A new case of a rare tick-borne disease has been detected in a human for only the fourth time on record.
A California resident tested positive for the bacteria Rickettsia lanei earlier this year, marking the third human case in the state and the fourth human case worldwide since the bacteria was first identified eight years ago.
Rickettsia lanei was first identified in 2018 in Sonoma County, California, in rabbit ticks.
The bacteria can cause severe life-threatening symptoms, including fever, gangrene – the death of body tissue – coma and brain swelling.
It is in the same family as Rocky Mountain spotted fever and other diseases known as spotted fever rickettsioses (SFR).
There is no known death rate for Rickettsia lanei because it is so rare and so newly described, but the death rate for the similar Rocky Mountain spotted fever is between five and ten percent.
The rabbit tick is known for its association with the bacteria Rickettsia rickettsii, which is the primary cause of Rocky Mountain spotted fever. However, a paper in 2018 detected ‘a novel spotted fever… genotype’ in California.
While it is similar to Rickettsia rickettsii, it ‘belongs to its own well-supported branch different from’ previously identified strains.
Pictured above is an aerial view of a town located in Sonoma County, California, which is home to nearly 500,000 residents and sees about 10 million tourists per year
Officials confirmed the new case to SF Gate, but did not reveal any details on the patient other than the person was diagnosed in California this year.
Sonoma County is located in northern California and is known for its rich wine region. It is home to nearly 500,000 people and sees about 10 million tourists per year.
While researchers have known about Rickettsia lanei since 2018, it wasn’t known it could infect humans until 2023, when a man visited a California hospital with fever-like symptoms and body aches.
According to a 2024 case report on that patient, doctors tested him for multiple diseases but his condition was quickly deteriorating. He was admitted to the intensive care unit, his oxygen levels declined, he experienced seizures and his blood became acidic.
When doctors finally considered an SFR disease, they gave him the powerful medication doxycycline.
He entered into a coma, developed severe kidney injury and gangrene in both hands.
It took him 22 days to recover and be released from the hospital.
In 2026, researchers identified Rickettsia lanei in ticks in Contra Costa County, where the patient had reported golfing prior to his infection.
Anne Kjemtrup, a research scientist and veterinarian with the California Department of Public Health, told SF Gate: ‘This is an important tick vector that we want people to be aware of.’
And Janet Foley, a professor of medicine and epidemiology at the UC Davis School of Veterinary Medicine, told the outlet: ‘It is the most dangerous, highly lethal vector-borne disease … in all of the Americas.’
The incidence of Rickettsia lanei is rare, but up to 7,000 people are infected with SFR diseases each year in the US.
And 5,000 people are infected with Rocky Mountain spotted fever.
According to data from the CDC, the group of diseases falling in the category of SFR are most concentrated in central and southern US, including Kansas, Oklahoma, Missouri, Arkansas, Mississippi, Alabama, Virginia, West Virginia, Tennessee, North Carolina and Kentucky, as well as Arizona.
From 2019 to 2023, five states accounted for over 50 percent of spotted fever cases, including Alabama, Missouri, North Carolina, Arkansas and Tennessee.
Shown above are heat maps displaying the number of Dermacentor occidentalis (A) and Haemaphysalis leporispalustris (B) ticks that tested for spotted fever group Rickettsia in 2024, by county in California. The color scale represents the number of ticks and the black stars represent where ticks tested positive for Rickettsia lanei
Rickettsia lanei was first identified in 2018 in Sonoma County, California in rabbit ticks (pictured above)
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North Carolina had the most SFR cases in 2023, the latest date available, with an incidence rate of 21.13 cases per 1 million people. It was followed by Arkansas with an incidence rate of 20.86 per 1 million people.
Kentucky was third with 20.77 cases per 1 million people; Missouri was fourth, with a rate of 18.08 cases per 1 million people and Alabama was fifth, with a rate of 15.66 cases per 1 million people.
California recorded just 0.31 cases of spotted fever diseases per 1 million people in 2023.
According to the CDC, SFR cases are more frequently reported in men than women and people over the age of 40 account for the highest number of reported cases.
However, children under 10 represent the highest number of reported deaths due to the group of diseases.
Symptoms of SFR can include fever, muscle aches, headache and a rash on the limbs.
The antibiotic doxycycline is commonly used to treat SFR. Delaying treatment by just a few days can greatly increase the risk of severe complications and death. Immediate identification and treatment is crucial.
