Heartworm Prevention in Dogs and Cats

By Katie McHenry

For responsible dog guardians, a monthly heartworm prevention pill (like Heartgard) is simply a way of life. Most dog guardians know that an untreated dog is vulnerable to heartworm, a mosquito-borne parasite that lives in the heart, lungs, and blood vessels of an infected animal. However, many cat guardians may not realize cats are also susceptible, albeit to a much lesser degree as their  physiology makes them undesirable hosts for heartworms.

According to VCA Hospitals, cats have only between 5 percent and 20 percent the rate of heartworm infection that dogs have, and while dogs can host up to hundreds of heartworms at once, cats usually have fewer than six (usually between one and three). However, while most heartworms in cats don’t survive to the adult stage, even immature heartworms can still cause a disease in cats known as HARD (heartworm associated respiratory  disease), and the worms that make it to the adult stage can live two to three years in cats (five to seven years in dogs).

While there are approved medications to treat heartworms in dogs, the same isn’t true for cats, although symptoms can be managed with medications. This means the only recommended treatment for heartworms in cats is prevention. When deciding whether to expose your cat to preventive treatments that can be expensive and have adverse side effects, you’ll have to ask yourself if the benefits outweigh the risks.

How Heartworm Is Transmitted

In an infected dog or cat, female heartworms release babies (called “microfilaria”) into the animal’s bloodstream, which are picked up by mosquitoes when they suck the animal’s blood. It then takes between 10 and 14 days for the microfilaria to become larvae, at which point they’re infectious. When a mosquito carrying heartworm larvae bites their next victim, the larvae enter the bloodstream through the new bite wound. It then takes about six months for the larvae to become adult heartworms. Female worms that reach the adult stage can range in size from six to 14 inches long and are about one-eighth inch wide; male worms are about half that size.

Because the parasite is transmitted via mosquitoes, heartworm disease is more prevalent in certain geographic regions and climates where mosquitoes remain active year-round. However, other factors – such as the country-wide relocation of tens of thousands of infected cats and dogs after Hurricane Katrina, or the migration of heartworm carriers such as coyotes – can also affect heartworm disease risk in other areas.

Symptoms and Treatment

In both cats and dogs, heartworm disease symptoms may include coughing, loss of appetite, weight loss, vomiting, difficulty breathing, fainting, lethargy, or even sudden death. The only way to diagnose heartworm is with a blood test administered at a vet’s office.

In heartworm-positive dogs, the earlier the disease is diagnosed, the greater the chances for survival. In cases of advanced heartworm disease, there is usually significant damage to the internal organs, which can sometimes mean a shortened lifespan. Treatment for dogs usually includes an antibiotic and an injection to kill the adult heartworms, followed by another drug to kill the remaining microfilaria a month later.

Because there is no approved treatment for heartworm-positive cats (the medication used for dogs can be dangerous when used on cats), the focus is usually on treating the symptoms, such as administering diuretics to help remove fluid from the lungs or prednisolone to reduce lung inflammation.

In cats and dogs alike, dying adult heartworms can get trapped in the lungs, leading to further respiratory complications.

Heartworm Prevention Options

For both cats and dogs, the options for heartworm prevention medication range from oral to topical, and from heartworm prevention only (such as Heartgard) to comprehensive heartworm/flea/tick/roundworm/hookworm prevention (such as Revolution). Keep in mind the number of parasites the medicine protects against will increase the cost of the treatment. Heartgard runs about $5 to $10 per pill per month, while Revolution, a topical medication, runs about $15 to $20 per vial per month.

Depending on the medication, side effects can range from mild to severe. These might include loss of appetite, vomiting, salivation, diarrhea, tremors, agitation, disorientation, shock, hives, seizures and, in rare instances, death.

Whether the medicine is oral or topical, it should be administered monthly for dogs year-round. For cats, there are additional considerations on whether to treat at all or only certain months of the year based on the much lower risk of a cat contracting heartworms, especially for indoor-only cats.

Is It Worth It?

Benjamin Franklin said, “An ounce of prevention is worth a pound of cure.” Because dogs are the ideal host for heartworms, and dogs usually spend some portion of their day outdoors, dog guardians should always opt for heartworm prevention. However, for cat guardians, the answer is, “Well, it depends.”

As with all decisions in life, it’s important to weigh the risks in terms of the rewards, and – as your cat’s guardian – only you can accurately answer that question. However, when determining the answer, factors to consider will include your geographic location, the likelihood of infection based on where you live, whether your cat is indoor/outdoor or indoor-only, whether your indoor-only cat has outdoor access (including a screened-in porch), and your budget.

To be clear, SPCA NOVA is a firm believer that the vast majority of cats should be indoor only, rather than indoor/outdoor. Most cats are happy indoors only, as long as they have the right indoor home environment with plenty of stimulation with humans, toys, and  other pets (if they like other cats or dogs). That being said, indoor/outdoor cats should absolutely be protected from heartworm and, since they would also need flea/tick prevention, you should consider a product that covers both (such as Revolution).

If you live in a hot, humid, mosquito infested area, and your indoor-only cat has limited access to the outdoors (such as a screened-in porch), it might make sense to invest in a heartworm prevention  medication since there is not yet an accepted treatment for heartworm-positive cats.

What SPCA NOVA Recommends

SPCA NOVA’s dogs are already on heartworm preventative medication. Depending on the timing of your dog’s adoption from us, we may give you the next month’s dose to administer for the sake of continuity. It is imperative that you keep your dog on a heartworm prevention medication and that you follow up with annual testing, visits, and vaccinations. When used properly, preventative heartworm medications for cats are considered safe – but read the instructions, consult your vet, and use it only when needed.

Because SPCA NOVA cats must be indoor-only, we have already eliminated the most important risk factor – being outdoors. However, if the adopter has a screened-in porch where the cat spends time during the hot, humid months when the risk for  exposure is higher, we recommend using a comprehensive product like Revolution because if a cat is exposed to mosquitoes, chances are they will be exposed to fleas as well.

Editor’s Note: This article has been updated for accuracy.