322 DISEASES OF SWINE 



The most common and the natural way of obtaining an ac- 

 quired immunity is through an actual attack of the disease. For 

 instance, if an animal has an attack of hog-cholera and successfully 

 passes through the sickness there will be left on recovery a protect- 

 ive power that will prevent the attack of the body by the germs of 

 this disease a second time. Almost every farmer and stockman has 

 noticed this fact. Sows that pass through one attack of cholera 

 and are kept over for breeding purposes are seldom attacked the 

 following year, even though the disease may make its appearance 

 in their htters about weaning time. It is an interesting fact, 

 worthy of mention here, that the litters of these immune sows are 

 furnished with a certain degree of protection which is in operation, 

 as a rule, up to about the time of weaning. This has a practical 

 importance in the management of hog-cholera which will be further 

 discussed under the head of Simultaneous Serum Treatment. It is 

 an interesting example of hereditary immunity and one that is of 

 considerable scientific interest and actual importance. 



There is. one great disadvantage to acquiring an immunity by 

 actual attack of the disease, in that there is always a large per- 

 centage of deaths among the hogs attacked. This is especially 

 the case with hog-cholera. For instance, out of a herd of 100 

 gilts only 1 or 2 may be left after an outbreak of the usual severe 

 type seen in the Corn-belt States. Immunity is a most desirable 

 condition, and it is our aim to secure as large a number of im- 

 mune hogs as possible, but if it has to be obtained at the expense 

 of losing 98 per cent, of the herd it can hardly be classed as of 

 practical advantage. 



In order to devise a method of securing this protective power 

 without entailing such an enormous loss among the healthy animals 

 various methods have been tried out from time to time in hog- 

 cholera, as well as numerous other diseases, for the purpose of 

 developing a less costly means of securing the same advantage. 

 The first real progress in this line was made in human practice 

 many years ago by Dr. Jenner, in the discovery of vaccination as a 

 means of preventing small-pox. This man found that by inoculat- 

 ing a healthy man with a weakened virus he was able to produce a 

 mild local reaction which resulted in the development of a perma- 

 nent protective power against small-pox. In veterinary practice 



