INCUBATION PERIOD OF CHOLERA 157 



attack than in other years. This is spoken of as virulency, and 

 the virus which is capable of making an animal sick in a very few 

 days is commonly called highly virulent. In some seasons we 

 see the hogs getting sick in a very few days after they have been 

 exposed to cholera, while in other years it takes the disease a long 

 time to get started in the herd. Often the incubation is so long 

 that the owner will think that his herd has entirely escaped the 

 disease. Just about the time he begins to feel pretty good about 

 it, however, he will note that one or two shoats are off feed and 

 do not seem to care to move about much. This is a pretty sure 

 sign that something is wrong, and in one or two more days cholera 

 is well started in the herd. 



Usually the first few weeks after cholera makes its appearance 

 in any community the period of incubation is quite a little bit 

 shorter than it is later on in the summer or fall. This is prob- 

 ably due to the fact that the disease-producing power or virulence 

 of the germ seems to gradually weaken toward the end of the season. 

 It is also quite likely that during the summer several of the herd 

 are repeatedly exposed to small doses of virus, and, while they do 

 not get sick, they do develop a certain amount of protective power 

 against the disease, and, when later on they are exposed to a large 

 number of the germs, they are able to put up a stronger fight against 

 the disease before they give up and get sick. 



It has also been commonly found by men who have worked a 

 great deal with cholera that the hotter the summer may be, the 

 shorter time it requires for the exposed animals to get sick. It 

 has also been found by those who have paid attention to the out- 

 breaks that occur during the winter months that the colder the 

 winter, the more quickly cholera will develop, and animals get sick 

 in just a few days. This short incubation period in cold winter 

 months is no doubt due to the fact that in very cold weather the 

 hogs tend to pile up more and get in closer contact with each other. 

 In this way it makes it easier for the germs of the disease to get 

 from one hog to another and set up the disease. The extreme heat 

 lowers vitality of the herd and hastens development of cholera. 



Many very interesting examples of how this incubation period 

 may differ are commonly noted, and the following cases will serve to 

 show what a big difference may be seen under various conditions : 



