Heartworm infection is a serious disease that can be fatal if left untreated. It is caused by worms that normally live in animals' heart, lungs, and blood vessels but can migrate to other tissues and organs, including the brain, kidneys, and eyes. These worms grow up to a foot long and can cause organ damage, heart failure, and lung disease. Cats and dogs can both get heartworm, though the disease looks different in the two species. 

What is heartworm disease? 

When a mosquito bites a dog with heartworm disease, the mosquito becomes infected with microfilariae, which is a heartworm in an early stage of development. In the mosquito, the microfilariae develop into larvae, and when the mosquito bites another animal, the larvae spread to their new host.  

The worm is a parasite, and it spreads through mosquito bites. In addition to cats and dogs, ferrets are also susceptible to heartworm. Wild animals like coyotes, raccoons, foxes, and wolves can also get heartworm and are considered disease carriers. 

Because mosquitoes can enter the home, even indoor-only cats are considered at risk.

Heartworm disease in dogs

Every year, more than 100,000 dogs in the United States test positive for heartworm.  

Dogs are the parasite's natural host, which means that heartworms mature and reproduce within the host. The life expectancy of a heartworm inside a dog is 5 to 7 years, and an average of 15 worms can be found in an infected dog.  

Heartworms are a serious problem, but infected dogs may not show symptoms at first, so it's important to test for heartworms annually and administer monthly preventatives. During heartworm testing, your veterinarian will perform a blood test to detect antigens produced by adult female heartworms and, if present, may also look for microfilariae in your dog's bloodstream.  

In dogs, heartworm disease has four stages:  

  • Stage 1: Symptoms are mild (like an occasional cough) or nonexistent.  
  • Stage 2: Symptoms are mild to moderate. For example, the dog may get tired more easily after moderate activity. 
  • Stage 3: Symptoms become more severe, like tiredness after mild activity and a cough that won't go away. The dog has trouble breathing and has signs of heart failure. 
  • Stage 4: There are so many worms in the dog's heart that blood flow is blocked; this is also called caval syndrome. Caval syndrome can only be treated with a risky surgery to remove the worms. Even with surgery, most dogs with caval syndrome die.  

Treatment for heartworm in stages 1, 2 and 3 requires several months of oral steroid administration and an antiparasitic injected into the back muscles. A topical drug that kills microfilariae in the blood and oral medications that kill the microfilariae are also available. 

"Treating heartworm infection is very serious for veterinarians, pet owners, and dogs. A dog's activity must be restricted for several months, they must take daily steroids for several weeks, and the side effects from the parasiticides can vary from mild to severe," said Rebecca Persons, clinical instructor at the Virginia-Maryland College of Veterinary Medicine's Small Animal Community Practice

Treatment often results in toxic side effects, so prevention is the best option to make sure your dog stays healthy!

Heartworm disease in cats

Cats also get heartworm disease, but they experience it differently than dogs. Cats are not the heartworms' ideal host, so heartworms inside cats live from 2 to 4 years, don't grow as long, and more heartworms in cats die before reaching adulthood than in dogs.  

However, even one heartworm is a serious problem because cats have smaller bodies.  

Cats with heartworms show nonspecific symptoms such as: 

  • Weight loss
  • Vomiting
  • Decreased appetite

Cats who show heartworm infection symptoms will become symptomatic when the heartworms, who have spent 3 to 4 months growing from larvae into immature heartworms, first enter the heart. During this time, infected cats will have respiratory symptoms like trouble breathing and a cough.  

"Heartworm infection can be an acutely fatal disease for cats, and many affected cats will develop a condition known as heartworm-associated respiratory disease. When immature heartworms die in the pulmonary vessels, cats may experience a life-threatening inflammatory response that causes lung damage and even sudden death," said Persons. 

Heartworms are harder to detect in cats through blood testing, so your veterinarian may need X-rays or a heart ultrasound to determine if your cat is infected.  

While symptoms can be managed with medication, there is no FDA-approved treatment for heartworms in cats, and surgery is risky. Just like dogs, prevention is the best treatment for cats.  

The importance of heartworm prevention

Pets in all 50 states have tested positive for heartworm, though it's more common in the southeastern United States. Heartworm is becoming more prevalent in many places, spreading north and west, so veterinarians recommend year-round heartworm preventative treatment to ensure your pet stays safe. 

"We often tell pet owners that prevention of heartworm disease costs substantially less than treatment of heartworm disease," Persons said. 

According to the American Heartworm Society, pet dogs should be tested for heartworm every 12 months, and all pets should be given heartworm preventative 12 months out of the year. Keeping your pet on a monthly heartworm preventative medication can greatly reduce the risk of infection. Monthly heartworm preventative is available in the form of an oral tablet, a liquid applied to the skin, or an injection administered once or twice per year.  

It is important to note that heartworm preventatives do not kill adult worms, so if your pet is already infected, a different treatment plan designed by your veterinarian will be necessary. 

It's essential to keep your pet protected against heartworm disease by testing and administering preventative medication. By staying informed about heartworm disease and protecting your pet, you can help keep them healthy and happy for years to come.

Written by Sarah Boudreau M.F.A. '21, a writer with the Virginia-Maryland College of Veterinary Medicine


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