Last reviewed on November 10, 2015


It’s snowing. We typically get our first snow of the year before Halloween in my neck of the woods, and this year’s is a doozy. The meteorologists are calling for 6-12 inches of the wet, heavy stuff before we’re done later today, and because many of our trees were still fully-leafed, we are seeing a lot of downed limbs. Our yard looks bad enough that my husband bought a chain saw on his way to work, fearing that they’d all be sold out by this evening.


All right, I’m done complaining. I actually love snow and all the good stuff that comes with winter — skiing, sledding, hot chocolate, taking a break from giving my dogs and cats their heartworm preventatives … NOT! (Just seeing if you were paying attention.)


I have to admit it does feel a bit ridiculous to be worrying about mosquitoes (the vector that transmits heartworm disease) with a half foot of snow on the ground, but you really can’t be too vigilant when it comes to this terrible disease. As I look at next week’s forecast, I see highs nearing 60 (you just gotta love Colorado weather), so those little buggers are probably going to be back before we know it.


I generally recommend that owners give heartworm prevention 12 months a year, and here’s why:

In many parts of the country, mosquitoes remain active year-round. Unless you live in an extremely cold and/or dry region, your dog or cat may be bitten during any month of the year. Travel can also complicate the situation. When I lived in western Wyoming, many of my clients did not give heartworm prevention during the winter, which was defensible in that extreme client. But, a lot of folks also liked to escape the frigid winter temperatures or the slop of spring and would forget to protect their pets when they traveled.


Keep in mind also that most preventatives do more than protect against heartworms. Some prevent flea and tick infestations, mange, lice, or intestinal worms, and stopping will leave your pet open to these problems during the winter months. This is especially important if your pet has contact with other animals in boarding facilities, doggy day care centers, at the groomers, etc.


Many owners misunderstand how heartworm preventatives work. They don’t actually prevent infections, they kill the parasites that your pet picked up during the previous month. Therefore, if you fail to give the last dose at the appropriate time, you are leaving your pet open to infection. This situation is complicated by the fact that monthly flea and tick preventatives work in the opposite way. They repel or quickly kill the parasites as they jump aboard your dog or cat and work for a month or more into the future. So, figuring out the timing of when to start and when to stop each product or combo formulation can get tricky.


Finally, no medication works 100 percent of the time. If your dog comes down with heartworm disease and you can prove that you bought enough preventative to protect him throughout the year, and that you followed appropriate testing guidelines, many manufacturers will pay for your dog’s treatment.


Heartworm prevention is not expensive. It typically costs between $5 and $15 per month, depending on which product you pick and the size of your pet. Is it really worth the risk to save a few dollars?



Dr. Jennifer Coates



Image: Adolph by Kim Carpenter / via Flickr