Do Lyme Ticks Exist in California

Lyme disease and ticks are often thought to be endemic to the east coast of United States. When I first started treating Lyme disease in 2003, my practice was located in Peoria, Arizona. I never thought that testing for Lyme disease should be on my radar. However, when I had a young patient who was experiencing joint pain that migrated between her wrists and knees, intermittent abdominal complaints, and a history of playing soccer, I ran labs ruling out juvenile rheumatoid arthritis and other inflammatory markers. We discussed dietary changes which seemed to moderately help the abdominal complaints. However, the migrating joint pain persisted. Recalling from my medical training that this key symptom can be associated with Lyme disease, I ran labs for Lyme disease and the results came back positive. I was puzzled. This particular patient had never traveled outside the state.


Like many clinicians, we may miss the diagnosis because of biases from our own personal beliefs, limited medical training, and limitations in available testing. Therefore, I took the time to search the literature, public health resources, and became a Lyme literate doctor.


Are cases of Lyme disease being reported in California?

According to the CDC, it is estimated that 476,000 individuals have been diagnosed and treated for Lyme disease between 2010-2018 and approximately 35,000 cases of Lyme are reported each year in the United States.The geographical region showing a high incidence of Lyme disease, as defined as 10 confirmed cases per 100,000 people per three years, remains in the Northeast states as well as upper Midwest states such as Wisconsin and Minnesota. According to the CDC, a low incidence of Lyme disease is reported outside this area, which includes the state of California.


However, California is a large state and areas in Northwestern California, around Mendocino County, have an incidence of more than 50 cases per 100,000 people per year.2  Other pockets of California (in Humboldt, Trinity, Shasta, Marin, Sonoma, Lake, Kern, Monterey, and Plumas County) have an incidence of 20-50 cases per 100,000 people per year making these areas technically high incidence. There are pockets of endemic Lyme disease incidence of more than 5 cases per 100,000 person-years seen in Southern California (in certain zip codes located in Imperial, Kern, Monterey, San Diego, and Tulare).


Are there ticks in California and do they contain pathogenic organisms?

In the west coast, there are different hosts and vectors in comparison to those seen on the east coast. In California, hosts may include squirrels, rabbits, mice and rats. A more common vector in the west coast is the Ixodes pacificus tick, also known as the western black-legged tick. This tick has been found in 56 of the 58 counties in California.3 A tick surveillance study has shown that Ixodes pacificus ticks from Alameda, Marin, Napa, Placer and Sonoma counties did harbor Borrelia miyamotoi, which causes disease characterized by relapsing fever.4 Additionally, Borrelia burgdoferi was found in Ixodes pacificus ticks collected from Lake, Placer and Sonoma counties.


In California, we have a large variability in habitats in different regions of the state. An interesting study completed in 2019 by Rose et al. demonstrated that these habitats may contribute to tick diversity because each habitat supports different hosts.5  They looked at adult and nymphal, Ixodes pacificus and Ixodes spinipalpis ticks gathered from 42 California counties from 2008 to 2015. What they found was that Borrellia burgdoferi ss, the bacteria causing Lyme disease, was only found in Ixodes pacificus ticks in northern and coastal counties and the Sierra Nevada foothill areas. What they found in the southern region of California (which includes Los Angeles, Orange, Riverside, San Bernadino, San Diego, Santa Barbara, and Ventura County) was different. Out of 2,636 Ixodes pacificus and Ixodes spinipalpis ticks collected, nine were determined to have a genospecies of B. burgdoferi s.l., two had B. americana, and seven had B. bissettiae. The pathogenicity of the latter two is unknown. Other Borrelia species have been found even in nontypically associated areas of northern California, coastal chapparal and prairie.6


A tick surveillance study completed in 2019, looked specifically at human-biting ticks submitted from the western states of California, Oregon and Washington between the time period of 2014 to 2017.7  This study showed that people are being bit by different species of ticks, such as the Ixodes pacificus, Ixodes spinipalpis, and Ixodes angustus, which are endemic to the west coast. Others in the study were being bit by nonendemic ticks and had a history of travel to endemic areas. So, travel history should be an important part of a screen when Lyme disease is suspected. This study also showed that adult Ixodes pacificus ticks from Oregon and California did carry B. Burgdoferi, B miyamotoi and A. phagocytophilum, all of which are known to cause disease. It was nymphal I. pacificus tick bites that were more likely to be seen in those under age 9, occurring within the months of March to August, with a peak in June. Such bites typically occurred on the lower extremities.


These studies do dispel the myth that Lyme doesn’t exist on the west coast. Certainly, different habitats have higher risks in exposure to tick-borne diseases but screening for Lyme and even associated co-infections should not be overlooked.


  1. Centers for Disease Control and Prevention. How many people get Lyme disease? gov.
  1. Eisen R, Lane R, Fritz C, Eisen L. (2006). Spatial Patterns of Lyme Disease Risk in California Based on Disease Incidence Data and Modeling of Vector-Tick Exposure. Am J Trop Med Hyg, 75(4) 669-676.
  1. California Department of Public Health – Vector-Borne Disease Section. Lyme Disease in California.
  1. Eshoo M, Carolan H, Massire C, Chou D, Crowder C, Rounds M, et. al. (2015). Survey of Ixodes pacificus Ticks in California reveals a Diversity of Microorganisms and a Novel and Widespread Anaplasmataceae Plos One, 10(9) 1-14.
  1. Rose I, Harstone M, Bonilla, D, Fedorova N, Lane R, Padgett K. (2019). Phylogeography of Borrelia spirochetes in Ixodes pacifcus and Ixodes Spinipalpis Ticks Highlights Differential Acarological Risk of Tick-Borne Disease Transmission in Northern versus Southern California. PLoS One. 14(4): 1-17.
  1. Salkeld D, Lagana D, Wachara, J, Porter W, Nieto N. (2021). Examining Prevalence and Diversity of Tick-Borne Questing Ixodes pacificus Ticks in California. Appl Environ Microbiol. 87: 1-20.
  1. Guang X, Pearson P, Dykstra E, Andrews E, Steven R. (2019). Human-Biting Ixodes Ticks and Pathogen Prevalence from California, Oregon, and Washington. Vector-Borne and Zoonotic Diseases. 19(2): 106-114.

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