Although there are 21 species of hard ticks in San Mateo County, the list below are the three most common ticks that people and pets contact in the environment.  These three species of ticks can and do bite humans.

As of April 2025, the District has not found lone star ticks (Amblyomma americanum) in San Mateo County.

Click to learn more about how our staff collect and test ticks from throughout the County.

Western Black-Legged Tick (Ixodes pacificus)

• Adults quest along trails primarily November through April
• Adults feed readily on humans, horses, dogs, and other large mammals
• Larvae and nymphs are most active March to early August, peaking in April to June
• Larvae and nymphs feed primarily on lizards but may also attach to rodents and people
• Primary vector of Lyme disease to humans in California

Pacific Coast Tick (Dermacentor occidentalis)

• Adults reach peak activity from April through May, but are present along trails from November through August
• Adults feed on cattle, horses, deer, dogs, and humans
• Larvae and nymphs are most active April through July
• Larvae and nymphs feed on small rodents and other small mammals in spring and summer
• Vector of tularemia and various uncommon Rickettsial diseases

American Dog Tick (Dermacentor variablis)

• Adults quest along trails primarily from March through July
• Adults feed on dogs and large mammals, including humans
• Larvae and nymphs are most active April through July
• Larvae and nymphs feed on rodents and other small mammals
• Vector of tularemia and various uncommon Rickettsial diseases

Very rarely, we receive a tick to identify that is a brown dog tick (Rhipicephalus sanguineus). To learn more about brown dog ticks, visit HERE or HERE.

If you've found a tick and would like it identified, our staff can help!  Learn more about our tick and insect identification service HERE.

Page last reviewed: April 11, 2025