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Veterinary Public Health


Veterinary Public Health

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Contact Information
Veterinary Public Health Program
313 N Figueroa St. Rm 1127
Los Angeles, CA 90012
Tel (213) 989-7060
Tel: (877) 747-2243
Fax (213) 481-2375
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Heartworm Disease

Updated 12.12.13

What is heartworm disease? Heartworm disease is an infection inside the bloodstream of animals. It is caused by a worm called Dirofilaria immitis. This worm is spread to animals by mosquito bites. Dogs, cats, ferrets, wolves, coyotes, seals, and sea lions can all become infected. The disease does NOT spread directly from animal-to-animal. This disease has been rare in Los Angeles County in the past. Infected dogs and coyotes are reservoirs for the disease.

Data on Local cases of Heartworm Disease
Between 2005-2012, veterinarians in Los Angeles County reported 100 cases. Of these, 12 cases were in cats and 88 in dogs.
  In 35% of these cases, the pet caught the infection in Southern California (local), in 43% of cases the pet was infected outside of Southern California (imported), and in 22% of cases there was not enough information about the animal to say where they caught it (unknown).  The graph seen upper right shows these cases by year.  The majority of the cases (72%) had no symptoms at the time it was diagnosed. In the other cases, symptoms included cough (20%), tiredness (11%), and heart failure (6%).

There are more heartworm cases in the county that are not reported.  The Companion Animal Parasite Council reports that 90 dogs tested positive for heartworm in Los Angeles County in 2012.  See their interactive map for details.

What are the symptoms of heartworm disease? Animals that are infected with heartworm may have tiredness, problems breathing, coughing, and heart failure. Infected cats may breathe hard and be more likely to vomit. It is important to know that infection can be present for awhile 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.

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 15% of the 100 cases in Los Angeles County, the animal was not treated. 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?  Humans can potentially get infected if they are bitten by a mosquito that is carrying the worm. However human infections with Dirofilaria immitis are very rare.  In most cases, the person has no symptoms, but small shadows ("coin lesions") may be seen inside the lungs on a chest X-ray.  No cases of human heartworm infection have been reported in our county.  See articles in blue box below for more information.

Reporting Heartworm Cases
VETERINARIANS: Report a case of heartworm disease by using this form or by using the online reporting portal.

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

Scientific Articles

Heartworm in California Coyotes
2004 - Modeling the Distribution and Abundance of the Non-native Parasite, Canine Heartworm, in California Coyotes pdf icon
2003 - Reconstructing the Spread of Dirofilaria immitis in California Coyotes

Heartworm Infection in Humans
2010 - Public Health Issues Concerning the Widespread Distribution of Canine Heartworm Disease
2005 - Public Health Aspects of Dirofilariasis in the United States
2002 - Human Pulmonary Dirofilariasis: Uncommon Cause of  Pulmonary Coin-lesion
2001 - Heartworm in a 28 year-old-man in California


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