Protecting Your Pet From Heartworms

According to the American Heartworm Society, “More than a million pets in the U.S. have heartworms. But heartworm disease is preventable.”

While one month each year—April—has been designated National Heartworm Awareness Month, keeping your pet safe from heartworms—although not difficult—is a year-round endeavor. 

About Heartworms

Heartworm disease is just what it sounds like it is, caused by a parasitic worm that lives in the heart, lungs and associated blood vessels of an infected animal. Mosquitoes spread the worms’ larvae after biting an infected animal. 

Heartworm disease affects dogs, cats and ferrets as well as wolves, coyotes, foxes, sea lions and other wildlife. 

Heartworm disease is not contagious, in that it can’t be directly transmitted by an infected host animal to others. It takes a mosquito bite to spread the disease. 

Once transmitted to a dog’s bloodstream, the larvae take 6 to 7 months to mature into adult heartworms. The adults mate and the females release their offspring into the host pet’s bloodstream. 

A heartworm’s lifespan inside a dog is 5 to 7 years. Adult male heartworms measure 4 to 6 inches long and adult females are 10 to 12 inches long. The mature worms look like strands of cooked spaghetti. 

The number of worms living inside an infected dog, on average, is 15 worms, but the number can range from 1 to 250. 

A heartworm infection can be well underway before a dog shows any symptoms. As the number of worms increases, so does the damage to internal organs. Symptoms may include a worsening and increasingly frequent cough, growing fatigue after activity, trouble breathing and signs of heart failure.

Cats Can Get Heartworm Disease, Too

Cats can get heartworm disease, too, but they are not as susceptible to infections as dogs. Heartworms in cats take longer to mature, have shorter lifespans and do not grow as long as heartworms in dogs. 

An infected cat might have only one or two adult heartworms, but because of their relatively small body size, cats with only one or two worms are considered heavily infected. Immature heartworms can cause a serious condition—heartworm associated respiratory disease (HARD)—in cats.

Heartworms are harder to detect in cats, compared to dogs. Blood tests can be inconclusive, and we may need x-rays and ultrasound images of the heart to determine if a cat has heartworm disease. 

Some infected cats spontaneously rid themselves of heartworms. Other infected cats die suddenly of heartworm disease without showing any signs of illness. The death of only one heartworm can trigger an inflammatory response severe enough to prove fatal to a cat. 

Symptoms of heartworm disease in cats include trouble breathing, increased respiratory rate, cough, vomiting, decreased activity and appetite and weight loss. 


In the United States, heartworm disease is most common along the Atlantic and Gulf coasts, but heartworms have been reported in all 50 states including Indiana. The Companion Animal Parasite Council sets the statewide prevalence rate among tested dogs in Indiana in 2023 at 0.66%. 

In Hendricks County, one of every 300 dogs tested for heartworms in 2023 tested positive, for an overall rate of 0.34%. County-wide, there were 49 reported cases based on 14,481 tests.


We test your pet’s blood for heartworms by detecting heartworm proteins, called antigens, released by adult females. The earliest we can detect these proteins is about 5 months after the bite by the infected mosquito. 

We use another test to detect microfilariae—an early stage in the heartworm lifecycle—produced by mating males and females and released into your pet’s bloodstream. If we find microfilariae, it tells us there are adult heartworms present. The earliest we can detect microfilariae is about 6 months after the infected mosquito bite. 

When and how often we recommend testing your pet for heartworms depend on several factors:

  • Your pet’s age when started on heartworm preventive 
  • If you skipped or forgot to give one or more heartworm prevention doses
  • If you switched from one type of heartworm preventive to another
  • If the pet traveled to an area where heartworm disease is prevalent

Dogs 7 months old and older should be tested for heartworms before starting a heartworm preventive and again 6 months later. Once using preventives regularly, dogs are usually tested annually. 

Testing is essential before we prescribe a preventive. Giving heartworm preventive to a pet already infected with heartworms can be deadly. 

Treating Heartworm Disease

Treatment for heartworm disease is expensive and time-consuming, requiring multiple visits to the clinic for repeated blood tests, x-rays and injections. The treatment process is hard on the pet as well as the owner. Serious, potentially deadly complications can develop. 

To treat heartworm disease, we use an injectable drug containing arsenic—melarsomine dihydrochloride—approved by the Food and Drug Administration to kill adult heartworms in dogs. Most adult worms die quickly and can be eliminated within 1 to 3 months. We prescribe cage rest and restricted exercise during this phase of the treatment to help minimize complications. 

Because a single course of treatment may not completely clear all heartworm infections, additional testing and injections of melarsomine may be needed. Your veterinarian may first administer heartworm preventives for 2 months to eliminate microfilariae in the dog’s bloodstream before treating with melarsomine. 

While we have medicines available to treat heartworm symptoms in cats, there is no FDA-approved drug to treat heartworm disease in cats.

Protecting Your Pet

Given how devastating and deadly heartworm disease can be for dogs and cats and how difficult and expensive it is to treat, we are fortunate to be able to offer easy-to-use, almost 100% effective preventive treatments by prescription. 

With very few exceptions, we strongly recommend heartworm preventives year-round for all dogs and cats in our care—including those who spend most or all of their time indoors. 

Heartworm preventives work by killing microfilariae and larvae in your pet’s blood. In as few as 51 days, heartworm larvae can molt into immature adults that, like adult heartworms, are not eliminated by preventives. That’s why it’s so important to keep to a strict schedule when administering preventives—to kill larvae before they have a chance to mature to adulthood. 

Preventives we prescribe can be given monthly, either as an oral tablet or applied to the skin as a topical liquid. Injectable products are available lasting 6 months to a full year. The dosage is based on your pet’s body weight. 

Many preventives have additional ingredients to control intestinal worms, such as roundworms and hookworms, as well as other parasites including fleas, ticks and ear mites. 

Our veterinarians will discuss the options for heartworm prevention and recommend the best choice for your pet. 

Recommended Resources

The American Heartworm Society has a Heartworm Resource Center offering authoritative information on heartworm disease in multiple languages. We suggest selecting and applying your preferred language in the search fields before your browse the Resource Center page.

See also our blog post, “Heartworm Prevention Is A Year-Round Commitment.”