A rare tick-borne disease caused by the bacterium Rickettsia lanei has been confirmed in a Northern California resident, marking the fourth reported human case in the state. The diagnosis was made in April, and while public health officials cannot disclose the patient’s exact location, they confirmed the individual lives and works in Northern California. The person was seriously ill and hospitalized but has since been discharged and is recovering.

Rickettsia lanei is part of the spotted fever group Rickettsia, bacteria transmitted to humans through the bites of infected ticks. In California, three tick species—the American dog tick (Dermacentor similis), Pacific Coast tick (Dermacentor occidentalis), and brown dog tick (Rhipicephalus sanguineus)—are known vectors for diseases such as Rocky Mountain spotted fever, which can cause fever, rash, and serious complications affecting internal organs and the nervous system.

Human cases of Rickettsia lanei are rare but may be underreported due to symptom overlap with Rocky Mountain spotted fever. Two prior cases in California were recorded nearly two decades apart, with one in 2004 and another in 2023. Both patients had severe symptoms requiring extended hospitalization, including stays in intensive care units.

One previously reported patient experienced symptoms such as fever, headaches, muscle pain, and abdominal issues after visiting multiple golf courses in Alameda and Contra Costa counties. This individual spent 22 days in the hospital, with 11 days in intensive care, and was diagnosed with Rocky Mountain spotted fever and acute kidney injury. Another patient who camped in San Mateo and Marin counties developed symptoms including headaches, vomiting, light sensitivity, neck pain, and confusion, and was comatose by their third day in hospital. After 13 days of treatment, the man was discharged with a diagnosis of severe Rickettsia infection.

Rickettsia lanei was first detected in rabbit ticks in Sonoma County in 2018, but its impact on humans was unclear due to the rabbit tick’s infrequent human bites. Researchers believe the Pacific Coast tick, which more commonly feeds on humans, may acquire the bacteria from infected rabbits, potentially leading to the rare human cases observed.

Experts note that increased awareness and public health investment have facilitated advanced molecular testing to identify infections caused by this emerging pathogen. Some researchers suggest the true number of infections may be higher, as mild or misdiagnosed cases might go unrecognized. Ongoing studies are limited, partly due to the newness of this disease and the lack of targeted research funding.

Health officials advise Californians to take precautions when spending time outdoors to reduce tick exposure. Recommended measures include wearing long pants and sleeves, avoiding tall grass, donning light-colored clothing to more easily spot ticks, showering immediately after outdoor activities, and treating pets with tick prevention products. These steps can help prevent not only Rickettsia lanei infections but also other tick-borne illnesses such as Lyme disease.