14 AN INTRODUCTION TO THE STUDY OF VIRUSES 
wild rats may be maintained in much the same fashion as trichina 
infection, by cannibalism or by ingestion of virus-containing tissues 
from other animals. The disease may be transmitted to swine when 
the carcass of a pseudorabies-infected rat is eaten by a hog. From 
this hog, the disease spreads to other swine in the same herd as a 
contagious disease. It is transmitted to swineherds on other farms, 
either by direct contact of infected pigs with normal pigs, or by the 
migration of infected rats. With two such efficient means of dis- 
semination of the virus among pigs, one would expect the disease 
to be prevalent in this species. However, because of the extremely 
mild nature of porcine pseudorabies, its existence is not suspected. 
Only when the virus breaks away from the swine reservoir and 
spreads to cattle is its presence made known. The transmission of 
the virus from swine to cattle is thought by Shope to take place when 
the noses of infected pigs come in contact with abraded areas of skin 
on cattle. It has been shown that in swine the nose serves both for 
the entrance and the exit of the virus. Furthermore, it has been observed 
that fatal pseudorabies infection in rabbits can be induced merely by 
bringing their abraded skin into contact with the noses of infected 
swine. The blood sera of swine on two farms where pseudorabies 
had occurred among the cattle were found to be capable of neutralizing 
pseudorabies virus. It is believed in these instances that the swine 
had a mild and unrecognized pseudorabies infection and transmitted 
their disease to the cattle with which they were associated, by transfer 
of the virus on their noses to the abraded skin of the cattle. The cycle 
of infection of the pseudorabies virus is thus thought to be as follows: . 
carcasses of cattle dead of the disease, if gnawed by rats, serve as a 
fresh source of virus from which a rat population can become infected. 
Pigs become infected through eating the virus-infected carcasses of 
rats and the infection then spreads through the herd as a mild, highly 
contagious disease. From the pigs the virus is transmitted to cattle 
by contact of the noses of infected swine, and so the cycle from rat 
to swine, to cow, and back to the rat can be completed. 
Birds are frequently suspected of being the passive vectors of various 
animal viruses, more especially of that of foot-and-mouth disease. 
The situation regarding the starling as a carrier of this virus has been 
very carefully examined by Bullough (1942) and some of his con- 
clusions are given here. Since the virus of foot-and-mouth disease is 
capable of retaining infectivity for long periods, two or three months, 
on such materials as feeding stuffs and bedding, it is theoretically 
