702 INFECTION AND IMMUNITY. 



the parasite, and either by his bite or by experi- 

 mental inoculation practically all animals, at least 

 mammalians, may be infected. 



incubation The incubation period in animals varies from 

 two weeks to several months. In man it varies 

 between twenty and sixty days usually, -but may be 

 as short as seven or ten days, or as long as twenty 

 months (rare). In children it is shorter than in 

 adults. The location of the bite is also of impor- 

 tance in determining the length of incubation. It 

 is shortest following wounds of the head and neck, 

 somewhat longer when the injury is in the hand or 

 arm, and still longer when in other parts of the 

 body. The degree of laceration is also a factor, de- 

 pending possibly on the introduction of large;- 

 quantities of virus, and on larger surfaces for its 

 absorption. The bite of the wolf is said to be most 

 virulent, and next in virulence is the bite of the 

 cat and dog. 



Not all who are bitten by rabid animals develop 

 hydrophobia. Correct figures on this point are dif- 

 ficult to obtain, since in many instances the ani- 

 mals are only suspected of being rabid. According 

 to Hogyes, from 15 to 16 per cent, of those who 

 are bitten contract hydrophobia. The percentage 

 is much higher following bites by the wolf. The 

 disease is invariably fatal to man. 



The symptoms of hydrophobia in man differ in 

 no essential respects from those seen in animals. 



Diagnosis The immediate determination of hydrophobia 

 in dogs which have bitten man is of the greatest 

 importance. In many instances the behavior of the 

 animal is sufficiently characteristic to justify clin- 

 ical diagnosis of the disease. The disposition of 

 the animal changes suddenly, it ceases to play, eats 



