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Heartworm Disease Is Preventable

Heartworm disease. The name says it all: These are worms that live inside the heart. Sounds gross, right?

The basics of this disease are simple. A mosquito bites an infected canine and can then transmit the microscopic larvae to another animal. In the dog, which is the primary host, larvae left by mosquitoes move from wounds in the skin to the surrounding tissues where they continue to mature. After about two months, the young adult worms (2 to 3 centimeters in size) migrate into the blood vessels and eventually make it to the heart and lungs where they continue to grow into fully mature, 15-to-30 centimeter (6 to 12 inches!) adult worms about 6 months after initial infection.

At this point, the worms start to reproduce, filling the blood with more larvae that can again be spread via mosquito. The infected dog is likely to suffer damage to the heart, lungs and/or kidneys. The only treatment for mature heartworm disease in dogs are one to two injections of an arsenic-containing compound called melarsomine that kills the worms. During treatment, dogs are kept on strict cage rest for one to two months to avoid the very real risk of a spike in blood pressure causing a mass of dead worms to exit the heart and cause complete blockage of one of the great vessels, which could result in sudden death.

Of course, most dog owners know that this disease can be avoided by keeping your dog on a monthly heartworm preventative. But here are some things you might not know:

Heartworm preventative medicine is retroactive. When you give a dose to your dog, it works by killing the earliest stage of larvae if they are present in the blood, preventing further maturation and full-fledged heartworm infection.

Canine heartworm tests pick up only mature infections, which means after a lapse in prevention, it’s important to test and make sure your dog is still negative about six months after the last missed dose.

Dogs aren’t the only hosts. Other canids such as wolves and coyotes can harbor it, too. In the Bay Area, dead coyotes have been found with heartworms and are therefore known to serve as a source for heartworm infection in domestic dogs.

Other animals can be sickened (or killed) by heartworm disease, even if they rarely host mature infections and do not spread the disease to other animals. These include cats, ferrets, and, rarely, humans.

Indoor dogs (and cats!) can get heartworm disease. Have you ever been tormented by a mosquito inside your home? Your pets can be, as well.

Good prevention is year-round. Guess what? In the Bay Area, mosquitoes aren’t just a summer phenomenon. In fact, there can be more mosquitoes during the mild, wet parts of the year.

Treatment is expensive. A year’s worth of generic heartworm preventative is typically less than $100. Treatment with melarsomine can range from several hundred to over $1,000 depending on the size of dog and the severity and stage of disease.

Heartworm infection is on the rise in California and across the United States. According to the Companion Animal Parasite Council, from 2013-2018, the rates of positive tests rose steadily and prevalence overall went up 20 percent, likely due to an increase in hot and wet weather. Another factor likely increasing prevalence is transport of heartworm-positive dogs after hurricanes and other natural disasters from the Southeast United States and the Caribbean (where heartworm disease risk is much higher) into rescues and shelters in California.

To see the number of positive heartworm tests by state and county, check out this page on the CAPC website:

If reading this is reminding you that your dog is not up to date on his or her heartworm preventative, don’t panic. Call your veterinarian and set up an appointment for a test and to discuss options for preventative measures.

Above is a photograph of a German Shepherd dog heart infested with heartworm. The heart is in a jar filled with formaldehyde. Its orientation is sideways, with the apex to the left and the great vessels to the right. The right ventricle is cut open, exposing the heartworms at the bottom.

 Elinor Granzow, D.V.M., is a small animal veterinarian at Alameda Pet Hospital.  She is originally from Seattle but has lived in the Bay Area since 2010. She shares her home with a 3-year-old cat named Tuna Bell and a 39-year-old Yellow-naped Amazon parrot named Harry.

Are you a San Francisco Bay Area veterinarian who would like to write an article for the Ask Dr. Dog column, which is authored by guest veterinarians practicing in the Bay Area? Bay Woof is accepting submissions. Send email to

Main article photo by: Photo by Joel Mills  / Creative Commons