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Incidence of Tularemia and Rocky Mountain Spotted Fever among Common Ticks of Arkansas

In the course of a year's field study in Arkansas, 576 pools totalling 28,661 lone star ticks (Amblyomma americanum) were tested for tularemia by animal inoculation, and 11 pools (1.9 per cent) were found positive—an average of one infected tick in 2,605, assuming only one infected individual to each positive pool. The distribution of naturally infected ticks was shown to be spotty, and the finding of a batch of infected, unfed larvae of A. americanum indicates the probable occurrence of transovarian passage of tularemia in this species.

The lone star tick was the only arthropod found naturally infected with tularemia, although the number of other ticks tested was insufficient to justify general conclusions. Tularemia-infected lone star ticks were collected in May, July, and September, coinciding with the reported seasonal incidence of human infections; and the data presented, taken in connection with previous epidemiologic investigations, point to ticks as the chief vectors of tularemia in Arkansas.

Pools of lone star, American dog, black-legged and rabbit ticks were tested by animal inoculation for Rocky Mountain spotted fever organisms (R. rickettsi), but only one successful isolation was made, from the tissues of a guinea pig inoculated from a pool of female American dog ticks. Reactions of several guinea pigs inoculated with material from pools of lone star ticks indicated rickettsial infections, but the organism was not recovered from the test animals.