SUMMARY 165 



Average Incubation Period. — From the above case histories 

 it can be seen that the length of time between exposure of healthy 

 animals to cholera and the appearance of signs of sickness is not 

 by any means always the same. We have found that symptoms of 

 illness may show up in three days, or they may not get sick for 

 ' nearly thirty days. As a general rule in actual practice, it is found 

 that this period of incubation will be from ten to fifteen days. 

 In other words, it is usually ten to fifteen days after the animals 

 are exposed to cholera before they commence to get sick. This 

 may then be stated as the average incubation period of cholera — 

 ten to fifteen days. It may be shorter, even as short as three days, 

 or it may be prolonged to four weeks. 



In serum-manufacturing plants, where the virus is injected 

 direct into the muscles of the hog, it is usually found that they get 

 sick during the first week. 



SUMMARY 

 Incubation periods vary from three to thirty days. 

 Average incubation period is ten to fifteen days. 

 Among those things which have a tendency to make the incu- 

 bation period shorter or longer are the following: 



(1) Condition the Hogs are In. — If hogs are run down they get 

 sick quicker. 



(2) Age of the Hogs. — Young shoats show sickness quicker than 

 do old animals. 



(3) Amount of Virus. — The greater amount of virus that is 

 taken into the body, the sooner the animal will get sick. 



(4) Strength of Virus. — Sometimes the virus is more powerful 

 than at others. In case the virus is real powerful (highly virulent) 

 it will, of course, produce disease quicker. 



(5) Period of Outbreak. — Animals usually get sick quicker at the 

 beginning of an outbreak than later on, when the strength of the 

 virus seems to sort of run out. 



(6) Temperature. — In very hot summer weather and in very 

 cold winter weather the animals get sick very soon after being 



