Tickborne relapsing fever (TBRF) is a zoonosis caused by spirochetes of the genus
Borrelia and transmitted to humans by ticks of the genus Ornithodoros. TBRF is endemic
in the western United States, predominately in mountainous regions. Clinical illness
is characterized by recurrent bouts of fever, headache, and malaise. Although TBRF
is usually a mild illness, severe sequelae and death can occur (1–4). This report
summarizes the epidemiology of 504 TBRF cases reported from 12 western states during
1990–2011. Cases occurred most commonly among males and among persons aged 10–14 and
40–44 years. Most reported infections occurred among nonresident visitors to areas
where TBRF is endemic. Clinicians and public health practitioners need to be familiar
with current epidemiology and features of TBRF to adequately diagnose and treat patients
and recognize that any TBRF case might indicate an ongoing source of potential exposure
that needs to be investigated and eliminated.
TBRF is not nationally reportable, and there is no standard case definition. For the
purpose of this report, a TBRF case was defined as a clinically compatible illness
with laboratory confirmation of infection or a clinically compatible illness epidemiologically
linked to a laboratory-confirmed case. In 2011, TBRF was reportable in 12 states:
Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, North Dakota, Oregon,
Texas, Utah, and Washington. TBRF case data for these 12 states for the period 1990–2011
were compiled, along with a single case reported to CDC from Wyoming, yielding 504
cases. Three states accounted for approximately 70% of all reported TBRF cases (California,
33%, Washington, 25%, and Colorado, 11%); the remainder were reported from Idaho,
7%, Nevada, 5%, Oregon, 4%, Arizona, 4%, Texas, 4%, New Mexico, 3%, Montana, 2%, Utah,
2%, and Wyoming, <1% (Figure). No cases were reported from North Dakota. County of
residence and county of exposure were known for 325 (64%) cases; 215 (66%) of these
cases were reported among nonresident visitors to the counties of exposure (Table).
The median number of cases per year was 20, with a range of 14 in 1993 to 45 in 2002.
Median age of patients was 38 years (range = 1–91 years). The age distribution was
bimodal, with peaks among persons aged 10–14 years and 40–44 years; 278 (57%) of the
patients were male. Race information was not available in the reported data.
Blood smear was indicated as the method of diagnosis for 184 (76%) of 243 cases for
which diagnostic information was available. Most (74%) patients had onset of illness
during June–September with a peak during July–August (52%). In Texas, cases occurred
more frequently (67%) during November–March, and 11 cases (61%) were associated with
spelunking.
Most TBRF cases in the United States are caused by Borrelia hermsii and transmitted
by Ornithodoros hermsi ticks. These soft ticks typically live in the nests of rodents
such as ground squirrels, tree squirrels, and chipmunks in coniferous forests at elevations
between 1,500 and 8,000 feet (457 and 2,438 meters) (5). Soft ticks can acquire TBRF
Borrelia by feeding on infected rodents, the reservoir hosts; once infected, soft
ticks remain infectious for life (6,7). The spirochete, which resides in the salivary
gland of the soft tick, can be transmitted within 30 seconds of initiation of a blood
meal (5). If the rodent reservoir host dies or vacates the nest, soft ticks seek other
sources of blood. In locations where rodents and humans are in close proximity (e.g.,
seasonally occupied lake or mountain cabins infested by rodents), human infections
can occur (8,9). Unlike hard ticks that embed in the host, soft ticks feed briefly
(up to 30 minutes) and typically at night, so most patients are unaware that they
have been bitten (5,6).
The characteristic clinical feature of TBRF is the occurrence of febrile episodes
lasting 3–5 days, with relapses after 5 to 7 days of apparent recovery. This pattern
is the result of antigenic variation in spirochete outer surface proteins, temporarily
evading the host immune response and allowing spirochete numbers to rebound (5). TBRF
is treated with antibiotics, which typically results in cure without sequelae (5).
However, complications such as acute respiratory distress syndrome have been described
(1,3). The risk for transplacental transmission has been documented and pregnant women
might be more susceptible to severe complications such as spontaneous abortion, preterm
delivery, and perinatal mortality (2,4). Clinicians need to consider TBRF in patients
with compatible clinical illness and a history of residence in or recent travel to
areas that are known foci for TBRF. A diagnosis of TBRF can be confirmed by observation
of spirochetes in a blood smear taken during a febrile episode and either stained
with Wright-Giemsa stain or examined with dark field microscopy (5,10). Testing for
serum antibodies is not valuable in the acute setting but might be useful for retrospective
identification in convalescent patients (5).
No overall increase or decrease in the annual number of cases reported was observed
during the reviewed time period. The bimodal age distribution could reflect differences
in clinical manifestations, health care seeking behavior, or exposure to infected
ticks. Most cases occurred during the summer months, consistent with arthropod vector
biology, reservoir host biology, human outdoor activity, and vacation seasons (5).
Outbreaks have been reported among groups of young persons on trips, particularly
those sleeping on floors, which might further explain the age distribution (9). Notably,
cases in Texas occurred more frequently in winter months and were associated with
time spent in caves, which likely represents infection with Borrelia turicatae, another
species of TBRF Borrelia transmitted by Ornithodoros turicata ticks (5).
This report is subject to at least two limitations. First, case ascertainment depends
upon state-specific practices, and there is no standard surveillance case definition
in the 12 western states where TBRF is reportable. Differences in case definitions
could lead to ascertainment and reporting bias. Second, TBRF cases likely represent
a fraction of the actual incidence because many patients might experience mild, self-limited
illness that goes undiagnosed.
Because tick-infested buildings can serve as a source of infection for years, it is
important to investigate all TBRF cases to identify the likely location of exposure
and guide remediation of rodent and tick infestations. Rodent control alone can increase
human risk because any remaining ticks, which can be long-lived, will repeatedly search
for alternative hosts. Therefore, it is important to consider tick control in concert
with rodent control. Personal preventive practices can include sleeping off the floor
and away from walls in rodent-infested buildings and eliminating incentives for rodent
residence (e.g., by storing food in tightly sealed containers).* Homeowners in areas
where TBRF is endemic can consult with local environmental health specialists and
pest removal services on strategies to discourage rodent activity in homes. Persons
living in or vacationing in areas where TBRF has been reported need to be aware of
the disease and seek medical attention if they develop febrile illness.† Educational
outreach would further public health objectives to increase awareness of TBRF prevention
measures and clinical signs and symptoms of disease.§
What is already known on this topic?
Tickborne relapsing fever (TBRF) is an uncommon cause of febrile illness in the western
United States. The most significant risk factor for infection is sleeping in a rodent-infested
cabin or house. In 2011, TBRF was reportable in 12 states.
What is added by this report?
During 1990–2011, a total of 504 cases of TBRF were reported to CDC. Cases occurred
most commonly among males and among persons aged 10–14 and 40–44 years. Three states,
California, Washington, and Colorado, accounted for approximately 70% of all reported
cases. In counties where most reported TBRF exposures occurred, most infections were
among visitors to the counties. Most TBRF infections occur during the summer months
during peak arthropod, host, and human activity.
What are the implications for public health practice?
Public health practitioners need to be aware of TBRF in locations where it is endemic,
and the importance of recognizing and eliminating foci of transmission. Clinicians
need to consider TBRF as a cause of febrile illness in visitors to, and persons living
in, areas where TBRF is endemic.