Heartworm in California dogs is less common than in the southeastern United States, but it’s far from nonexistent. Studies have found an overall infection rate of roughly 3.5% in tested dogs in parts of the Central Valley, and heartworm-positive dogs have been identified in 56 of the state’s 58 counties. Northern California, in particular, is seeing rising risk.
Infection Rates Across the State
California’s heartworm picture varies dramatically by region. In a study of Fresno and Madera Counties in the Central Valley, overall prevalence among tested dogs was 3.47%, with Madera County slightly higher at 3.8% and Fresno at 3.5%. Those numbers are lower than in heartworm hotspots like the Mississippi River Valley, where infection rates can exceed 10%, but they still represent thousands of dogs statewide.
The foothill regions carry the highest risk. Heartworm became established in the foothills of the Sierra Nevada mountains roughly three decades ago and has persisted there since. In coyotes, which serve as a wildlife reservoir for the parasite, infection rates in foothill areas range from 24% to 57% depending on the region. Those coyotes keep the parasite circulating in the environment, giving local mosquitoes a constant source of infection to pass along to dogs. In Madera County, researchers found that heartworm prevalence in dogs increased progressively with elevation, meaning foothill and mountain communities face a meaningfully higher risk than flat valley towns.
The Companion Animal Parasite Council’s 2025 forecast notes that Northern California continues to see increasing heartworm risk, stretching from the northern coast inland through the Central Valley. This trend makes the infection relevant even for dog owners who have historically considered California “low risk.”
Why California’s Climate Allows Transmission
Heartworm spreads exclusively through mosquito bites. A mosquito picks up microscopic larvae from an infected dog or coyote, the larvae mature inside the mosquito, and the mosquito passes them to the next dog it bites. That maturation step inside the mosquito requires about two weeks of temperatures consistently above 57°F. Much of California, especially inland valleys and foothill areas, easily meets that threshold for large portions of the year.
Once the larvae enter a dog, they migrate through the body and eventually grow into foot-long adult worms that lodge in the heart and pulmonary arteries. The entire process from mosquito bite to detectable adult infection takes about six months.
The Mosquitoes Driving Transmission
Four mosquito species have been linked to heartworm transmission in California, each dominating a different part of the state. In Northern and Central California, the most significant vector is a species commonly found around suburban neighborhoods and rural homes. Central California also sees transmission driven by two additional species, including the invasive yellow fever mosquito that has expanded its range in recent years. In Southern California, the Asian tiger mosquito is the primary driver.
Both the yellow fever mosquito and the Asian tiger mosquito are invasive species that have been spreading through California’s urban and suburban areas. Their presence in residential neighborhoods, particularly in less densely developed areas with mixed vegetation and housing, creates transmission risk right in your backyard. These aren’t wilderness mosquitoes. They breed in small containers of standing water: plant saucers, clogged gutters, forgotten buckets.
How Heartworm Is Detected
Vets test for heartworm using a blood test that detects proteins produced by adult female worms. The test is quick and can be done during a routine visit, but it has some important limitations. Its sensitivity is around 78%, meaning it catches about four out of five true infections. It’s much better at confirming a dog is clean: the negative predictive value exceeds 99% in low-prevalence areas like most of California.
Where things get tricky is with positive results. In a state with relatively low prevalence, a positive antigen test has a positive predictive value of only 15% to 54%, depending on the local infection rate. That means a positive result doesn’t automatically confirm infection. Your vet will typically factor in your dog’s symptoms, travel history, and where you live before deciding on next steps, which may include additional testing.
The test can’t detect heartworm until about six months after infection, since it picks up proteins from mature adult worms. A dog bitten by an infected mosquito in July won’t test positive until January at the earliest. This is one reason annual testing matters even for dogs on preventive medication.
Year-Round Prevention Is the Standard
The Companion Animal Parasite Council recommends that all dogs, regardless of lifestyle or location, stay on heartworm prevention year-round. In California, where mild temperatures allow mosquito activity across many months, seasonal gaps in prevention create real windows of vulnerability. Every dog should also be tested annually using both antigen and microfilariae tests, and dogs in high-exposure areas (foothill communities, homes near irrigation canals or standing water) may benefit from testing twice a year.
Preventive medication typically costs around $185 per year. Treating an established heartworm infection is dramatically more expensive, often running $1,000 to $3,000 or more, and the treatment itself carries health risks for the dog. The worms must be killed gradually to prevent dangerous blockages in the blood vessels, and dogs need weeks of strict exercise restriction during treatment. Prevention is cheaper, safer, and far less stressful for both you and your dog.
Foothill and Suburban Dogs Face the Most Risk
If you live in the Sierra Nevada foothills, the North Coast Ranges, or the hills around the San Francisco Bay, your dog’s risk is higher than the statewide average. These areas have established coyote reservoirs and mosquito populations well suited to heartworm transmission. Researchers have noted that the San Joaquin Valley floor, by contrast, has a low enough transmission rate that a modest increase in dogs on preventive medication could potentially eliminate local transmission entirely.
Suburban neighborhoods in less urbanized parts of the state also warrant extra attention. The mosquito species driving transmission in California tend to thrive in areas with a mix of housing and vegetation, single-family neighborhoods rather than dense urban centers. If your home has a yard, backs up to open space, or sits in a community surrounded by agriculture or foothills, your dog’s mosquito exposure is likely higher than you’d expect for a California address.