Virulent strains of Yersinia pestis, Y. pseudotuberculosis and Yersinia enterocolitica invariably autoagglutinated in tissue culture media when grown at 36°C. Avirulent strains did not possess this property. Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica are animal pathogens which can cause a diversity of human disease. Y. pestis, the causative agent of bubonic plague in humans, is highly infectious and lethal to most rodents and many small mammals. Y. pseudotuberculosis and Y. enterocolitica, which pro

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An 8 year study of the influence of climate on the seasonal prevalence of bubonic plague in the Republic of Vietnam has been completed. Climatic conditions were found to influence the course of plague epidemics in 2 ways: 1) by regulating the density of the flea population; and, 2) by regulating the efficiency of Xenopsy/la ebieopis in transmitting the plague bacillus. Slight variations in temperature, relative humidity, and vapor pressure deficits either permit an epidemic to flourish or cause a decline in its intensity.

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The modes of stylostome formation by larvae (chiggers) of Leptotrombidium intermedium, L. fletcheri, L. arenicola, and L. deliense in parasitized mouse skin were studied histologically in relation to their capacity to transmit Rickettsia tsutsugamushi. Three types of stylostome formation were recognized among the different species: the epidermal stylostome formed by the larva of L. intermedium; the mesenchymal stylostome formed by the larva of L. fletcheri; and, the mixed stylostome formed by the larva of both L. arenicola and L. deliense. Dermal inflammations related to the three types of stylostomes were histologically defined. The possible importance of stylostome characteristics to the transmission of rickettsial organisms is discussed.

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