562 PATHOGENIC MICROORGANISMS 



The curative plague serum prepared by Yersin and others by the 

 immunization of horses with plague cultures has been extensively used 

 in practice and though often disappointing, a definitely beneficial in- 

 fluence on the milder cases has been noted. The sera are standardized 

 by their protective power as measured in white rats. 



THE PLAGUE-LIKE DISEASE OF RODENTS (McCOY) 1 

 Bacterium Tularense (McCoy and Chapin) 2 



McCoy has described a disease occurring in Californian ground 

 squirrels (Citellus beechyi) which presents lesions very similar to those 

 of plague in these animals. In fact the disease was noticed in the 

 course of the systematic examination of rodents by McCoy at the 

 Federal Laboratory in San Francisco. Although McCoy was able to 

 transmit the disease to guinea-pigs, mice, rabbits, monkeys, and gophers, 

 and plague-like lesions could be produced in most of the animals, he was 

 at first entirely unable to cultivate any organism from these lesions. 

 In 1912 McCoy and Chapin finally succeeded in growing the specific 

 bacterium on an egg medium made entirely of the yolk. Mor- 

 phologically it is a very small rod, 0.3 to 0.7 micron in length and often 

 capsulated. The rods stain poorly with methylene blue, better with 

 carbol fuchsin or gentian violet. They are found in large numbers in 

 the spleens of animals dead of the disease. 



1 McCoy, U. S. Public Health Bull. 43, 1911. 



2 McCoy and Chapin, Jour, of Inf. Dis., x, 1912. 



