EPIZOOTIC DISEASE 21 



(2) The infection must be followed by a certain period of 

 incubation before the development of symptoms. This is the 

 time necessary for the invading microorganism to become es- 

 tablished in the body and to bring about the first symptoms of 

 the disease. The incubation period varies in different diseases 

 and to a certain degree in the same disease according to the 

 mode of infection and the resistance of the individual. Usu- 

 ally the incubation period of a given disease is practically the 

 same for all individuals of the same species when subjected to 

 the same mode of infection. Exceptions, however, are not rare. 



(3) The course of an infectious disease is usually nearly 

 the same in animals suffering in the same outbreak especially 

 when they were infected at or about the same time. It is more 

 common for only a few individuals in a herd to be infected in 

 the beginning and from these first cases other animals con- 

 tract the disease. In many epizootics, the disease appears in 

 an acute form in the first animals attacked while those infected 

 later in the course of the outbreak suffer from a chronic form 

 of the disease. In other outbreaks, the first cases are chronic 

 in nature and the later ones acute. 



(4) In animals, as in man, most of the infectious diseases 

 are self limiting, but, as a rule, the percentage of fatal cases is 

 much larger among animals than in the human species. The 

 period of convalescence is not so well marked in the lower 

 species as in man. It frequently happens that the course of 

 the disease is so changed that an acute case which appears to 

 recover, or at least to pass into the stage of convalescence, be- 

 comes chronic or sub chronic in nature and eventually termin- 

 ates in death. The lateness in the development of the modified 

 lesions often causes the nature of the terminal disease to be 

 unrecognized. 



(5 ) Finally, it is necessary in making a positive diognosis 

 to find the specific organism, or to prove the transmissibility 

 of the malady from the sick or dead to healthy animals. The 

 extent of the spread of the virus of the disease, through the 

 available channels for its dissemination, will also aid in deter- 

 mining the infectious or non-infectious nature of the disease in 

 an outbreak among animals. 



