Never Stop Fighting – Why You Should Give Heartworm Preventatives All Year Long.


Author: Karen L. McGlothlin, DVM, PhD


One evening last week after I had mowed my yard (in February), I was sitting out on my back porch taking in a beautiful sunset. It was such a peaceful way to end the day, until I heard a familiar high-pitched buzzing by my left ear and realized that I was being stalked by a mosquito. Again, in February.

As a veterinarian and the gracious caretaker of three dogs and a cat who were sitting on the porch with me, the first thing that popped into my mind was the thought that this little mosquito that was currently harassing me could easily be carrying infective stages of the heartworm. I didn’t dwell on that thought too long that evening because on the 10th of every month, I give my three dogs their monthly doses of Sentinel and I apply a topical dose of Revolution Plus on my cat. Every single month, all year long.


I must confess, however, that I did pause to think about all the dogs and cats in the region that may not get their heartworm preventatives all year long. I don’t hear it as often as I used to, but I do still have people tell me that they discontinue their pets’ heartworm preventatives during the winter because the cold weather provides protection from exposure to the vectors of heartworm (and other diseases, since heartworm preventatives also provide protection against other parasites). The reality is that the climate does not provide protection. Unfortunately, mosquitoes, fleas, and ticks don’t simply disappear during the winter months and our current pattern of milder temperatures combined with abundant rainfall increases the likelihood that our pets will encounter mosquitos, fleas, ticks, and other pests during this time of year.


Heartworms are transmitted via a bite from an infected female mosquito. A mosquito becomes infected when she takes a blood meal from an infected animal and ingests microscopic larval heartworms, called microfilariae, that are present in the blood. Once in the mosquito, microfilariae undergo further development for a period of about two weeks in the gut and then migrate to the mosquito’s mouth parts. As the mosquito feeds on a susceptible host such as a dog or cat, the infective larvae are deposited onto the skin and enter into the bite wound, where they undergo further development and migration through the host’s body. Eventually, heartworm larvae reach the blood vessels and are carried to the lungs and heart, where they mature into reproducing adults that begin breeding and releasing larvae into the blood stream, which can then be ingested by the next mosquito that comes along and bites. Heartworm disease is not directly transmissible from one pet to another, the larval heartworm has to go through the short period of development in the mosquito before it is infective to pets.

Photo: American Heartworm Society

The time that it takes a heartworm to reach maturity from the larval stage that is injected into the host to reproducing adult is about 6-9 months. This means that if a pet is bitten by an infected mosquito in February and doesn’t receive its monthly preventative for several months afterward, heartworms could be well on their way to maturity. Mature heartworms can live 5-7 years in the dog.  If a dog develops a heartworm infection, there are treatment options. The gold standard is a series of injections of melarsomine, an arsenical compound that kills adult heartworms, deep into the muscles of the lower back. Adulticide therapy can cost anywhere from $850-1200, depending on size of the dog. If a cat develops a heartworm infection, there is no approved treatment.

At this time, cases of heartworm disease have been reported from all 50 states and all it takes is a single bite from an infected mosquito to infect an animal. There are a variety of different heartworm preventatives available and they come in oral, topical, and injectable formulations. The costs of heartworm preventatives vary, but will typically run between $10-25 per month, depending on the size of your pet. Considering the alternative, monthly heartworm preventatives are a bargain.


Brooks, Wendy. 2019. Heartworm Disease in Dogs.

Brooks, Wendy. 2018. Heartworm Infections in Cats.

Brooks, Wendy. 2019. Heartworm Preventative Comparison Chart for Dogs and Cats.

Companion Animal Parasite Council Forecasted Prevalence of Heartworm, 2019.

Companion Animal Parasite Council. Heartworm.