422 DISEASES INVOLVING NERVOUS SYSTEM 



Natural Infection. — ^Rabies is essentially a disease due to the 

 bite of a rabid animal, the saliva of which contains virulent 

 virus. Such saliva coming in contact with any fresh wound 

 could produce infection. The saliva of an animal may be 

 virulent as early as eight days before the termination of the 

 period of incubation and before sjTnptoms of the disease 

 develop. The virus, as in tetanus, extends along the nerves 

 to the brain and cord. It may also be carried by the blood 

 and lymph. The danger of the bite of a rabid animal 

 depends upon the virulency of the saliva, character of the 

 wound and the number of lymph vessels and nerves injured. 

 Bites inflicted by carnivorous animals are more dangerous 

 than those produced by herbivora on account of the form of 

 the teeth and the character of the wound they produce. 

 Wounds near the brain and cord are especially apt to be fol- 

 lowed by infection. In horses, bites in the lips, nose, and 

 cheeks are therefore dangerous. The length and thickness of 

 the hair or wool covering the part bitten are important fac- 

 tors, a heavy, thick growth catching most of the saliva and 

 preventing its entering the wound. Recently shorn sheep are 

 thus much more susceptible than when wearing the full wool 

 coat. The infected wound usually heals as any other wound, 

 quite often by first intention. Infection through the intact 

 skin has not been demonstrated. Not over 30 to 50 per cent, 

 of the animals bitten by rabid animals take the disease. 



Necropsy. — ^There are no characteristic lesions of rabies. 

 In herbivorous animals fairly constant are the empty stomach, 

 congested lungs, and larynx. The other organs are either 

 normal or show secondary lesions not directly due to rabies. 



Symptoms. — The period of incubation is very varied. Gen- 

 erally the disease breaks out two to eight weeks after inocula- 

 tion. Much longer periods have been observed. An incuba- 

 tion period of one to two years is probable. The length of 

 the period is determined by the virulency of the virus, the 

 character and location of the wound, and the age of the ani- 

 mal. Young animals are more susceptible than adult or aged 

 ones. In general the symptoms of rabies are much the same 

 in all animals. They are modified only by the natural pecu- 

 liarities of the different species. In practically all animals 



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