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Recurring fever? Consider soft tick relapsing fever (STRF), a Borrelia bacteria infection transmitted to humans by way of soft-bodied ticks that, while rare, has been seen across western and southwestern states.

There were about 24 cases per year, totalling 251 cases reported from 2012 to 2021 across 11 states. One-third of cases occurred in California, followed by Washington, Colorado, and Oregon, with a smattering of cases in Arizona, Texas, Idaho, Utah, Montana, Nevada, and New Mexico.

More than half of those infected patients were hospitalized, although none died, reported Amy M. Beeson, MD, of the CDC's National Center for Emerging and Zoonotic Infectious Diseases in Atlanta, and colleagues in .

STRF arises from Borrelia species infection transmitted by Ornithodoros ticks, which commonly live in rodent nests and burrows, through brief and painless bites that can go undetected. Infection starts with an incubation period of about 7 days, then leads to a high fever with headache, nausea, and muscle or joint pain lasting about 3 days. If left untreated, the symptoms recur in 7-10 days, again lasting for about 3 days, with repeated recurrence until treated, according to the .

Early antibiotics -- doxycycline, beta-lactam antibiotics (e.g., penicillin or ceftriaxone), and azithromycin -- are critical to prevent rare complications such as neurologic and ocular disease, myocarditis, and acute respiratory distress syndrome. Infection during pregnancy can result in transplacental transmission, loss of pregnancy, and neonatal death.

"Clinicians should be aware of the clinical symptoms associated with STRF, risk factors, options for diagnostic testing, and public health reporting requirements," Beeson and colleagues wrote. Twelve states currently have reporting requirements when cases are seen (the 11 listed above, plus Montana).

Although diagnostic testing provides diagnosis at different stages of the illness, the emergence of hard tick relapsing fever (HTRF) caused by Borrelia miyamotoi in some of the same geographical areas as STRF can complicate accurate diagnosis, since serologic assays do not distinguish the two infections. Lyme disease is also often a misdiagnosis for this infection, they added.

For the 221 cases in which laboratory test data were available, microscopy of peripheral blood smear was used to identify spirochetes in 59%. Antibodies were detected by serologic testing in 41%, and relapsing fever Borrelia DNA was detected by PCR testing in 15%.

"Though molecular diagnostic testing has become increasingly available in recent years, microscopic examination of peripheral blood smears remains an important diagnostic test; microscopy is most sensitive when performed during febrile episodes because fever is associated with coincident high levels of spirochetemia," the group wrote. "Increased awareness of and access to molecular diagnostic testing for symptomatic patients with suspected STRF might improve recognition of cases at different stages of illness."

Progress in surveillance, prevention, and disease recognition would also help reduce STRF incidence, the researchers noted. "A regional standardized case definition has been developed by vector-borne disease epidemiologists in several states with endemic diseases; broader adoption of this case definition would enhance STRF surveillance."

Coordinated improvements in those areas "have the potential to prevent morbidity and mortality from STRF in the United States in the next decade," Beeson and colleagues added.

They also urged avoiding situations where one is likely to come in contact with infected ticks: "People should avoid sleeping in rodent-infested structures such as rustic mountain cabins and caves where STRF is common in the western United States; if rodents are present, infestations should be addressed by licensed pest control professionals."

In western states, 74% of cases occurred following exposure to cabins and 8% after camping trips. Not surprisingly, there was a summer (June-September) peak in cases there.

Endemic in Africa, Asia, Europe, and the Americas, the two species of ticks that most commonly transmit the infection in the U.S. are the Ornithodoros hermsi tick that spreads Borrelia hermsii (most likely to be found in mountainous areas of western states, with infection often associated with rustic, rodent-infested cabins) and Ornithodoros turicata that spreads Borrelia turicatae (commonly found in the south-central U.S., often in caves).

However, Beeson's group noted: "A large proportion of cases continue to occur in nonresident visitors to areas where the disease is endemic (such as vacationers to mountain cabins); cases in returned visitors who live in areas where STRF is not endemic or reportable would be more likely to be missed by clinicians and public health authorities."

Among 232 cases where the patient's state of residence and exposure were known, 14% occurred in out-of-state visitors; among 210 cases for which the county of the patient's exposure was available, 56% occurred when people were outside their county of residence.

In areas where the infection is endemic, residents and visitors should be educated how to prevent getting bitten by soft ticks, Beeson's group wrote. "Owners of tick- or rodent-infested cabins should be made aware of recommendations for remediation of these structures."

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