Pathogenesis of Tularemia in Monkeys Aerogenically Exposed to Francisella tularensis 425

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Abstract
The pathogenesis of tularemia was studied in groups of rhesus monkeys (Macaca mulatta) that inhaled graded 10-fold doses ranging from 10 through 106 organisms of Francisella tularensis 425, a strain highly virulent for the white mouse but of reduced virulence for the domestic rabbit. Mean incubation periods ranged from 3 to 6 days followed by acute illness lasting 5 to 11 days with subsequent recovery of most animals. The higher inhaled doses resulted in shorter incubation periods, longer and more severe acute illnesses, and 18% mortality at the highest dose. Strain 425 multiplied in the lungs, disseminated to the regional lymph nodes, and became systemic. Maximal bacterial populations in tissues were reached by the 7th day after exposure of the animals regardless of the number of organisms inhaled. F. tularensis was no longer recoverable from any of six tissues examined 2 months after exposure. The most significant tissue changes occurred in the lungs; these consisted of foci of liquefaction necrosis, lobular consolidation, and pleural effusion and adhesions. The data indicate that the inhaled dose of strain 425 determined the maximal growth of the organism in the lungs which in turn influenced the severity of the usually self-limiting pneumonia and systemic infection. Although the monkey is less resistant to tularemia than is man, this laboratory animal when infected with F. tularensis 425 provides a useful model for the self-limiting type of human pulmonary tularemia usually observed in Europe and Asia but to a lesser extent in North America.