THE NERVOUS SYSTEM 267 



of this disease, according to Wehrbein, occurs only in pigs up to 

 six months of age and usually affects more than one pig in the same 

 litter. 



Etiology. — This condition does not appear to be the result of any 

 one specific causative agent. In some instances the disease on a farm 

 or communitjr occurs in pigs from one particular boar. Inbreeding 

 may be a factor in the causation of pig paralysis. Tuberculosis of 

 the vertebral column may produce symptoms that cannot be readily 

 differentiated from paralysis. Parasitic invasion of voluntary mus- 

 cular tissiie, spinal cord or nerves may produce paralysis. Malnu- 

 trition may also be an important factor in paralysis. The disease 

 is not infectious, or at least experiments in which blood and nerve 

 tissue v^fire injected into susceptible pigs proved negative. The dis- 

 ease has not been transmitted by contact of affected and susceptible 

 pigs, vsrhich is further evidence of its non-infectiousness. The causa- 

 tive agent exerts an influence on the motor nerves. 



Lesions. — The lesions found in those cases in which there is a tuber- 

 cular infection are characteristic of tuberculosis. In the cases of 

 paralysis due to tubercular infection there is usually involvement 

 of the vertebra, resulting in a rarefying osteitis and diminishing of 

 the lumen of the neural canal and usually pressure upon the spinal 

 nerves and spinal cord. In typical cases of pig paralysis there are 

 no gross lesions. Microscopically the motor nerve-fibers are found 

 to be involved. The fibers degenerate and there is a fibrous prolifer- 

 ation between the nerve-fibers. The extent of the degeneration and 

 fibrous proliferation, varies in different cases and also in different 

 stages of the disease. These changes are designated as polyneuritis 

 parenchymatosa. 



Symptoms. — The first symptoms evidenced in some cases consist 

 of disturbances of locomotion. The affected animal may appear stiff 

 and knuckle over and it will weave or wabble and may fall to the 

 ground. In other instances the onset of the disease is rapid, the 

 first indication being paralysis and inability to arise. The affected 

 animals are unable to stand even though they be raised to their feet. 

 The paralyzed pig will assume the decubital position with little con- 

 cern and apparently without discomfort. They will have a normal 

 appetite and may even develop, though usually less rapid than nor- 

 mal pigs if feed and drink is provided. Their temperature remains 

 ncrmal unless some complication arises. Defecation and urination are 

 usually nonnal. 



