought you might want to learn about them. The crucial factor is the NUMBER of microfilaria and the patient’s likelihood of having a reaction. Your daughter’s veterinarian may have already determined that the microfilaria count is low, OR he may have already treated for microfilaria, OR the patient might be microfilaria-negative (just as not all married couples have children, not all adult heartworms have offspring) OR he may have already administered the first dose of heartworm preventive under supervision, ensuring that any reactions that might occur were treated while being observed. Your daughter is free to address her concerns with the doctor(s) and receive clarification. Thanks for reading, Dr. Randolph.

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