California is not the Northeast. Our primary tick is different, our reservoir hosts are different, our seasons are different, and our case counts are dramatically smaller. If you’ve been applying East Coast Lyme advice to your California dog, much of it is wrong — including the assumption that summer is the high-risk window.

Here is what California-specific surveillance, vet research, and CDC data actually show — and what it means for hiking, vaccinating, and prevention products.

California vs East Coast Lyme: A snapshot

The numbers tell the story. From the CDC’s 2023 case-map data:

• New York: 22,173 cases
• Pennsylvania: 16,671 cases
• Massachusetts: 9,715 cases
• Wisconsin: 6,283 cases
• Connecticut: 3,239 cases
• California: 109 cases

California reports roughly 1/200th the volume of New York despite having about twice the population. This isn’t an artifact of underreporting. CDPH’s 2013–2019 epi summary documented just 904 total California Lyme cases over seven years statewide.

Within California, the risk is highly concentrated:

• Northern California: 0.9 cases per 100,000 people (739 cases over the seven years)
• Southern California: 0.1 per 100,000 (165 cases)
• Northern California is 6.6× higher risk than Southern California
• Top counties by rate: Santa Cruz (4.0/100k), Humboldt (3.4/100k), Sonoma (2.4/100k)
• 40.1% of case-patients had hiked, camped or picnicked in wooded/brushy/grassy areas during incubation

This isn’t a “California is safe” message — if you live in Sebastopol or Eureka or hike Annadel and Armstrong Redwoods, the risk for your dog is meaningful and year-round. But the East Coast playbook (“watch out from Memorial Day to Labor Day”) is wrong here in two big ways: wrong region (CA risk is concentrated in the north and northwest, not statewide), and wrong season (more on that below).

The ticks that actually live in California

Four tick species matter for California dog owners:

• Western black-legged tick (Ixodes pacificus) — the only Lyme vector on the Pacific Coast. Found in 56 of California’s 58 counties; Borrelia-positive ticks confirmed in 42 of the 58 (per CDPH). This is the tick that matters most for Lyme.
• Pacific Coast tick (Dermacentor occidentalis) — common along the western coast; vector for Pacific Coast tick fever (Rickettsia rickettsii subsp. californica) and a possible RMSF vector. CDPH calls it “one of the tick species most commonly recorded from humans” in the Pacific Coast states.
• American dog tick (Dermacentor variabilis) — present in CA but more dominant elsewhere; primary RMSF vector in the eastern U.S.
• Brown dog tick (Rhipicephalus sanguineus) — the unusual one that “can survive indoors,” per VCA. Transmits ehrlichiosis and a secondary anaplasmosis form. Often the relevant tick in residential / kennel settings rather than wild trail settings.

Note what’s missing: the deer tick (I. scapularis) is the East Coast Lyme vector and is not present in California. East Coast tick advice often references the deer tick — ours behaves differently.

Where the high-risk areas are

• North Coast (Humboldt, Mendocino, Del Norte): Highest I. pacificus density and human Lyme rate in the state. UC Davis specifically calls out Humboldt as where dogs face the most Lyme exposure.
• Bay Area / Wine Country (Marin, Sonoma, Napa, Santa Cruz, San Mateo): Documented year-round I. pacificus activity. Santa Cruz and Sonoma are top-10 CDPH counties; oak woodland and chaparral edges are the danger zones.
• Sierra foothills and Sierra Nevada lower elevations: I. pacificus habitat correlates strongly with oak woodland and the dusky-footed woodrat / western gray squirrel reservoir hosts.
• Central Coast (San Luis Obispo south to Santa Barbara): Pacific Coast tick is common; SLO is one of only two SoCal counties with documented Borrelia-positive I. pacificus.
• Southern California (LA, OC, Inland Empire, San Diego): Low Lyme risk. Orange County is the other SoCal county with documented Borrelia-positive ticks. Pacific Coast tick and brown dog tick are more relevant than I. pacificus.
• Central Valley: Lower I. pacificus density (less oak woodland); brown dog tick possible in residential settings.
• Desert regions: Lowest tick burden of any California region.

Tick season in California is year-round (and opposite of the Northeast)

This is the part East Coast advice gets most wrong. Per a study summarized by the Bay Area Lyme Foundation:

• Adult I. pacificus peak: October–May — the rainy/cool season. Opposite of the Northeast assumption.
• Nymphal I. pacificus peak: January–October. Nymphs are tiny (“smaller than a poppy seed”) and account for most human cases.
• Larvae: April–June, sometimes through October.
• “Tick season in Northwestern California is longer than even we expected and quite different from patterns in the Northeast USA.”

The driver: California’s main reservoir hosts (western gray squirrels, dusky-footed woodrats) live multiple years and stay active year-round — unlike the Northeast’s white-footed mouse cycle. So a California dog who hikes Mt. Tam or Annadel in February has higher seasonal tick exposure than the same dog in July. Plan prevention accordingly.

The Lyme vaccine: does my California dog need it?

The Lyme vaccine is classified as noncore by AAHA — meaning it’s risk-based, not blanket-recommended. The 2022 AAHA guidelines note: “Vaccination for Lyme borreliosis should be considered for dogs that live within or travel to regions with emerging or endemic Lyme disease. In endemic regions, traditionally noncore vaccines may be considered a core vaccine by veterinary practices in those locations.”

What the evidence base looks like, honestly:

• Efficacy is variable: The Merck Vet Manual states “Reported efficacies are variable (50%–100%), and evaluation of efficacy is challenging because of the lack of a disease model. The duration of immunity is 1 year.”
• Schedule: Two initial doses two to four weeks apart (puppies at risk vaccinated at 9–12 weeks), then annual boosters — ideally just before tick season starts in your region.
• Vaccine types: Whole-cell bacterins, recombinant OspA, and bacterins with both OspA and OspC.

For a California dog, the vaccine decision is regional:

• North Coast, Bay Area, Wine Country, Sierra foothills: Real exposure risk. Worth a serious vaccine conversation with your vet, especially if your dog hikes off-trail or in oak woodland year-round.
• Central Coast (SLO area): Moderate consideration; Borrelia-positive ticks are documented.
• Southern California, Central Valley, deserts: Documented Lyme risk is low. AAHA’s “noncore, risk-based” framing typically lands on “skip” for these regions unless travel patterns change the math.

Whatever you decide on the vaccine, year-round tick prevention products carry more day-to-day weight than the vaccine does — because the vaccine only covers Lyme, while products cover all four California tick species and most of the diseases they carry.

Prevention products (categories, not brand rankings)

Per the Merck Veterinary Manual: “Isoxazolines have revolutionized external parasite control and are currently the dominant parasiticide drug class in small animal veterinary medicine worldwide.”

• Oral isoxazolines (afoxolaner, fluralaner, lotilaner, sarolaner) — sold as monthly or 12-week chews. Highly effective, well-tolerated for most dogs, but the FDA has issued an alert on potential neurologic side effects in some dogs (muscle tremors, loss of muscle control, seizures). FDA still considers them safe and effective; talk to your vet if your dog has a history of seizures.
• Topical spot-ons — fipronil-based and imidacloprid + permethrin-based monthly applications. Effective but require keeping the dog out of water for 24–48 hours after application.
• Collars (imidacloprid + flumethrin) — multi-month coverage. Per VCA, should not be combined with other flea/tick products.

The right product is the one your dog will actually wear or take consistently. A perfect oral isoxazoline that gets skipped because the dog hates the taste does less than a topical that goes on every month without complaint. Talk to your vet about matching the product to your dog’s lifestyle.

How to remove a tick correctly

Per CDC’s official guidance:

• Use fine-tipped tweezers — not your fingers, not a “tick key,” not a lit match.
• Grasp the tick as close to the skin as possible.
• Pull straight up with steady, even pressure.
• Don’t twist or jerk the tick — that can leave mouthparts embedded.
• Don’t crush the tick with your fingers; place it in rubbing alcohol or seal it in a bag.
• Save the tick for species identification and engorgement assessment if you need it for later medical decisions — but the CDC explicitly does not recommend commercial tick testing as the basis for treatment decisions.

Transmission window matters: Merck cites that “at least 24 hours are required for transmission of spirochetes from tick to host,” and VCA cites approximately 12–24 hours. The faster you find and remove the tick, the lower the risk — which is why a hands-on tick check after every wooded-trail outing is the single most effective thing you can do.

Symptoms after a tick bite

Most tick bites cause nothing. When they do cause illness, the classic Lyme presentation in dogs (per AKC and the Merck Vet Manual):

• Fever
• Loss of appetite
• Reduced energy / lethargy
• Lameness — often “shifting” from one leg to another
• Generalized stiffness, discomfort, or pain
• Swollen joints
• Walking “as if on eggshells”

In advanced cases: kidney failure (Lyme nephritis), cardiac issues, neurologic signs.

When to test: the C6 antibody can be detected three to five weeks after an infected tick bite. VCA recommends testing no earlier than four weeks post-bite. The 4Dx SNAP test (annual heartworm + Lyme/anaplasma/ehrlichia screen) is the standard in-clinic tool and is typically run with annual heartworm tests.

Treatment for confirmed Lyme is doxycycline at 10 mg/kg every 12 hours for four weeks per Merck.

The other tick-borne diseases that matter in California

Lyme is the famous one, but it’s not the only thing California ticks transmit. CDPH 2019–2021 surveillance of I. pacificus showed:

• Borrelia burgdorferi (Lyme): 1.05% adults, 4.40% nymphs
• Borrelia miyamotoi (relapsing fever): 0.65% adults, 1.27% nymphs
• Anaplasma phagocytophilum (anaplasmosis): 0.56% adults, 2.0% nymphs

Anaplasmosis presents in dogs as lameness, joint pain, fever, lethargy, and loss of appetite. Treatment is doxycycline for two to four weeks, and per VCA, “dogs are often markedly better 24 to 48 hours after therapy is started.”

Pacific Coast tick fever (caused by R. rickettsii subsp. californica) is, per CDPH, “one of the most frequently reported spotted fever group rickettsioses in California” — specific to our state and worth distinguishing from Rocky Mountain spotted fever in clinical conversations.

Ehrlichiosis in California is most often brown-dog-tick-mediated — relevant for dogs in residential/kennel exposure rather than wild trails.

The takeaway for California dog owners

If you live in or hike the North Coast, the Bay Area, the wine country, or the Sierra foothills, your dog has meaningful year-round tick exposure — with the highest activity during cool, wet months that East Coast advice ignores. Use a year-round tick prevention product matched to your dog’s lifestyle. Have an honest vaccine conversation with your vet (the answer for a Sebastopol dog is different from a San Diego dog). Do a hands-on tick check after every wooded-trail outing, and if you find a tick, remove it correctly with fine-tipped tweezers. The classic shifting lameness three to five weeks later is your cue to ask for the 4Dx SNAP test.

Sources

• CDC. Lyme disease case map (2023) and After a tick bite.
• CDPH. Tick-borne diseases page, 2013–2019 epi summary, and spotted fever fact sheet.
• AAHA. 2022 Canine Vaccination Guidelines.
• Merck Veterinary Manual. Lyme borreliosis, Ixodes spp., and Ectoparasiticides.
• VCA Hospitals. Lyme disease in dogs, Ticks in dogs, and Anaplasmosis in dogs.
• AKC. Lyme disease in dogs and Tick-borne diseases.
• Bay Area Lyme Foundation. Lyme risk year-round in NW California.
• UC Davis Center for Companion Animal Health. Dog health resources.