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Heartworm Disease

mosquito freeHeartworm Disease--the Heart of the Matter

As I write this article, it is still cold outside and It may seem like a strange time of the year to think about heartworm disease and the mosquitoes that spread it, but given new factors playing out with heartworm disease, it is important for us to be thinking of this disease year-round.

What is heartworm disease?

Heartworm disease is a serious and sometimes fatal condition caused by parasitic worms living in the arteries of the lungs and sometimes the right side of the heart in dogs, cats, and other species of mammals. These parasites are “filarids”, which are one species of roundworm (classified as nematodes). The type of filarid that causes heartworm disease in dogs and cats is known as Dirofilaria immitis.

How is heartworm disease spread?

Mosquitoes spread heartworm disease and dogs and other mammals that host adult worms are the reservoir population. Mosquitoes become infected with “microfilaria” while taking a blood meal from an infected dog or other host. Within the mosquito the microfilaria mature into the infective larval stage. When the mosquito bites another cat, dog, or susceptible animal, larva are deposited on the skin and actively migrate into the new host through the break in the skin. For about 2 months, the larva can migrate through connective tissue under the skin, and then into the venous blood stream where they end up in the arteries of the lungs as a juvenile worm. Juvenile worms mature to adulthood around 6 months post infection and these adult worms begin to produce offspring, the microfilaria that can then infect another mosquito. The cycle can then repeat itself over and over.

How do I prevent heartworm disease in my dog or cat?

At Westgate Pet Clinic we have several options for preventing heartworm disease in dogs. Presently, the most common product we prescribe as a preventive is Heartgard Plus (ivermectin and pyrantel).   It is a chewable tablet and most dogs take these readily like a treat. In cats, we usually rely on a topical product called Revolution (selamectin) to prevent heartworm disease. All approved heartworm medications work by eliminating the larval stages of this parasite—the juvenile and adult worm are not susceptible. Because of the unique life cycle of the heartworm, prevention should be administered on-time once monthly. Administering medication late can allow larvae to molt to adulthood.


If it is cold outside and mosquitoes are not present, why should my dog or cat be on year-round prevention?

The American Heartworm Society is now recommending year-round heartworm prevention. According to their website, dogs have been diagnosed with heartworms in almost every county of Minnesota, and there can be differences in the duration of the mosquito season from the north of the state to the south of the state (and often we are caught by surprise by sudden changes in weather which help mosquito populations thrive). On top of prevalence worries and the inability for us to predict sudden changes in the weather, some mosquito species have the ability to adapt to cold climates and there are some species of mosquitoes that can even successfully overwinter indoors as well. Beyond heartworm disease, an additional bonus of using most year-round preventatives is protection in removing certain intestinal parasites, including hookworms, roundworms, whipworms, and occasionally tapeworms. Some products are even effective in treating or preventing external parasites such as fleas, ear mites, and the sarcoptic mange mite.

Can my indoor cat get heartworm disease?

Yes, both indoor and outdoor cats are susceptible to Dirofilaria immitis, as some species of mosquito are more likely to come inside to look for a blood meal. According to the American Heartworm Society, multiple scientific studies have found a significant number of heartworm infections in cats living exclusively indoors.  Although cats appear to be more resistant hosts to heartworm disease than dogs (adult worms do not live as long, these adult worms less commonly produce microfilaria, and many cats may be able to clear infections on their own), cats can be easily exposed to the disease and some will become very ill.   Unfortunately, there is no approved effective treatment for cats that have heartworm disease. Prevention is truly at the heart of the matter for our feline companions.


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