Add a new spotted fever group Rickettsia to the long list of pathogens carried by West Coast ticks.

Researchers with the California Department of Public Health (CDPH) have identified a new species of rickettsial bacterium, called Rickettsia sp. CA6269. It is now confirmed to cause severe illness in humans.

The report, published in the July 2024 issue of CDC’s Emerging Infectious Diseases journal, documents two severe cases of Rocky Mountain spotted fever-like illnesses in patients residing in Northern California.

First found in rabbit ticks

The pathogen was first detected in rabbit ticks (Haemaphysalis leporispalustris) in Northern California in 2018. The researchers who discovered the pathogen have proposed naming it Candidatus Rickettsia lanei, after Robert S. Lane, PhD, Professor Emeritus of Medical Entomology, at the University of California, Berkeley.

Professor Lane, internationally recognized for his research on ticks and tick-borne diseases since the mid-1970s, has served on California’s Lyme Disease Advisory Committee since its inception in 2000 to the present.

For this study, CDPH researchers examined blood samples taken from a 2023 patient (first case below) and eight confirmed rickettsiosis cases collected over the past 20 years.

They used triplex real-time reverse transcription PCR (rRT-PCR)—a highly specialized tool to quickly detect specific RNA sequences. With this technique, researchers identified a new species of Rickettsia that very closely resembles Rickettsia rickettsii—the cause of Rocky Mountain spotted fever (RMSF).

Both patients were in San Francisco Bay Area

In the first case, the patient had been golfing several times in the San Francisco Bay Area but had no recollection of a tick bite. In the second case, the patient had been camping at two different parks in the San Francisco Bay Area. He remembered seeing a tick crawling on his body but did not recall a bite. (Note: In nearly half of all reported Rocky Mountain spotted fever cases, individuals do not recall a tick bite.)

Neither patient had traveled outside the San Francisco Bay Area in the two to three weeks prior to becoming ill.

In both cases, they were admitted to the hospital for high fever, severe headache, nausea, vomiting, diarrhea, abdominal pain, and other symptoms. One patient had a rash characteristic of RMSF. The other developed cutaneous necrosis (irreversible injury to skin cells) and gangrene, and lost portions of several fingers on both hands.

Each patient was started on a triple-combination of antibiotics that included ceftriaxone and vancomycin but did not include doxycycline.

Within days of being hospitalized, each patient went into a coma and respiratory failure and was transferred to intensive care. Each was then given a presumptive diagnosis of RMSF and started on doxycycline.

The first patient spent 22 days in the hospital, the second 13 days before being sent home. Both had continuing symptoms upon discharge.

Severe illness

Based upon these cases, Rickettsia CA6269 (lanei) can progress into a severe life-threatening illness. While the rabbit tick rarely bites humans, it may have been the vector in at least these two cases although that awaits confirmation.

Rickettsial infections are prevalent worldwide but remain significantly under-diagnosed because clinicians are not aware of them, the general public also is not aware of them, and diagnostic tests are either not available, slow to detect infection, and/or non-specific.

At a “public engagement meeting,” on June 11, Anne Kjemtrup, DVM, MPVM, PhD, with the California Department of Public Health, gave an update on RMSF.

In the image below, the number of reported California cases of RMSF is summarized by county of residence. Of those cases acquired outside of California, 36% were from tick bites in Mexico, and 34% were from visits to the Southeastern U.S.

Signs and Symptoms of RMSF

Early signs and symptoms of RMSF can be vague and non-specific, including fever and headache. However, the disease can rapidly progress to a life-threatening illness, even before a rash appears.

Signs and symptoms can include:

• Fever
• Headache
• Rash
• Nausea or vomiting
• Stomach pain
• Muscle pain
• Lack of appetite

The CDC advises immediate treatment with doxycycline whenever rickettsiosis is suspected.

LymeSci is written by Lonnie Marcum, a physical therapist and mother of a daughter with Lyme. She served two terms on a subcommittee of the federal Tick-Borne Disease Working Group. Follow her on Twitter: @LonnieRhea  Email her at: lmarcum@lymedisease.org.

Resources

• CDC: About Rocky Mountain Spotted Fever
• CDC: Clinical Signs and Symptoms of Other Spotted Fever Rickettsioses
• CA Dept. Public Health: Spotted Fever Group Rickettsia (Rocky Mountain Spotted Fever & Pacific Coast Tick Fever)
• LymeSci: A Closer Look at Rickettsial Infections
• Rocky Mountain spotted fever warning for travelers to Mexico

References

Probert WS, Haw MP, Nichol AC, Glaser CA, Park SY, Campbell LE, et al. Newly recognized spotted fever group Rickettsia as cause of severe Rocky Mountain spotted fever–like illness, Northern California, USA. Emerg Infect Dis. 2024 Jul [date cited]. https://doi.org/10.3201/eid3007.231771

Eremeeva ME, Weiner LM, Zambrano ML, Dasch GA, Hu R, Vilcins I, Castro MB, Bonilla DL, Padgett KA. Detection and characterization of a novel spotted fever group Rickettsia genotype in Haemaphysalis leporispalustris from California, USA. Ticks Tick Borne Dis. 2018 May;9(4):814-818. doi: 10.1016/j.ttbdis.2018.02.023. Epub 2018 Mar 1. PMID: 29545107.