Do Dogs Need Heartworm Medicine All Year?

Dogs exposed to mosquitoes are at risk for heartworm.

Dogs exposed to mosquitoes are at risk for heartworm.

If you have a canine buddy, you’re familiar with heartworm prevention. You give your pal his monthly dose right on schedule to keep him safe. Still, mosquitoes carry this parasite, so it’s easy to think preventive treatment could be discontinued during the coldest months. The experts' answer: It's needed year-round.

Heartworm Lifecycle

Heartworm larvae are carried to your dog by mosquitoes that have picked them up while feeding on infected wild or domestic canines or other host animals. When a carrier mosquito feeds on your dog, it injects one or more heartworm larvae into the dog. These larvae may live in the dog's tissue for months before migrating to the bloodstream. If the larvae are not killed in time by a preventive medication, they eventually set up housekeeping in the dog’s lungs and heart, where they mature into adults. There, they are set to be permanent guests. An individual heartworm can live about seven years and grow to be as long as 14 inches. Dogs can acquire a mass of these parasites, which interfere with blood flow. The burden of heartworms can easily end up causing heart failure and killing the dog. Adult heartworms produce offspring, the microscopic larval form called microfilariae, which circulate in the infected dog's bloodstream. Larvae at this stage cannot develop into an adult heartworm without passing through a mosquito. A mosquito picks up one or more microfilariae during a meal. Inside the mosquito, the larva develops toward adulthood until it is ready to be injected into the next warm-blooded host the mosquito feeds on.

Heartworm Medicine

Heartworm preventive interrupts the heartworm’s lifecycle. The preventive kills the infective larvae before they have a chance to become adults. The preventive kills only those circulating larvae that are in a specific life stage at the time it is given, so it’s important to give your dog his heartworm medication on a regular schedule. Only an unbroken preventive schedule can reasonably ensure no heartworm larvae will mature into adults. Before giving your dog his first dose of heartworm medicine, your vet will check your dog for the presence of heartworms. He may perform a microscopic search of a blood sample for the microfilariae or he may use an antigen test. Microfilariae found in the blood are a sure indication adult heartworms are present and reproducing in the dog. The antigen test indicates the presence of heartworms by reacting to any heartworm cells in your dog's blood.


Never skipping the monthly dose of heartworm preventive is the best way to ensure that exposure to heartworm-carrying mosquitoes will not harm your dog. Because the parasite is only transmitted by mosquitoes, it may seem safe to skip the preventive treatment for a time in regions where winter weather gets cold enough to kill off most mosquitoes. Stopping the preventive, even in the winter months, puts your dog at risk of infestation, according to the American Heartworm Society. It's much safer, for your canine best friend and for your pocketbook, to give him his monthly dose on schedule, all year long.


If your dog contracts heartworm, he must undergo an expensive treatment regimen consisting of a series of injections of a poison to kill the heartworms. Treatment generally requires hospitalization. The treatment itself is life-threatening: The more advanced the disease, the higher the risk that the mass of dead and dying worms in the dog's heart and lungs will cause serious additional health problems. The dog must not be allowed exercise for several months after the treatment. If you take a winter break from your dog's medication, he must tested for the presence of heartworms before he can resume his monthly preventive.

Other Factors

Most types of heartworm preventive will also prevent infestation by some other important parasites, including roundworm and hookworm. In addition, some preventives are formulated to help prevent or control a variety of other pests, including whipworm and even fleas and ear mites. If you live in an area where heartworm is prevalent, your pet depends on the efficacy of his heartworm preventive. Discuss with your vet his experience with and opinion of the total reliability of the specific product you select for heartworm prevention. Whichever product he recommends, he’s also likely to suggest annual testing, because heartworm prevention is not always 100 percent effective.

About the Author

A recipient of a business and technology degree from the master's program at West Coast University, Cindy Quarters has been writing professionally since 1984. Past experience as a veterinary technician and plenty of time gardening round out her interests. Quarters has had work featured in Radiance Magazine and the AKC Gazette.

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