PLAGUE AND BACILLUS PESTIS 811 



for as long as ten years in the ice chest were found living and 

 virulent at the end of this time. 



In regard to the viability of plague bacilli in air at different 

 atmospheric temperatures and conditions of humidity, there are 

 many important sanitary problems involved which are of particular 

 significance in connection with the spread of pneumonic plague. 

 Teague and Barber 7 worked on this subject in connection with the 

 Manchurian epidemic of pneumonic plague, and found that plague 

 bacilli contained in fine droplets of pneumonic plague sputum would 

 suffer death from drying in a few minutes unless they were sus- 

 pended in an atmosphere with a very small water deficit; in other 

 words, the humidity or the degree of saturation of the atmosphere 

 with water is a very important factor in determining the length of 

 time for which plague bacilli will remain alive in such droplet- 

 spray. Such atmospheres under ordinary circumstances are common 

 in cold climates and droplets of sputum will, therefore, remain 

 infectious longer in cold, wet climates than in warm ones. 



Animal Pathog"enicity. Bacillus pestis is extremely pathogenic 

 for rats, mice, guinea-pigs, rabbits, and monkeys. The most sus- 

 ceptible of these animals are rats and guinea-pigs, in whom mere 

 rubbing of plague bacilli into the unbroken skin will often produce 

 the disease. This method of experimental infection of guinea-pigs 

 is of great service in isolating the plague bacillus from material 

 contaminated with other microorganisms. For the same purpose, 

 infection of rats subcutaneously at the root of the tail may be 

 employed. Such inoculation in rats is invariably fatal. 



The studies of McCoy 8 of the United States and Public Health 

 Service upon guinea-pigs and white rats show that individual plague 

 cultures may vary considerably in virulence. The size of the dose, 

 always excepting enormous quantities such as a whole agar culture, 

 seems to make little difference in the speed with which the animals 

 die. There may be considerable variation in the susceptibility of 

 individual animals. Prolonged cultivation on artificial media may 

 gradually reduce the virulence of plague bacilli, though, as stated 

 above, this has not been the experience of all observers. 



T Teague and Barber, Philippine Jour, of Science, B, 7, 1912. 



8 McCoy noted the surprising fact that, in San Francisco a considerable per- 

 centage of wild rats especially old ones, showed a high natural immunity to 

 plague. 



