14 DISSEMINATION 



tious ones largely by eliminating the characters of the latter 

 and finding, if possible, the causative agent. 



The necessity for an early and positive diagnosis in all 

 outbreaks of epizootic disease, is to assure the enforcement of 

 all possible measures to prevent its spread. The essential 

 problem for the practitioner or sanitarian in the presence of 

 these diseases, is to restrict the number of cases to the indi- 

 viduals already infected. In order to do this, it is of much 

 importance that modified or chronic cases of any infectious 

 disease should not escape detection if there is danger of their 

 spreading the virus or exposing susceptible animals. 



§ 7. Dissemination of infectious diseases. Al- 

 though the discussion of the means by which each of the vari- 

 ous diseases are disseminated will be found under the considera- 

 tions of the individual affections, it is important to consider 

 the general ways and means by which these different vital 

 causative factors are spread from an infected individual to a 

 non-infected one in the same herd and from one herd to 

 another. As we understand them at the present time, each 

 virus is dependent for its perpetuation upon its escape from 

 one host (sick or dead) to another. As these organisms are 

 without power of their own for such migration, they are depend- 

 ent upon other forces and carriers to take them. In finding 

 the cause of their spread, we must consider first how they 

 escape from the infected individual and secondly how they are 

 carried rom one individual to another. 



1. Escape of virus from infected individuals. The infect- 

 ing organisms escape from the living body either with (a) the 

 excreta, (b) the external discharge of ulcers and abscesses or 

 both, and (c) the blood by sucking and possibly biting insects. 

 After the death of the host they can escape only by the dis- 

 integration of the dead body or by its being consumed by 

 other animals or birds. The bacteria of several diseases 

 can pass through the digestive tract of such animals un- 

 injured. 



2. Dissemination of infectiyig orga^iisjns. Pathogenic 



