Disease Control and Environmental Epidemiology Division

Relapsing Fever

Tree squirrels, chipmunks, and other wild rodents found in coniferous forests in the higher elevations of the Western United States serve as the primary reservoirs for the relapsing fever spirochete. The soft-bodied ticks (Ornithodoros sp.) that associate with these rodents can remain alive and infectious for years without feeding.

People with tick-borne relapsing fever suffer cyclical high fevers and other symptoms such as headaches and pain in the joints or muscles that easily can be mistaken for a severe flu. These episodes usually last several days, alternating with periods when the symptoms cease. In most patients, the infection responds to treatment with antibiotics. The disease is under recognized and underreported, and often mistaken for Lyme disease.

Human cases of illness tend to peak in the warmer months, but they can occur year-round. A tick population often becomes established with rodents that inhabit rustic mountain cabins. If the rodents die off, leave, or hibernate, the ticks look for other hosts. In winter, people will stay in these cabins and warm them up for a week. The rodents are not active, the ticks get warmed up, and they become hungry and start moving around looking for a food source. A person who's breathing is basically a carbon dioxide generator. The ticks actually orient to a carbon dioxide gradient, and this is one of the ways they find their hosts.

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