Pet Education

Heartworm Disease in Southern California.

Heartworm disease 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.

By Mickila Collins, DVM
Diplomate ACVIM

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. The only way to know if your pet has Heartworm is by having a blood test for performed at a veterinary hospital.

Heartworm: Canine

IMAGE: Alan R Walker (Wikipedia)

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?

Between 2005-2015, veterinarians in Los Angeles County reported 257 cases – in 18 cats and 239 dogs. The majority of the cases (70%) had no symptoms at the time they were diagnosed. In 29% 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.

2005-2015 Heartworm Map

2005-2015 Heartworm Graph

Images: County of Los Angeles Public Health


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.
  3. Treat your pet! In 21% 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 also be reservoirs for the disease.


While Heartworm disease is fairly uncommon in Southern California the number of cases we are seeing is rising! It is important to remain vigilent about prevention and routine testing.


Animal Specialty Group

DVM, Diplomate American College of Veterinary Internal Medicine

Dr. Mickila Collins received her Doctor of Veterinary Medicine degree from Ross University in 2005, completing her clinical year at Louisiana State University. After graduation, she completed a one-year rotating internship, followed by an internal medicine internship, both at ASG.