heartworm dog image 250pxBeing a Midwest native, one very common thing I hear in exam rooms is, “Why should I give heartworm preventatives in the winter? There are no mosquitoes!”. According to the American Heartworm Society, individual vet clinics in Madison diagnosed an average of 6-25 cases of heartworm disease in 2016 alone. But why does this matter? Heartworm disease is a very serious and potentially fatal condition where blood borne worms (Dirofilaria immitis) live in the heart, lungs, and blood vessels of pets. These parasites can be up to a foot long, and one dog can be infected with up to 300 worms at one time. As you can imagine, this can lead to clogging of the heart and interfering with heart valves.

However, when we talk about preventatives, it is not the adult worms we are aiming at. Adult female worms lay larvae known as microfilaria. These baby heartworms are spread between dogs by different species of mosquitoes, and it is immature heartworms that we are targeting with our preventatives. Prevention is effective against larvae in the first few months of development. Since it is impossible to know when the first mosquito of the season is going to appear and when the last one will die off, you may miss the window to kill off the larvae as they are maturing if you stop your preventatives during the cold months. I mean, this is Wisconsin. No one can predict the weather! So just because there are no mosquitoes, it does not mean there are no maturing larvae circulating through your dog’s blood stream ready to settle in with their new host.

dog MudgeHeartworm disease has been reported in all 50 states. Rescue organizations from all across the United States are constantly relocating dogs throughout the country. This allows for the movement of heartworm disease inside infected dogs. Most dogs are tested for heartworm disease prior to transfer out of state, but false negative results are often seen due to timing of exposure relative to testing. This has led to the spread of heartworm disease to areas that previously were disease free. Treatment of adult heartworms is a very long, painful, and expensive process that can also be deadly. Although I know this sounds dramatic, I personally have lost a pet to heartworm. Mudge came to me from Alabama and was already heartworm positive when I adopted him. He made it about a month into his treatment. Unfortunately, his body was no longer able to compensate for the disease and he passed away. With a disease that is so easily prevented, yet can be so serious if contracted, year around preventatives should be made a vital part of your pet’s health and wellbeing.


Dr. Mackenzie Kelling, DVM, enjoys spending time with family, friends, and her dog. Check out Dr. Kelling’s full bio here!