Don't stop now! Heartworm preventatives are most effective when given year round

Dr. Julie Hunt

Most dog owners are aware of heartworm disease and have been instructed to give a heartworm preventative pill to their dog once monthly. You also probably know that heartworm is passed from dog to dog through the bite of a mosquito. So how do the preventatives work? And why do we recommend heartworm prevention year round, even here in New England, where mosquitoes do not live in the dead of winter?

Let’s review the heartworm parasite life cycle. Adult heartworms live in the arteries that go from the heart to the lungs (pulmonary arteries). Adult heartworms reach many inches in length and if the infection is heavy, they start to “back up” into the heart chambers. Symptoms occur when the worms cause inflammation within the lungs, causing your dog to cough. As the worms start plugging up the heart, heart failure develops. Once settled within the pulmonary arteries, the adult male and female heartworms mate and produce baby heartworms (called microfilaria) which circulate within the infected dog’s blood stream. When a mosquito bites an infected dog, it ingests the microfilaria contained within the blood meal. The microfilaria develops inside the mosquito until it becomes a larval stage capable of being transmitted back to a dog. When the mosquito bites another dog, it passes the larval stage of the heartworm into the skin. From there, the larval heartworm develops into a mature worm and migrates to its favorite place, the pulmonary arteries, where the life cycle begins all over again. It is important to understand that microfilaria cannot mature within the dog. The mosquito is the only place microfilaria can develop into a larval stage capable of infecting another dog. Therefore mosquitoes are essential for transmitting heartworm from dog to dog.

Current heartworm preventatives (Heartgard, Tri-Heart, Interceptor, Advantage Multi, Trifexis) kill the larval forms of heartworm before they mature into the adult worm. Let’s say a dog is bitten by an infected mosquito and the larval stages begin to mature. When the heartworm preventative is given, it kills the larvae before they have a chance to develop into adult heartworms which cause the damage and symptoms of heartworm disease. The preventative is actually working backwards to kill any developing larvae that your dog was exposed to over the course of the previous month.

So why give heartworm preventatives in the winter when mosquitoes are dead? As luck would have it, current medications used for heartworm prevention have added benefits. Given once monthly, they also prevent many common intestinal worms. These worms are passed through the feces from one infected dog to another. For example, Dog A passes feces which contain intestinal worm eggs onto the ground. These eggs develop into larval worms which are then ingested by Dog B when he inadvertently steps on the feces and licks his paws. Next the larval intestinal worms develop and mature within Dog B’s intestinal tract, causing vomiting, diarrhea, and weight loss. Fortunately, the same medicines used for heartworm prevention also prevent the larval stages of many common intestinal worms from maturing into adults. Also, new information suggests that there is a strain of heartworm that originated in the Southeast that is especially good at resisting heartworm medications. Instead of one monthly dose of heartworm preventative killing 100% of larvae, studies show that it takes up to 3 consecutive months of heartworm medication to completely eliminate this strain. This is another very important reason to continue preventative medication all year long.

We currently carry Heartgard and Tri-Heart, which are both oral chewable monthly medications that prevent heartworm, roundworm, and hookworm. We also carry Interceptor, another monthly chewable oral medication that prevents all of those parasites plus whipworms. Unfortunately, Interceptor is currently unavailable from the manufacturer. Advantage Multi differs in that it is a topically applied monthly heartworm preventative medicine. It prevents heartworm, roundworm, hookworm, and fleas! The latest addition to our arsenal of heartworm preventatives is Trifexis, a once monthly chewable that prevents all of the same parasites as Advantage Multi.

All of these easily-administered monthly medications are capable of preventing both heartworm disease (which causes coughing and heart failure) and intestinal parasites (which cause vomiting, diarrhea, and weight loss). That is why we strongly recommend all dogs in our practice take a monthly heartworm preventative year round.