DISEASES OF UNKNOWN ETIOLOGY 425 



occasionally assuming a chronic form. Post mortem exam- 

 ination of acute eases usually show congestion and acute 

 swelling of the internal organs and hemorrhages on the 

 serous and mucous membranes and not infrequently a 

 serous transudate in the pericardium. In the chronic type 

 necrotic and ulcerated areas are commonly found in the 

 intestines, frequently associated with pneumonia in the 

 lungs. It has not proved possible to produce the disease in 

 animals other than swine. The organism Bacterium cholera 

 suis is undoubtedly an important secondary invader in the 

 disease and possibly may in some cases have the ability to 

 produce a choleralike disease in the entire absence of the 

 virus. 



The work of Dorset, MacBryde and Niles in the United 

 States showed that the blood serum from a hog that has 

 recovered from hog cholera will confer some degree of im- 

 munity when injected into susceptible animals. This fact 

 has been made use of in the development of a practicable 

 method of prevention of the disease. An animal is first 

 secured that has already been immunized or has recovered 

 spontaneously from hog cholera. This animal is hyper- 

 immunized by injection of blood from a so-called virus pig, 

 that is, one suffering from acute attack of hog cholera as a 

 result of the injection of virus. Usually about 5 cubic 

 centimeters of this defibrinated virulent blood is injected 

 for each pound of body weight of the animal to be im- 

 munized. Some days later this animal is bled either from 

 the carotid or by cutting off the tip of the tail. The blood 

 thus secured is defibrinated and preserved by the addition 

 of one-half per cent of phenol. The animal may again be 

 hyperimmunized and bled several times; eventually all of 

 the blood is removed from the body. The potency of the 

 serum thus secured is tested upon young pigs usually weigh- 

 ing from 50 to 60 pounds. A serum sufficiently potent 

 should in a dose of 15 cubic centimeters or less, protect 



