﻿IV. ANIMALS EMPLOYED— THEIR SUSCEPTIBILITY TO 

 PEST INFECTION— LETHAL DOSE— TECH- 

 NIQUE OF INFECTION. 



Eats and monkeys, in addition to guinea pigs and rabbits, mainly 

 have been employed in the experimental work. 



Rats. — The rats used in the majority of cases were the ordinary wild 

 ones found in Manila belonging to the species Mus decumanus, and in 

 a few instances to the species M. rattus. They contract pest infection 

 naturally and have been known to be concerned in the spread of plague 

 in Manila since 1902. During that year the pest bacillus was found 

 in the organs of 0.34 per cent and in the following year in 0.0625 per 

 cent of these rodents collected from various portions of the city of 

 Manila and sent by the Board of Health to the laboratory for examination. 

 The majority of the rats so infected had been found dead or were captured 

 in the houses where cases of human plague had occurred. Since the 

 decline of plague in Manila, only an occasional rat has been found to 

 be infected. Wherry reported from this laboratory one such instance 

 in 1905, and during the present year (1906) the pest bacillus has been 

 isolated but twice from rats collected in Manila and sent to the laboratory 

 for examination. These wild rats could be obtained alive in great abun- 

 dance and on account of the frequent scarcity and the difficulty of breeding 

 white rats here in this city, the wild species was used in the majority of 

 the plague experiments and the}' proved very satisfactory animals. The 

 rats were always kept for some time after their capture before being 

 employed for experimental purposes. They were satisfactorily handled 

 during the inoculations by two Filipino assistants, who protected them- 

 selves from being bitten by wearing extra heavy leather gloves. Eat 

 tongs or pincers were not employed because of the danger of traumatic 

 injuries to the animals. As was expected, the species M. decumanus 

 proved itself to be readily susceptible to plague infection. In the 

 experimental subcutaneous inoculations of these rats a second transfer 

 of a 48-hour culture of "Pest Virulent" was suspended in 5 cubic cen- 

 timeters of bouillon or saline solution and a syringe needle from a 5 

 cubic centimeter syringe dipped in this s^^spension and then thrust 

 beneath the skin near the base of the tail of the rat. Alter such an 

 inoculation the animal almost invariably succumbed to pest infection, 

 usually in from three and one-half to five days, although life sometimes 

 was prolonged to from the seventh to the fifteenth day after inoculation. 

 170 



