14 CONTROL 
purposes, it is necessary to know how it can be detected and the best 
method for its cultivation or demonstration. 
Specific reactions. In the study of immunity it has been found 
that animals suffering from certain infectious diseases will react 
to the injection into the body of the specific protein of the organism 
with which they are infected. Thus, if mallein is injected into a 
horse suffering from glanders a definite reaction will occur which 
can be considered specific for glanders. There are also certain 
substances given off hy the tissues into its blood that have a specific 
affinity for the antigen or species of organisms that cause the infec- 
tion. There are a number of these so-called specific reactions em- 
ployed in diagnosis. They are reactions to mallein, tuberculin 
and other like substances, the agglutination, the precipitation, 
complement fixation and others. These are all quite largely em- 
ployed in diagnosis. When a reaction takes place with any of these 
tests it is generally considered positive evidence of infection. 
The limitations of the specific tests are not determined, but experi- 
ence has shown that with certain of them, especially tuberculin, 
a failure to react is not conclusive evidence of the absence of infection 
while a reaction is a very definite indication that the animal is 
tuberculous. The sera tests seem to be more accurate in negative 
cases. The methods employed in making a diagnosis will be described 
in connection with the diseases to which they apply. 
Control. The knowledge necessary for the control of specific 
infectious diseases is one of the most important acquisitions of the 
veterinarian. It is obtained by a careful study of the etiology and 
morbid anatomy of these affections. The control is accomplished 
by following out definite procedures along one or both of the follow- 
ing lines, namely: (1) by preventing the spread of the virus or 
(2) by immunizing susceptible animals against it. 
By checking the spread of the virus. This involves an intimate 
knowledge of the life cycle of the virus for each of the diseases. 
It requires that one should know, (1) when in the course of the 
disease the virus is eliminated and the channels through which it 
escapes. (2) What happens to the virus after it leaves the body 
and how long it will resist the action of external influences such 
as drying and sunlight. (3) Through what channels it gains entrance 
to the body of the uninfected animal. When the facts regarding 
these points are ascertained it is not difficult to determine what 
