350 Annual Report for 1913 of Royal Veterinary College. 
contact, and it is generally assumed that ingestion or swallow- 
ing of the virus is the common natural method of infection, 
although it is not improbable that tbe disease may be contracted 
through inhalation. 
As is the case in some other contagious diseases, the viru- 
lence of swine fever varies considerably, the virulence being 
measured by the severity of the illness and the mortality among 
the pigs attacked or exposed to infection. When first intro- 
duced into a country the mortality may reach 90 per cent, or 
even more, but after a time the disease often becomes less 
virulent, with the result that the majority of the pigs attacked 
recover, and many never become visibly ill although exposed 
to infection. In the outbreaks of ordinary severity the period 
which elapses between infection and the commencement of the 
actual illness is usually about ten days, but* it may be consider- 
ably less than that or it may be as long as three weeks. And 
here it is important to note that in mild cases there is no 
definite period of incubation, since although infected and 
actually diseased the animal may never become obviously ill. 
The disease is spread to fresh premises mainly by the traffic in 
pigs in the incubation stage of the disease, or in pigs which, 
although actually diseased, do not present definite symptoms. 
In the majority of cases a pig which contracts swine fever 
either dies within a few weeks or it recovers rapidly and com- 
pletely and is afterwards immune against a further attack. 
Unfortunately there are cases in which diseased pigs make a 
recovery which is incomplete, since they remain capable of 
spreading the disease. 
The method of dealing with swine fever which was 
adopted in 1892, when the disease was taken over by the 
Board of Agriculture, may be said to be based on the follow- 
ing contentions : — 
1. The disease is a purely contagious one, like cattle-plague 
and bovine pleuro-pneumonia, and it is therefore capable of 
being stamped out. 
2. To stamp the disease out will be economically sound 
because the cost of eradication will be less than the sum of 
which the interest would equal the annual loss inflicted by the 
disease. 
dhe hist of these contentions appears to be perfectly sound, 
and at any rate it has not been disproved by the fact that an 
effort which has now been sustained for twenty-one years has 
not stamped the disease out. The severe measures which 
eradicated cattle-plague and pleuro-pneumonia of cattle have 
not been consistently employed against swine fever, and 
the late Sir George Brown prophesied that no milder 
plan would succeed iin stamping out the disease. The two 
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