How To Protect Dogs And Cats From Mosquito Bites — And The Diseases They Might Transmit

Mosquito bites aren’t just a problem for humans. They can put our pets at risk.

Heavy rainfall in the spring, combined with warming summer temperatures, mean more mosquitoes are breeding in Southern California. And your pet’s thick fur won’t protect them from the blood suckers, which can cause discomfort and disease.

Here’s what you can do to protect them.

Why should I try to keep mosquitoes from biting my cat or dog?

Mosquito bites are just as painful and itchy for pets as they are for people, and local mosquitoes can carry heartworms, which can kill dogs and cats.

Local veterinarians are noticing the effect of more mosquitoes in Southern California.

“We’ve also seen an increase in irritation from mosquito bites and hypersensitivities,” said veterinarian David Clark, director of the Pet Health Center at Western University of Health Sciences.

Clark said products containing permethrin act as a mosquito repellant and kill fleas and ticks when used as directed on dogs. But too much of it can be toxic to canines, and it can’t be used at all on cats.

His best advice is to keep mosquitoes out of your home and yard by dumping standing water where they breed and putting up well-fitting screens on all windows and doors.

What are heartworms?

Heartworms are parasitic roundworms spread by mosquitoes. They enter an animal’s bloodstream through mosquito bites, and can grow from microscopic larvae up to footlong adult worms that live in the heart and large blood vessels in the chest.

They commonly infect dogs, but they can also infect cats, ferrets, wolves, coyotes, seals and sea lions. Human infections are rare.

Southern California may not be a heartworm hot spot — authorities say the regions with the highest infection numbers are the Gulf Coast, the Southern Atlantic and along the Mississippi River — but cases are rising nationwide as mosquitoes continue to thrive farther north in warmer temperatures brought on by climate change.

“I strongly recommend heartworm preventative medication, even for local animals,” Clark said. “It's not as big a risk as it is in some other areas, but there's definitely risk.”

Which mosquitoes spread heartworms?

The L.A. County Department of Public Health points to three drought-resistant species of mosquitoes that can transmit heartworm and have been spreading in Southern California over the past decade.

• Asian tiger mosquito 
• Australian backyard mosquito 
• yellow fever mosquito 

They are also potential vectors for multiple human viruses such as dengue fever.

What are the symptoms of heartworm disease?

Infection can be present for a long time in the pet before symptoms appear, according to the American Heartworm Society. Symptoms may include tiredness, problems breathing, coughing, and eventually heart failure.

Infected cats may breathe hard and be more likely to vomit.

Blood tests for heartworms are performed at a veterinary hospital. Veterinarians say it’s best to have your vet run an annual blood test for heartworms so treatment can begin before your pet shows symptoms, even if they are on heartworm prevention medication.

The most recent data available show between 2012 and 2021, veterinarians in L.A. County reported 604 cases: 24 cats and 580 dogs. The majority of those cases had no symptoms at the time they were diagnosed, and 80% had traveled outside of L.A. County, according to the Department of Public Health.

What is the treatment for heartworm infection?

Veterinarians treat infected pets by giving medications to kill the worms in the bloodstream.

If a dog becomes infected, they may need to undergo an expensive, painful and potentially life-threatening series of arsenic injections. As the worms die, there is a risk of the pet having a bad reaction to the dead worms, so treatment is only administered by vets.

“You're going to spend a couple thousand dollars over four to six months to actually treat that. And treatment's a big deal,” Clark said.

There’s no cure for heartworms in cats, said Chris Duke, a Mississippi veterinarian and past president of the American Heartworm Society.

“Unfortunately, the way I've diagnosed it in some cats is on autopsy,” he said.

Heartworms are parasitic roundworms. They enter an animal’s bloodstream through mosquito bites, and live in the heart.

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Courtesy of the American Heartworm Society

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I’m taking my pet on a trip. How can I protect my dog or cat?

Preventive medication is the most cost effective and least invasive protection against heartworm disease, Clark said. Even just a short trip heightens their risk.

“Even in Northern California, you go north of the Central Valley and there are more heartworm cases,” Clark said.

A whopping 80% of locally reported heartworm cases were in pets that traveled outside of L.A. County, according to public health officials.

Dog owners have multiple heartworm medications to choose from, including monthly chewable tablets, topical medications and annual injections at a veterinary clinic.

Heartworm disease in cats is different from heartworm disease in dogs, because cats are atypical hosts. There is no treatment for heartworms in cats once they're infected, so prevention is the only means of protecting them from the effects of the disease.

Duke recommends topical monthly medications for his feline patients.

“We have mainly Revolution and Advantage Multi in my practice that both do a wonderful job,” he said. “I have my cat on one of them each month. My trick is just to wait until he's over the food bowl eating and then squirt the drops on the back of his shoulder and we're done for the month.”