Every dog owner knows that their pet should be on monthly heartworm prevention. Heartworms sure sound like a bad thing – I mean worms that live in the heart – not cool, right? Right!
I have discovered lately that some thoughts and general knowledge vary among my dog-owning friends and clients and felt it was time to comment on a few facts.

A gray square with the numbers 5 2 and 3 2 written in it.

Thank you to those of you that give your dog monthly heartworm prevention because your vet recommends it. However, I also want you to know WHY! Thank you also to those that purchase your product from your vet – there’s a reason you should do that… do you know why?
Heartworms come from mosquitoes – the worm has a life cycle that involves something called an “intermediate host” and it’s a mosquito. Canids (dogs, foxes, wolves, etc.) are the “primary host” and most susceptible to infection, though cats can get heartworms and even immunocompromised humans have been known to get them. In the wrong host, the worm tends to migrate around and often ends up in the lungs, causing chronic and severe damage to the host.

A gray square with the numbers 5 2 and 3 2 written in it.
thanks to oasisanimalclinic.com

A heartworm is a pretty good parasite. I mean, a good parasite uses its host to the utmost, so therefore does not want to cause the death of the host too soon. In the dog, a young stage of the heartworm is introduced via the mosquito bite, it enters circulation and goes through several larval stages before it becomes an adult. As an adult, the worm lives in the heart (and also lungs and liver) and causes slow, irreparable damage to the dog. The dog looks and acts great and can do so for several years. Eventually the dog may develop a cough and that is often the first clinical sign. By this time, the worm has done its damage.

A gray square with the numbers 5 2 and 3 2 written in it.
thanks to buckcreekvet.com

Heartworm prevention kills a larval stage of the worm. Think of it like a monthly dewormer that acts for a short time, then is cleared out. The reason we give it every 30 days is to catch those larval stages that are susceptible to the medicine before they grow up past the point of having that medicine work on them. Once they are adults, the prevention will not kill them.
Heartworm testing is done annually because the sooner we discover a case, we can treat and prevent the long-term damage undetected worms do to our dogs. One down-side to the test is that it does not detect those larva, so there must be actual adult heartworms for the test to be positive. For this reason, if a dog has not been on prevention we test them initially, start prevention, then test them again in 6 months. After 6 months, any larva stages that were in the dog prior to the start of prevention will have grown up into adults that we can detect via the test.
For the naysayers – heartworm disease is a real problem, really does occur, and really can be prevented for pennies a day. Vets are not trying to sell you product your pet does not need. For prevalence maps see the Companion Animal Parasite Council website.
For the online shoppers – the companies that manufacture prevention will only guarantee product purchased through a veterinarian. If a pet has been proven heartworm negative via appropriate testing, you have purchased prevention from your vet, and you have given it as directed, then IF (heaven forbid) your pet gets heartworm disease the company that manufactured the product will reimburse you for the treatment.
Heartworms can be treated, yes. I have actually heard poor logic stating that if a pet tests positive for heartworm disease then treating it when/if that happens is less costly than prevention. Wow. Remember that stuff about the damage done to your pet in the meantime? I don’t recommend this tactic. Prevention costs less than 50 cents per day. It should be considered part of the cost of owning a dog.
There is, of course, much more to the story. There are many types of heartworm prevention out there that work in different ways and prevent a variety of parasites, not just heartworms. Talk to your vet about your choices. For more great information and a wonderful FAQ please see the American Heartworm Society Website.

Dr. Amy Hellard
WCVC (West Chester Veterinary Care)