183 



EXPERIMENTS UPON RAT VIRUS IN THE HYGIENIC LABORATORY. 



In 1901 1 made an investigation of this pathogenic microbe (B. typhi 

 murium) applied to the destruction of rats under laboratory condi- 

 tions^ One hundred and fifteen rats were fed with the cultures in 

 various ways during the course of these experiments with the virus. 

 Of these, 46 died less than half. 



Most of the rats used were the gray rat (M. decumamis or norve- 

 gicus] and the tame white rat. A few (8) of the wild black or house 

 rats ( M. rattus) were used. 



The virus is in reality pathogenic for these three kinds of rats when 

 ingested. No special difference was noted in its effects upon the 

 various species. 



As the work progressed it soon became evident to me that the 

 result depended largely upon the amount of the culture ingested. 

 By starving rats for a day or two and then giving them all they could 

 be induced to eat and drink of the cultures, a very positive result 

 was obtained. In one instance of 27 rats so fed all died within a 

 week. If the rats are given a small amount the effect is uncertain 

 only a few die. In one instance I fed 70 rats with 4 agar tubes and 

 only 7 died. Feeding them a second time with very large quantities 

 9 more died. The survivors were then fed with all they could be 

 induced to eat every day for a week without effect. 



It seems plain, therefore, that a large primary dose proves fatal, 

 and a small dose is not only uncertain, but produces an immunity. 

 This is a very important factor, for it is likely that in the wild state 

 rats would often partake of an amount too small to cause death. 

 Such rats may then subsequently eat large amounts of the culture 

 with impunity. 



It would seem then that, after all, the virus is not so different from 

 the laying of a chemical poison, depending as it does for its effect 

 upon the amount ingested. A chemical poison, however, does not 

 possess the disadvantage of producing an immunity. Another dis- 

 advantage possessed by the virus is the rapid deterioration in viru- 

 lence which occurs when it is exposed to the action of air and light, 

 or when it becomes dry, as is very apt to happen when laid out for 

 rats in the wild state. 



Since these early experiments, tests have been made of various rat 

 viruses in the Hygienic Laboratory, and the results are given in the 

 following pages: 



a Rosenau, M. J. : An investigation of a pathogenic microbe (B. typhi murium Danysz) 

 applied to the destruction of rats. Bull. No. 5 of the Hygienic Laboratory, U. S. Mar. 

 Hosp. Serv., Washington, 1901, 11 p. 



