Everything You Need to Know About Heartworm

Heartworm disease is as gnarly as it sounds. Here’s how to prevent a horror story from coming true for your pet.

by Oneal Bogan, DVM
September 12, 2021
a pug is cradled by a person in scrubs with a stethoscope

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Have you seen that horror film about foot-long worms that invade a heart, reproduce at warp speed, and can ultimately kill their host if not stopped in their tracks? Probably not. Because that’s actually a true story of how heartworm disease progresses in pets. Still: Scary stuff! The good news (there is good news, thankfully) is that you can prevent heartworms from ever infecting your pet and, if administered properly, heartworm preventatives are 99.9% effective against the parasite. 

How do heartworms infect pets anyway?

Heartworm disease is spread by mosquitoes. (I’m about to get gross again — bear with me.) The life cycle begins with a mosquito biting a dog that already has heartworm disease. The parasite then replicates inside of the mosquito, and within 14 days, when that mosquito bites another dog or cat, the parasite is passed on to the animal. Once inside the blood system of a dog, the heartworm will not only mature, multiply, and make your pet very sick, but it can also be passed to other mosquitoes (and dogs). In cats, heartworm just causes them to get sick, but they are not contagious like dogs.

So, how do I know if my pet has heartworm disease?

As the heartworms mature in a dog, they begin to invade the chambers of the heart — cue horror-movie music. If left unchecked, dogs with heartworm disease that has been allowed to progress will have their whole heart and arteries full of worms. Yes, this is gory. Dogs with the disease will cough and have difficulty breathing, have low energy and little interest in exercise, lose their appetite and, thus, lose weight. They may also develop heart murmurs and swollen abdomens due to excess fluid.

In cats, heartworm can cause similar symptoms, but since they can mimic those of other diseases, they are often not caught early enough and progression of the disease can cause blindness, convulsions, and sudden death. For either species, this is a serious disease and can cause lasting damage to the cardiovascular system.

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What is the treatment for heartworm disease?

The good news? There is a protocol for treating heartworm. The bad? It’s not pretty. It involves oral steroids, antibiotics, and typically three rounds of intramuscular injections of an potent anti-parasitic medication called Ivermectin. (Where have you heard that before? Oh yes, some misinformed people think it will protect them from Covid.) While this treatment is usually effective at killing the worms, it is a grueling process for a pet. Plus, for the six months (or more) while they are going through treatment, they must refrain from normal dog activities and be kept very calm. As any pet parent can understand, this is difficult for most dogs and can affect their quality of life. Oh, and then there’s the cost: while this varies by region, most treatments can add up to $3,000 when all is said and done. 

How can I prevent heartworms in the first place?

The short answer: it’s really easy. Monthly heartworm medication such as Heartgard is safe, affordable, and effective at preventing heartworms from maturing and multiplying if your pet becomes infected. There are also several options available that combine heartworm and intestinal parasite medication. The monthly cost for heartworm prevention ranges from $10-25, depending on the size of your pet.

Before starting on preventatives, your dog (if over six months of age) will need a heartworm test, called a 4Dx Snap Test. (This in-house blood test also tests for other tick-borne diseases including Lyme, anaplasmosis, and ehrlichia.) Most heartworm prevention is in the form of a chewable tablet given as a treat every 30 days, which should only be purchased from a licensed veterinarian (there are some dangerous knockoffs online). There’s also an injection, ProHeart, that provides six months of prevention. 

How at risk is my dog?

You may be wondering if all of this is necessary, especially in regions of the country where mosquitos are scarce. Logically, the lower the mosquito population, the lower the probability of your dog getting heartworm, but most veterinarians agree that if there is any risk of a mosquito bite, then the cost of prevention is well worth the peace of mind. It is also important to remember that in areas where heartworm disease is low, there still may be animals traveling in and out that may be infected with heartworm, such as rescue dogs imported from regions where heartworm is common that could be the source of infection. That said, if you live in a cold climate, ask your veterinarian if they think it is safe to skip heartworm preventative in the winter months.

Got it. How at risk is my cat, though?

Indoor cats are very low risk, but if you have an outdoor cat, then they need the same protection as dogs. If infected, treatment is aimed at management of the clinical disease, but there is no cure-all as there is with dogs.

The final word on heartworm: weighing pros and cons is a key part of pet parenthood. When it comes to heartworm protection, prevention far outweighs the cost of treatment if your pet was to be infected.

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Oneal Bogan, DVM

Oneal Bogan, DVM, is a mixed animal veterinarian from Colorado. Dr. Bogan loves the variety of animals she gets to work with. She owns her own mobile practice which provides at-home care to large and small animals. Dr. Bogan also works at a local small animal clinic. In her free time, Dr. Bogan loves to hike, ride horses, and read. She also loves writing and hopes her advice helps all pets live a happy, healthy life.