Heartworm Prevention for Dogs

Medications to Prevent Heartworm Disease

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Heartworm disease is a life-threatening parasitic infection that affects the hearts and lungs of dogs. Because heartworms are spread solely by the mosquito, any dog exposed to mosquitos is at risk. Heartworm disease is a relatively common disease in dogs everywhere in the United States. Because of this, the American Heartworm Society recommends year-round treatment with a heartworm preventative for all dogs in the US regardless of climate.

It’s important to note that dogs living in areas that are warmer and more humid are at the highest risk.

Though termed "prevention," heartworm preventatives are actually insecticides that kill microscopic heartworm larvae present in the bloodstream. These medications are given on a monthly basis to remove heartworm larvae before they can develop into adult. Because these medications destroy early heartworm infections, it is essential to administer them on a regular basis. If you occasionally miss a dose, it is recommended to give the missed dose immediately and continue to give the medication monthly.

All dogs should have a heartworm test done by a veterinarian once per year. If you miss two or more months of heartworm prevention, contact your vet. Your dog may need to be tested for heartworms sooner (usually about six months after that point). It is important not to give heartworm prevention to a heartworm positive dog unless specifically instructed to do so by a veterinarian. In some situations, giving heartworm prevention to a dog that is already heartworm positive may be harmful and your veterinarian may recommend their involvement in that so if you suspect your dog is heartworm positive, talk to your vet about the next steps recommended.

Giving your dog heartworm prevention is an essential part of being a responsible dog owner. In addition, it is safer and less expensive to prevent heartworms compared to putting your dog through heartworm treatmentHeartworm treatment can cost over $1000 and is risky to the dog. While heartworm prevention costs about $35-$250 per year depending on the size of the dog and the type of medication used.

Talk to your veterinarian about the ideal heartworm prevention for your dog. There are several different medications that can be used on a regular basis to prevent heartworm infestation.


Some Brand Names: Heartgard Plus (by Merial) Iverhart Plus and Iverhart Max (by Virbac)
Tri-Heart Plus (by Merck)

Ivermectin-based heartworm medications are given orally once per month. Ivermectin is a broad-spectrum antiparasitic drug used in humans and animals. In monthly heartworm prevention, the dose of ivermectin is extremely low, making it effective at killing heartworm larvae but not other common parasites. In the above brands of heartworm prevention, the "plus" refers to the addition of pyrantel pamoate, which helps control the common intestinal parasites hookworms and roundworms. The "max" indicates the addition of pyrantel pamoate as well as praziquantel, which kills tapeworms.

Side effects of ivermectin are rare. If side effects occur, they are usually neurological in nature (central nervous system depression, wobbly gait). Some dog breeds, such as the Collie and Shetland Sheepdog, are known to be sensitive to ivermectin. However, at such a low dose, ivermectin heartworm prevention medications rarely cause side effects or reactions, even in sensitive breeds.

Milbemycin Oxime

Some Brand Names: Trifexis (by Elanco)​
(by Virbac)

Milbemycin-based heartworm medications are also given orally once per month. Like ivermectin, milbemycin oxime is a broad-spectrum antiparasitic drug. It is used only in animals, not in humans. Milbemycin also helps control roundworms, hookworms, and whipworms. Both Trifexis and Sentinel include an additional drug to prevent flea infestation. The flea prevention offered by Trifexis kills fleas. The one in Sentinel keeps fleas from reproducing but does not actually kill fleas.

There are generally few side effects of milbemycin if given as directed though side effects with any medication are possible. Symptoms of milbemycin overdose tend to be neurological in nature (stupor, tremors, wobbly gait). There is no specific breed sensitivity noted at this time.


One Brand Name: Revolution (by Pfizer)

Selamectin is applied topically once per month. It is an anti-parasitic drug used in animals only that kills fleas, prevents heartworm infections and treats and controls hookworms, roundworms, and ear mites. The product comes in a small tube that is applied to the skin (not the hair) on the back, between the shoulder blades. Proper application is essential to ensure effectiveness.

Though rare, the side effects of selamectin may include loss of appetite, excessive drooling, vomiting, diarrhea, muscle tremors, fever, and rapid breathing. At the site of application, there may temporarily be stiff or clumped hair, hair discoloration, hair loss, skin irritation, or a powdery residue.


Brand Names: Advantage Multi (by Bayer) Proheart (by Zoetis)

Moxidectin is an anti-parasitic drug used in animals only that prevents heartworm infections and helps control hookworms, roundworms, and whipworms. Moxidectin is administered one of two ways: Advantage Multi is applied topically; Proheart 6 and Proheart 12 are injectable drugs.

Advantage Multi comes in a small tube that is applied once a month to the skin (not the hair) on the back between the shoulder blades. It must be applied correctly to work properly. In addition to moxidectin, Advantage Multi contains imidacloprid, which kills fleas. Though rare, the side effects of Advantage Multi may include lethargy, itching, and hyperactivity. At the site of application, there may temporarily be stiff or clumped hair, hair discoloration, hair loss, skin irritation, or a powdery residue.

Proheart is given as a subcutaneous (under-the-skin) injection once every six to 12 months depending on the variety used. The moxidectin is in a sustained release formula that allows it to last for the labeled duration. This form of heartworm prevention is ideal for many owners because it eliminates the need to remember monthly treatments. Potential side effects, though uncommon, include mild to severe allergic reactions, lethargy, seizures, vomiting, diarrhea, weight loss, and fever.

Choosing the right heartworm prevention for your dog can be a tough decision. It is important to talk to your vet about the right options for you and your dog. No matter what you decide, make sure you keep your dog on heartworm prevention all year long. It can make the difference between life and death for your dog.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  2. Morchón, Rodrigo et al. “Heartworm Disease (Dirofilaria immitis) and Their Vectors in Europe - New Distribution Trends.” Frontiers in physiology vol. 3 196. 12 Jun. 2012, doi:10.3389/fphys.2012.00196

  3. Merola, Valentina M, and Paul A Eubig. “Toxicology of avermectins and milbemycins (macrocylic lactones) and the role of P-glycoprotein in dogs and cats.” The Veterinary clinics of North America. Small animal practice vol. 42,2 (2012): 313-33, vii. doi:10.1016/j.cvsm.2011.12.005

  4. NIH. Revolution(selamectin). Updated August, 2014.

  5. NIH. Advantage Multi. Updated December 2, 2020.

  6. NIH. Proheart. Updated October 26, 2020.