Our investigation found that transmission of sylvatic typhus to humans occurred during 3 consecutive winters at the camp. All 4 cases among counselors were epidemiologically linked to cabin A. All had slept in a specific bunk, bunk B, next to a wall that had evidence of flying squirrel infestation. Other counselors sleeping in the same cabin but without direct exposure to bunk B had no evidence of infection. The finding that 10 (71%) of 14 flying squirrels collected at multiple sites showed evidence of R. prowazekii infection indicates that the pathogen is well established among these squirrels.
Although inhalation and transdermal or mucous membrane exposure to infected louse feces are well-established routes of transmission during epidemics of louse-borne typhus, the mechanism by which R. prowazekii
is transmitted from flying squirrels to humans is not well understood. The lack of detectable antibodies to R. prowazekii
in household members of documented sylvatic typhus case-patients (6
) has been used to support the hypothesis that risk for sylvatic typhus in the absence of direct exposure to flying squirrels and their nests is low (9
) and thus may explain why human disease has been reported only sporadically. The cluster of cases described here suggests that where repeated and prolonged close exposure to flying squirrels and their nests occurs, potential for transmission can be high. Only 1 case-patient described here reported direct contact with flying squirrels; however, all case-patients slept for many nights next to a wall that was continuously inhabited by flying squirrels. Nesting material, dander, arthropod feces, or ectoparasites may have been introduced to the living area of the cabin through the many openings in this cabin wall and provided a source of infection for those sleeping in the bunk adjacent to the wall but not elsewhere in the cabin. Because cabins at this camp were occupied continuously during the fall and winter months by the same group of counselors, epidemiologic linking of several infections occurring at different times during consecutive winters was possible. No identified cases occurred during the summer when staff and students lived outdoors in tents, further suggesting that exposure was highly focal to cabin A.
During spring of 2006, cabin A was vacated by staff and students for remediation. External openings were closed by using wire hardware cloth to exclude squirrel entry. The inside wall boards of the cabin were removed, insulation was replaced, and new wall boards were installed. The entire cabin was professionally treated to eliminate ectoparasites. Staff and students were educated about sylvatic typhus and the need to avoid contact with flying squirrels and their nests. Since then, no additional cases of sylvatic typhus have been identified.
In contrast to classic epidemic typhus, which typically causes severe disease and mortality rates up to 4% despite antimicrobial drug therapy, fatal cases of typhus associated with flying squirrels have not been reported (15
). Although flying squirrel isolates of R. prowazekii
are reported to have biological, biochemical, and molecular properties similar to those of other typhus isolates (16
), most cases of sylvatic typhus in the United States are apparently less severe than classic louse-borne epidemic typhus. All 4 patients with confirmed cases reported here had fever, headache, and malaise typical of sylvatic typhus; however, only 2 (case-patients 1 and 2) were hospitalized. None of the case-patients reported having had a rash with their illness. Rash has been reported for only half of patients with flying squirrel–associated typhus (10
) and cannot be considered a reliable sign of the disease. Milder disease associated with sylvatic typhus may result from better nutritional and overall health status of most persons in the United States compared with those in populations affected by louse-borne epidemic typhus during war or other catastrophe. Because sylvatic typhus was not initially considered for any case-patients reported here, despite their occupational history, diagnosis and treatment were delayed. Given the nonspecific signs and symptoms associated with sylvatic typhus, the disease is likely underdiagnosed.
Clinicians should consider sylvatic typhus when evaluating a patient with compatible signs and symptoms and should inquire about potential exposures to flying squirrels. Clinicians who suspect sylvatic typhus on the basis of clinical presentation and history of potential exposure should empirically treat with doxycycline and not withhold treatment pending laboratory confirmation by serologic testing. Submission of blood or skin punch biopsy samples (when rash is present) for PCR analysis may provide options for earlier diagnosis, but these assays are not widely available. Serologic confirmation of infection is based on 4-fold IgG titer increases between acute and convalescent samples; both IgM and IgG can persist for years after infection with R. prowazekii.
Because southern flying squirrels are distributed throughout the eastern United States, other woodland settings frequented by humans during winter may present a similar risk for sylvatic typhus. Parks and campgrounds that maintain rental cabins should be aware of risks associated with flying squirrels and take steps to exclude these animals from structures occupied by humans by sealing openings at attic vents and around roof joists with heavy-gauge screen or similar products. In addition, use of repellents may keep arthropods from biting humans, and premise sprays may be useful for reducing arthropods in building structures. Squirrel removal without concomitant arthropod control is not recommended because the presence of potentially infected external parasites may increase the risk for disease transmission to humans.