Relapsing Fever in Emergency Medicine 

Relapsing fever is an acute febrile illness caused by spirochetes of the genus Borrelia. The high fevers of presenting patients spontaneously abate and then recur. This characteristic pattern of remission and relapse not only gives relapsing fever its name but also allows it to be differentiated clinically from other febrile illnesses as it has since the 1840s.

Large outbreaks of louse-borne relapsing fever have occurred throughout the past century. These outbreaks usually occur following man-made breakdowns in public health, as typified by the epidemic following World War II that involved about 10 million people.

Relapsing fever is transmitted to humans by 2 vectors, ticks and lice. The human body louse, Pediculus humanus, is the specific vector (Pediculus pubis is not a vector). Louse-borne relapsing fever is more severe than the tick-borne variety.

Louse-borne relapsing fever is caused by Borrelia recurrentis. No animal reservoir exists. Lice that feed on infected humans acquire the Borrelia organisms that then multiply in the gut of the louse. When an infected louse feeds on an uninfected human, the organism gains access when the victim crushes the louse or scratches the area where the louse is feeding. B recurrentis infects the person via either abraded or intact skin (or mucous membranes) and then invades the bloodstream.

Soft ticks of the genus Ornithodoros spread the tick-borne variety. The responsible Borrelia species are identified closely with its tick vector and they share parallel nomenclature. (For example, Borrelia parkeri infects Ornithodoros parkeri; Borrelia hermsii is the agent transmitted by tick bite by Ornithodoros hermsii.) Soft ticks feed for short periods of time (an hour or so), and the Borrelia organisms are inoculated within minutes. This is an important distinction from other tick-borne diseases such as Lyme disease. Transplacental transmission has been reported.

Regardless of the mode of transmission, a spirochetemia develops. Borrelia organisms then invade the endothelium. This can produce a low-grade disseminated intravascular coagulation and thrombocytopenia. The relapse phenomenon occurs because of genetically programmed shifting of outer surface proteins of the Borrelia that allows a new clone to avoid destruction by antibodies directed against the majority of the original infecting organisms. Thus, the person clinically improves until the new clone multiplies sufficiently to cause another relapse. Tick-borne disease tends to have more relapses (average of 3) compared with the louse-borne variety (often just 1).

The recent resurgence of interest in Borrelia because of Lyme disease and, especially the recent publication of the genomic sequence of B burgdorferi, has led to advances in the understanding of the host-parasite interactions of the relapsing fever Borrelia.

Frequency

United States

Louse-borne relapsing fever is not endemic in the United States, although an occasional traveler presents with an imported case.

Few cases of tick-borne relapsing fever are reported in the United States; however, sporadic cases continue to occur. It is highly focal, with 13 counties producing 50% of cases. Most of these are found in the late spring and summer in the western mountainous states, south into Texas, and northwest into Washington.

Undoubtedly, many cases occur that either are misdiagnosed or go unreported.

Clusters of cases are reported; often groups of campers share a rustic facility infested with rodents on which the ticks feed.

International

Endemic foci of louse-borne relapsing fever occur in much of the world where war, poverty, and overcrowding exist; all of which are conditions that favor louse infestation. Civil wars, which result in large refugee camps, are also fertile ground for lice and relapsing fever.^{[1, 2, 3]}

Mortality/Morbidity

• Mortality rates from 30-70% are reported in untreated patients during epidemics of the louse-borne variety; the mortality rate falls to about 5% with treatment. This striking figure probably reflects the underlying malnutrition and coexisting infections that exist in these situations.
• The mortality rate of patients with tick-borne relapsing fever who are treated is less than 1%.

Sex

A slight preponderance of female patients exists in louse-borne epidemics (60%); tick-borne relapsing fever occurs more often in males (60%) than in females. The latter figure probably reflects the greater likelihood of males being exposed to ticks through recreational and occupational activities.

Age

A trend toward pediatric cases of both forms of relapsing fever exists. In the case of the louse-borne variety, this may reflect the general state of health in populations where relapsing fever is endemic. Regarding the tick-borne disease, this may reflect activities that lead to tick exposure.