TRANSMISSION AND SPREAD OF VIRUSES 13 
tissues of another animal. The virus is then transmitted along a nerve 
trunk to the central nervous system; the localization of the virus is 
highly selective, for only nervous tissues are affected. Transmission 
therefore occurs in nature only by the bite of a mad dog or some other 
rabid animal. The paralytic form of rabies occurs in South America, 
frequently in cattle, less frequently in man. In these cases the vector 
is the vampire bat and the virus has been isolated from the saliva of 
these animals. The bats apparently affect each other by bites given 
during quarrels in their roosting places and while a certain number 
die of the disease, others remain healthy but carry the virus for long 
periods. 
Shope (1941) has carried out some interesting experiments on swine 
influenza in which he demonstrated that the swine lungworm acts as 
a reservoir and intermediate host for swine influenza virus. The 
actual cycle of events seems to be as follows: the developing eggs 
are deposited by the female lungworm in the lungs of the pig; these 
are coughed up and swallowed by the pig and eventually passed out 
with the faeces. The eggs are next swallowed by an earthworm in 
which they hatch and undergo two further developmental stages. 
When they reach the third stage, the larvae are ready to infect the pig, 
and these third-stage larvae become localized in large numbers in 
the heart, gizzard, and certain glands of the earthworm. Here the 
larvae must wait until they are swallowed by a pig and they can, if 
necessary, wait as long as four years in this third-stage larval condition. 
If the eggs came originally from the lungs of an infected pig, the larvae 
from these eggs are carrying the influenza virus. The virus, however, 
is present in the lungworm in a masked, non-infectious form and to 
induce infection it must be rendered active by the application of a 
‘ provocative stimulus to the swine it infests. Multiple intramuscular 
injections of the bacterium, H. influenzae suis, furnish a means of 
provoking infection. Shope found that whilst swine influenza in- 
fection can be provoked in properly prepared swine during the 
autumn, winter, and spring, this could not be achieved in summer. 
No explanation for this failure has yet been discovered. 
Pseudorabies is a very fatal but non-contagious disease in cattle and 
the common laboratory animals, whereas it is a relatively mild but 
highly contagious disease in swine. The spread of the virus in cattle 
has been attributed to rats but Shope (1935) has suggested that the 
part played by rats is not the whole story. He visualizes the epide- 
miology of the disease as follows: Pseudorabies infection among 
