Clinically mild tularemia associated with tick-borne Francisella tularensis

Between May 9 and July 3, 1979, 12 cases of glandular or ulceroglandular tularemia occurred in residents of the Crow Indian Reservation in southcentral Montana; only 13 cases had been reported from this geographic area in the preceding 25 years. The illness was mild, characterized by fever and cervical or occipital adenopathy. Systemic symptoms were self-limited although residual lymphadenopathy was common. Francisella tularensis was isolated from ticks (Dermacentor variabilis), the suspected vector. The strains of F tularensis did not ferment glycerol and thus were identified as type B rather than the more virulent type A. None of 83 adults hospitalized in an urban area 50 miles from the reservation had agglutination titers of antibody to F tularensis of greater than or equal to 1:40 compared with eight of 77 patients at the reservation hospital (P less than 0.01). Mild tularemia in reservation residents may have gone unrecognized; similar illness due to type B F tularensis may occur elsewhere.