Five Things to Know
Poodle pet dog with beef chewables for heartworm protection and treatment on white background
Poodle pet dog with beef chewables for heartworm protection. Photo: iStock/ThamKC

Every summer we either read, see, or hear stories about the transmission of disease by mosquitoes to humans. These insects also pose a serious health risk to our beloved pets. Here are five things to know about heartworm. 

What is heartworm? 

Heartworms, Dirofilaria immitis, can be found in the blood vessels and lungs of infected dogs and cats. When a mosquito sucks blood from an infected animal and bites its next victim, the larvae are transmitted through the skin and make their way to the chambers of the heart or lungs. They can grow up to 12 inches in length and wreak havoc on your pet’s heart and lungs by laying very tiny larvae called microfilariae, which travel throughout the bloodstream.


Symptoms in dogs most commonly include breathing difficulties, coughing, reduced appetite, weight loss and lethargy. Symptoms in cats are similar and may include gagging and vomiting. Signs associated with the first stage of heartworm disease can often be mistaken for feline asthma or allergic bronchitis. Cats can show signs of breathing problems with only one adult heartworm in their lungs. However, if you’ve been diligent with heartworm preventives, you will have little to worry about.


Most cases of heartworm can be effectively treated in dogs. It is a complicated and expensive process requiring a series of treatments over a couple months. Adult heartworms in dogs are killed using an adulticide drug that is injected into the muscle. Treatment may be administered on an outpatient basis, but hospitalization is often recommended. During the recovery period, you will need to limit your dog’s physical exercise to leash walking. This will reduce the chance that blood flow through the lungs would become blocked by dead worms. Preventive medications are also administered to avoid heartworm reinfection and to eliminate any larvae that may be present.

Currently, there are no products in the United States approved for the treatment of heartworm infection in cats, so disease prevention is critical. A cat’s immune system can often eliminate the infection on its own. Unfortunately, complications may develop as the dead worms are being cleared by the body, which can result in a life-threatening shock reaction. Veterinarians will often attempt to treat an infected cat with supportive therapy measures to limit this reaction.


Prevention includes reducing exposure to mosquitoes by spraying your yard and removing standing water, where they like to breed.

Heartworm preventive therapy is safe, inexpensive, and, when administered properly, can be extremely effective. There are a variety of options for preventing heartworm infection in both dogs and cats, including: 

  • Oral pill or tablet (ivermectin and milbemycin, taken monthly)
  • Topical liquid that you apply monthly to the pet’s back
  • Injectable (six-month product for dogs only)

Heartworm preventatives work to kill only the heartworm larvae that have infected the dog within the previous one to two months. Any larvae that have been in your dog longer are more likely to survive the treatment and develop into adult worms that will require adulticide treatment (discussed above). Your veterinarian will complete a blood test to confirm that your pet is heartworm-free before writing a prescription for heartworm preventive medication. Many heartworm preventives can cause illness if given to a dog with larvae in the bloodstream. 

We recommend that preventive medication be given year-round. If you choose to discontinue preventives in the winter, treatment should be started one month before the mosquito season begins and continued for one month beyond the first frost. 

How is routine heartworm testing performed?

For dogs, a prevention program should be started at 6–8 weeks of age. If not started at an early age, the American Heartworm Society recommends that all adult dogs be tested before starting an initial heartworm preventive. In consultation with your veterinarian, dogs should be tested annually for heartworm infection through an antigen test. 

Similarly in cats, it is recommended that heartworm preventives be started before the kitten reaches nine weeks of age. Cats older than six months should be tested for heartworms prior to starting prevention, and annually thereafter.

By keeping as many pets as possible free of heartworm disease, we will reduce the risk of disease in the overall population. Preventives are worth every dollar spent when you consider what it will cost to treat an infected pet—or risk its life. For more information, consult your veterinarian or visit the links below. 

FDA Animal and Veterinary

American Heartworm Society

WebMD: Healthy Pets

Dr. Fosters and Smith Pet Education

Dr. Vicky Yang is a veterinary cardiologist and a researcher in the field of regenerative medicine at Cummings School of Veterinary Medicine, where she also serves as an associate professor in the Department of Clinical Sciences.