86 AN INTRODUCTION TO THE STUDY OF VIRUSES 
of the lower animals and to eliminate a focus of infection, it is some- 
times necessary to slaughter infected animals and their contacts. This 
policy of slaughter has been ruthlessly applied and rigorously carried 
out in the case of foot-and-mouth disease of cattle and pigs. It has 
been severely criticized from time to time, but there is no doubt that 
it has saved this country from much greater epidemics such as have 
been experienced on the continent. As soon as an outbreak of foot- 
and-mouth disease on a farm has been confirmed, the infected animals 
and any that have been in contact with them are slaughtered and the 
carcasses burned. The farm buildings are sprayed with disinfectant, 
any heavily contaminated material is burned, and the clothes of the 
farm workers are fumigated. At the same time, a “standstill” order 
on the movement of cattle into or out of the prohibited area comes 
into force. 
The slaughter policy has recently been extended to the domestic 
fowl in an attempt to deal with another extremely infectious virus 
attacking fowls and turkeys. This disease is known as fowl-pest or 
Newcastle disease. 
Another group of intensely infectious viruses are those attacking 
caterpillars and causing the so-called “polyhedral diseases.” This type 
of disease is particularly destructive to silkworms and it has often 
been found more satisfactory to destroy the whole of the stocks of 
silkworms when the disease has become established and to start afresh 
rather than to try to clean up the stocks. Very careful disinfection is 
necessary, since the polyhedra are very resistant and retain their 
infective power for long periods. 
Selective Breeding. There does not seem to have been very much 
selective breeding carried out in animals against virus diseases but 
Wilson Smith (1939) quotes the work of Webster (1937) in selective 
breeding of mice. By intensive in-breeding he produced four strains 
of mice: 
(1) virus resistant, bacteria susceptible; 
(2) virus resistant, bacteria resistant; 
(3) virus susceptible, bacteria susceptible; 
(4) virus susceptible, bacteria resistant. 
Each of these types might be suitable for some problem in virus 
research. 
Protection from Insect or other Vectors. Screening, or protecting in 
other ways, the susceptible animal from the vector of a virus is 
