Veterinary Public Health

Heartworm Disease in Animals in Los Angeles County

About Heartworm Disease

What is heartworm disease?

Heartworm disease in animals caused by a worm called Dirofilaria immitis, which is spread between animals by mosquito bites. The adult worms live in the heart and large blood vessels in the chest. Dogs, cats, ferrets, wolves, coyotes, seals, and sea lions can all become infected.

What are the symptoms of heartworm disease?

Infection can be present for a long time in the pet before symptoms appear. Symptoms may include tiredness, problems breathing, coughing, and heart failure. Infected cats may breathe hard and be more likely to vomit.

How do you know if your pet is infected with heartworms?

By blood tests for heartworms performed at a veterinary hospital.

What is the treatment for heartworm infection?

Veterinarians treat infected pets by giving medications to kill the worms in the bloodstream. As the worms die, there is a risk of the pet having a bad reaction to the dead worms. Therefore, heartworm disease is treated only under the close supervision of a veterinarian.

Untreated animals

Untreated animals may become "reservoirs" for the disease. This means they can infect mosquitoes, and then the mosquitoes can infect more pets. Infected coyotes can be reservoirs for the disease.

Is there any heartworm disease in Los Angeles County?

Graph LA County heartworm cases 2012-2021 map of heartworm cases in animals in LA County 2012-2021

Yes. Between 2012-2021, veterinarians in Los Angeles County reported 604 cases - in 24 cats and 580 dogs. The majority of the cases (80%) had no symptoms at the time they were diagnosed.

In 19.5% of these cases, the pet had not traveled outside of Southern California, so they had acquired the infection locally. The graph seen at the right shows these cases by year. The amount of reports received per year increased in 2014 because laboratories began to report cases.

How Can I Prevent Heartworm in My Pet?

1. Mosquito Control. Mosquitoes breed in stagnant water. Stop mosquito breeding by dumping any standing water on your property every 2 days. Mosquitoes feed the most at dawn, dusk and at night, so keep your pet indoors at night.

2. Heartworm Preventative Medication. Heartworm preventative medications are generally regarded as safe and help prevent infection with additional parasites. Discuss the issue with your pet’s veterinarian.

Protect Your Pet from Heartworm FAQ - English and Spanish (PDF) (10.25.23)

Invasive Aedes mosquitoes in Los Angeles (LA) County

  • 3 drought-resistant species of mosquitoes that may transmit heartworm have been spreading in LA County in the past decade: The Asian Tiger Mosquito (Aedes albopictus), the Australian Backyard Mosquito (Aedes notoscriptus) and the Yellow Fever Mosquito (Aedes aegypti). They are also potential vectors for multiple human viruses.
  • Learn more about Aedes mosquitoes in California. (California Dept of Public Health)
  • Your help is needed! Be a mosquito-fighter.

Can humans catch heartworm?

Human infections with Dirofilaria immitis are very rare. Clinical findings in humans with heartworm infection have included having a small shadow ("coin lesion") in the lungs as seen on a chest X-ray, or finding a heartworm in a lump under the person's skin. No cases of human heartworm infection have been reported in LA County.

Tracking and Reporting Heartworm

Tracking Heartworm in LA County

Heartworm in animals is reportable in LA County. In 2014, laboratories were required to begin reporting cases, and the reports available increased. Cases are categorized as Confirmed, Probable, or Suspected based on the Heartworm Case Definition for LA County.

Reporting Heartworm Cases

VETERINARIANS: report a case of heartworm disease through our reporting page, or email vet@ph.lacounty.gov or fax to 213-481-2375.