Does Your Dog or Cat Need Heartworm Medication?

Heartworms are parasitic roundworms that can infect both cats and dogs. The worms are life-threatening and found in all 50 states. The latest data available showed that over 1 million dogs were infected with heartworm in 2019.  

Fortunately, protecting your cat or dog from heartworm is relatively simple. Giving your pet a preventative heartworm medication on schedule can help them avoid this serious and common health condition. 

Is heartworm medication necessary for all dogs and cats?

Most dogs and even cats will benefit from heartworm medication. 

Dr. Amy Attas, DVM, the founder of City Pets, a Manhattan home veterinary practice, explains that heartworm is transmitted by mosquitoes. So all cats and dogs that go outside or could be exposed to mosquitoes should take heartworm prevention medication — ideally year-round. 

You might assume that your indoor-only cat is safe from being bitten. But a single mosquito that gets into your home could still infect your cat. 

“It takes just that one carrier that transmits the heartworm,” says Dr. Attas. In fact, studies found that 25% of all cats infected with heartworm were indoor-only cats.

You might be tempted to take your pet off of heartworm medication during the winter, when mosquitoes are rare. But the American Heartworm Society recommends that dogs should be on heartworm medication year-round.

There are many factors to consider when deciding what’s right for your pet. Your vet can provide advice specific to your location and your pet’s risk factors. 

How does medication prevent heartworms in pets?

Heartworm medication works by killing the heartworms when they are in the larvae stage. The medication kills heartworm larvae already in a pet’s blood as well as any larvae that begin to form in the blood. 

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As larvae mature, they enter a juvenile adult stage, and medication can’t eliminate the heartworms once they reach that stage. Because of this, it’s important to give your pet heartworm medication according to the product’s directions.

Giving prevention medication late means that it won’t be able to kill off the adult worms. Those worms may grow and infest the pet.

What can happen if you don’t give your dog or cat heartworm medication?

Mosquitoes can inject heartworm larvae into a pet through a single bite. The heartworms mature and reproduce in the heart and lung blood vessels. They can damage the blood vessels that carry blood from the heart to the lungs. 

That damage can limit blood flow and decrease the amount of oxygen the blood carries. Less oxygen in the blood can lead to life-threatening symptoms.

A pet with heartworm may experience several symptoms: 

• Difficulty breathing

• Weight loss

• Lethargy

• Collapse

How often should you administer heartworm medication?

Heartworm prevention medications for dogs are all given monthly. The medications may consist of a topical liquid that you apply to your dog’s skin, or a chewable or non-chewable oral tablet. 

If you’re looking for a longer-term solution, there is an injection available that your veterinarian administers to your dog every 6 or 12 months. 

For cats, topical and oral products are available. These products must all be administered monthly, and they all require a veterinarian’s prescription. 

As an added bonus, some of these products also help to protect your pet against other parasites like: 

• Roundworms

• Hookworms

• Fleas

• Ticks

• Ear mites 

Your vet can help to determine the best medication choice for your pet, their breed, and their medical history.

How much does heartworm medication cost?

The cost of heartworm medication will vary depending on the product you choose and your pet’s weight. Heartgard, a chewable heartworm medication for dogs, costs between $5 and $10 per pill. Revolution, a topical heartworm preventative for cats and dogs, costs $15 to $20 per vial. These medications are designed to be given monthly.

Don’t forget to factor in the cost of a vet visit, since heartworm medications require a prescription. Your vet may need to test your pet for heartworm to confirm that there isn’t an active infection. Blood heartworm tests can cost $40 or more.

Keep in mind that these costs are far less than the expenses you could face if your pet needs to be treated for heartworm. The American Heartworm Society estimates the cost of treating heartworm in a 40-pound dog to be $1,200 to $1,800. It is a complex, long-term process.

In contrast, paying for 12 months of heartworm prevention for a dog of the same size would cost just $70 to $200.

Are there any dangers or side effects of heartworm medication?

Heartworm prevention side effects range from mild to severe. Pets may experience: 

• Loss of appetite

• Vomiting

• Salivation

• Diarrhea

• Agitation

• Tremors

• Disorientation

• Shock

• Hives

• Seizures

• Death (rare)

Your pet’s risk of side effects may depend on the brand or type of medication. Dr. Attas explains this is because heartworm medications aren’t all made with the same chemicals. Ivermectin is one medication that is extremely effective in preventing heartworm. But some dog breeds, like Australian shepherds, can be particularly sensitive to it.

A vet can help to monitor side effects and adjust your pet’s heartworm medication, if necessary. “All heartworm medications are prescription medications, and this is one of the reasons why,” says Dr. Attas. 

Can you naturally prevent heartworms without medication?

Dr. Attas notes that there aren’t natural ways to prevent heartworm without medication. 

There are, however, natural ways to help reduce a pet’s exposure to mosquitoes. “We know that mosquitoes are a lot more active at dusk,” says Dr. Attas. “Avoid having your pet outside at dusk for fewer exposures.” 

The same is true of avoiding bodies of water, where it’s more likely that a mosquito may bite your pet. 

“Having some smarts about avoiding mosquitoes may help your pet to be less likely to be bitten,” says Dr. Attas. 

You can take additional steps to discourage mosquitoes from breeding in your backyard: 

• Empty, wash, and refill any outdoor water bowls every day to remove any mosquito eggs. 

• Empty items that hold standing water like bird baths and baby pools when they’re not in use.

• Fill in any holes in your yard where rainwater might pool. 

• Keep screens on all of your windows and doors to keep mosquitoes out of your home.

But you can’t completely remove your dog or cat’s potential exposure to mosquitos. So it’s still important to keep your pet on heartworm prevention. 

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Written by:

Paige Cerulli

Paige Cerulli is a freelance writer who specializes in pet and animal topics. She studied English and music performance at Westfield State University.

Edited by:

Renée Fabian, MA

Renée Fabian is the senior pet health editor at GoodRx. She’s worked for nearly 10 years as a journalist and editor across a wide range of health and well-being topics.

Reviewed by:

Ghanasyam Bey, DVM

Ghanasyam Bey, DVM, is from Cleveland, Ohio, and attended Princeton University for undergraduate studies. After a year of biology research at Duke University, he attended Ohio State College of Veterinary Medicine.

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