Outbreak of Tick-Borne Tularemia -- South Dakota

Between May 30, and July 15, 1984, 20 definite and eight probable cases of tularemia were reported among residents of the adjoining Lower Brule and Crow Creek Indian Reservations in central South Dakota. All the patients were native Americans ranging in age from 2 years to 31 years (median 6 years). The attack rate for reservation residents 0-9 years of age was 2%; for those 10-19 years of age, 0.2%; and for those 20 years and older, 0.2%. Sixteen of the patients were male. Twenty-two (79%) of the patients reported a tick bite, and none had contact with rabbits or dead animals or had eaten rabbit meat.

Most patients presented with fever, headache, and adenopathy. All the patients for whom a tick-bite location was known had been bitten on the head or neck. These patients presented with regional adenopathy draining the area of the tick bite. All 28 patients had either cervical, submandibular, occipital, or preauricular adenopathy. Four patients also appeared to have enlarged parotid glands and presented with a clinical picture that resembled mumps. Seven patients had pharyngitis. Eight had a fourfold rise in serum agglutination titer of 1:160 or greater to Francisella tularensis; 12 patients had a single convalescent titer of 1:160 or greater; and eight with pending convalescent serology had compatible clinical illnesses. Three lymph-node aspirates did not yield F. tularensis. Twenty-six patients were treated with streptomycin; two, with tetracycline. All responded to antimicrobial therapy.

Environmental investigation revealed few ticks on vegetation near the homes, but ticks were found on vegetation around the streams and rivers on the reservation. Forty-six (73%) of the 63 dogs that were examined on the two reservations were infested with ticks. Ticks collected from both vegetation and dogs were identified as Dermacentor variabilis. These ticks, as well as three mud and three water samples from areas where children play on the reservation, were cultured for F. tularensis. Tick lots from eight (17%) of the 46 dogs were positive. Mud and water samples were negative. Biochemical analysis of the F. tularensis isolates revealed that seven were type B, and one was type A.

Most families owned several dogs, and stray dogs were abundant on the reservations. It is likely that tularemia was seen predominately in children because of their increased exposure to ticks through their frequent contact with dogs and outdoor activities in tick-infested areas.

Recommended prevention measures included continuing an educational program on tularemia for reservation residents, dusting dogs with tick powder (6% malathion), and cutting grass around the homes to prevent tick harborage. Reported by P de la Cruz, MD, L Cummings, D Harmon, D Mosier, MS, P Johannes, J Lawler, MS, F Pintz, MD, Aberdeen Area Indian Health Svc, K Senger, State Epidemiologist, T Dosch, South Dakota Dept of Health; Div of Bacterial Diseases, Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This outbreak is similar to two previously reported tick-borne outbreaks of tularemia in the United States. In 1966, 12 cases occurred on the Pine Ridge and Rosebud Indian Reservations in South Dakota (1); in 1979, 12 cases occurred on the Crow Indian Reservation in Montana (2). Infection in those two outbreaks also occurred predominately among children, and the presentation was mild glandular or ulceroglandular tularemia. Adenopathy in the head and neck areas, similar to the clinical picture in this outbreak, was also described in those outbreaks. In both prior outbreaks, D. variabilis was the tick vector, and F. tularensis was isolated from ticks.

Two subtypes of F. tularensis have been recognized (3). Type A strains, which have been found only in North America, are more virulent and cause illness that, without treatment, has a 5%-7% mortality rate. Type B strains are less virulent. These strains differ biochemically in that type A utilizes glycerol and is citrulline ureidase positive (4). In this outbreak, seven of the eight F. tularensis isolates from ticks were type B. Although no human isolates were obtained, the mild clinical illness was consistent with disease caused by type B F. tularensis. Disease caused by type B strains have been most commonly associated with exposure to contaminated water or aquatic animals, rather than insect vectors. However, type B strains were also isolated from ticks in the 1979 outbreak in Montana (2).

Because glandular tularemia can be mild, as in the current outbreak, and can mimic other illnesses such as pharyngitis or mumps, cases may be misdiagnosed. Physicians in areas endemic for tularemia should be aware of the manifestations of glandular tularemia so that cases can be identified and appropriately treated.

References

1. Saliba GS, Harmston FC, Diamond BE, Zymet CL, Goldenberg MI, Chin TDY. An outbreak of human tularemia associated with the American dog tick, Dermacentor variabilis. Am J Trop Med Hyg 1966;15:531-8.

2. Schmid GP, Kornblatt AN, Connors CA, et al. Clinically mild tularemia associated with tick-borne Francisella tularensis. J Infect Dis 1983;148:63-7.

3. Jellison W. Tularemia in North America, 1930-1974. Missoula, Montana: University of Montana 1974.

4. Marchette NJ, Nicholes PS. Virulence and citrulline ureidase activity of Pasteurella tularensis. J Bacteriol 1961;82:26-32.

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