Veterinary Public Health

Pet Health Calendar
Contact Information
Veterinary Public Health Program
313 N Figueroa St. Rm 1127
Los Angeles, CA 90012
Tel (213) 288-7060
Fax (213) 481-2375
Adobe Reader
Get Adobe Reader icon
Note: PDF documents on this site were created using Adobe Acrobat 5.0 or later. If you are using an earlier version of Adobe Acrobat Reader (4.x or less), document functionality may be reduced.
Heartworm Disease in Animals in Los Angeles County

What is heartworm disease?
It is an infection in animals caused by a worm called Dirofilaria immitis. This worm is spread 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. The disease does NOT spread directly from animal-to-animal. 

What are the symptoms of heartworm disease?
Infected animals may have tiredness, problems breathing, coughing, and heart failure. Infected cats may breathe hard and be more likely to vomit. Infection can be present for a while in the pet before symptoms appear.

How do you know if your pet is infected with heartworms? The only way to know is by having a blood test for heartworms performed at a veterinary hospital.

What is the treatment for heartworm infection?
Veterinarians treat infected pets by giving medication 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.

Is there any heartworm disease in Los Angeles County?
Yes. Between 2009-2018, veterinarians in Los Angeles County reported 521 cases - in 30 cats and 490 dogs. The majority of the cases (77%) had no symptoms at the time they were diagnosed.


In 27% 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.

Untreated animals
In about 20%  of the cases in LA County, the animal had not been treated for heartworm infection at the time of the report. 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.

Can humans catch heartworm? 
However human infections with Dirofilaria immitis are very rare.  In most cases, the person has no symptoms, but a small shadow ("coin lesion") may be seen inside the lungs on a chest X-ray.  In a few cases in the world, a heartworm has been found in a lump under the skin of a person. No cases of human heartworm infection have been reported in LA County.  See articles in blue box below for more information.

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. Of the 521 cases reported between 2009-2018, 49% were Confirmed, 39% were Probable, and 13% were Suspected.

Reporting Heartworm Cases
VETERINARIANS: Report a case of heartworm disease by using this form and email it in to or fax to 213-481-2375.

Invasive Aedes mosquitoes in Los Angeles (LA) County

  • Three new species of drought-resistant mosquitoes 
    are spreading LA County.
    Aedes aegypti - CDC

  • They are black with white stripes, and
    bite in daytime.

  • Two of the new mosquito species have transmitted heartworm in other countries, and therefore may spread heartworm here.

  • The Yellow Fever mosquito and Asian Tiger mosquito are potential vectors for multiple human viruses, such as West Nile, Zika, chikungunya, and dengue. The California Department of Public Health has an interactive map showing places in California where these two mosquitoes have been detected.

  • Your help is needed! Be a mosquito-fighter.

Graph 2009-2018 Heartworm cases in Los Angeles County by location where exposed

Graph 2009-2018 Heartworm cases in Los Angeles County by location where exposed



American Heartworm Society (find "Pet Owner Resources" at bottom of page)


Heartworm in California Coyotes

Heartworm Infection in Humans

Last updated:
July 31, 2020
Public Health has made reasonable efforts to provide accurate translation. However, no computerized translation is perfect and is not intended to replace traditional translation methods. If questions arise concerning the accuracy of the information, please refer to the English edition of the website, which is the official version.
Los Angeles County Seal: Enriching lives through effective and caring services