HOW DOUBLE TREATMENT IS GIVEN 385 



can be used at practically the same expense, and which gives a 

 protection which will last as long as the animal lives. 



There is one possible exception to the statement that immunity 

 produced by the double method is lifelong. This is in the case of 

 young pigs. Young, sucking pigs, when injected by the double 

 treatment, often prove to lose this protection later on in life. 

 This is probably due to the fact that the cells of the body are so 

 young and imperfectly developed at this time that they are not 

 in a condition to take up permanently the work of forming germ 

 fighters or antibodies. Just as a young child or a young animal 

 forgets the things that it learns in its early childhood unless they 

 are constantly practised, so the cells of the body of these young 

 animals seem to lose the knack of manufacturing the germ 

 fighters as they grow older. This can be overcome by treating 

 again a little later on in life, say, at the age of three months. 



HOW DOUBLE TREATMENT IS GIVEN 



In giving the double treatment it is necessary to make use 

 of both serum and virus blood. This is how the treatment gets 

 the name of double treatment, there being a double injection made 

 of serum and virus. These two injections are made at the same 

 time, and on this account it is sometimes known as the "simul- 

 taneous" treatment. 



Source of Virus Blood. — It will be recalled that virus, or virus 

 blood, as it is more properly called, is the blood which is obtained 

 from the body of the sick animal when the disease is well advanced 

 and the animal in practically a dying condition. This virus blood 

 has already been described, as have also the methods by which it 

 is drawn. 



Virus blood which is intended for use in the field for giving the 

 -double treatment is handled in just the same manner at the serum 

 plant as that which is intended for immediate use in hyper- 

 immunizing animals from which serum is to be produced. The 

 blood is drawn in the same manner, and shaken and strained 

 to remove the fibrin which would cause clotting if not re- 

 moved. . 



Carbolized y{rws.— The only difference in this virus blood 

 and that used for hyperimmunizing at the plants is that a small 



