4 
of this bacillus, sold under the name of “Eat Virus,’’ is efficient for i 
the destruction of these rodents. 
The following work was undertaken in order to test the validity of 
this claim. 
THE ORIGIN OE THE VIRUS. 
In 1889 Loeffler discovered the bacillus typhi murium, which he isolated 
from a spontaneous epidemic among white mice, and which he applied 
with success to the destruction of harvest mice (m. arvicola). Other 
bacteriologists have observed similar epidemics and have isolated 
the microbes thereof, morphologically identical with the bacillus of 
Loeffler, but more or less virulent for the several genera and species of 
the little rodent. 
For example : The B. typhi murium of Loeffler was only frankly patho- 
genic for mice (m. musculus) and for harvest mice (m. arvicola). A ; 
bacillus isolated by Laser was pathogenic for the m. agrarius, that iso- | 
lated by Merechkowski for the spermophiles, and finally that isolated by , 1 
Issatchenko for white rats. 1 
Each of these various bacilli is of such variable virulence that it | 
can not be used practically for the destruction of all species of these I 
rodents. 
Danyz, therefore, conceived the notion that it would be of great i 
interest, first to extend the field of action of one of these organisms by 
increasing its virulence so that it would attack other species of rodents i 
and then, this virulence increased, to maintain it at its highest point. 
This is how he proceeded to solve this problem. 
He first isolated a bacillus from a spontaneous epidemic among har- 
vest mice. This organism was a cocco -bacillus, presenting in general 
the characteristics of the colon bacillus, and resembling the bacillus 
of Loeffler — B. typhi murium. From the first this bacillus showed a 
slight pathogenicity for gray rats (m. decumanus). Out of 10 animals 
fed with a culture of this microbe 2 or 3 would die ; several others 
would sicken and recover ; others still appeared completely refractory. 
The fact that a certain number of the rats fed with these cultures always 
succumbed led to the hope that it would be possible to increase the vir- 
ulence of this particular microbe by the generally accepted methods — 
that is to say, by a certain number of passages from rat to rat. 
Danyz first tried to increase the virulence of the organism by this 
means, but he found that successive passages from rat to rat, whether 
by feeding or by subcutaneous injection, ended by enfeebling rather 
than increasing the virulence of the microbe. He found that it was 
rarely possible to go beyond 10 to 12 passages. Sometimes the series 
was stopped at the fifth passage, or even sooner, by the survival of all 
the animals undergoing experiment. The result was exactly the same, ' 
if, instead of alternating each passage through the animal by a culture 
