DIFFERENTIATION 9 
to itself. They may vary within rather definite limits. They may 
also be acute or chronic in nature. They are due to the action upon 
the tissues of exogenous and endogenous toxins, bacterial proteins, 
or mechanical blocking of vessels. In many epizodtics, the disease 
appears in an acute form in the first animals attacked while those 
attacked later in the course of the outbreak suffer from a chronic 
or modified form of the affection. In other outbreaks, the first 
cases are chronic in nature and the later ones acute. It is important 
to distinguish between the lesions due to the virus and secondary 
tissue changes that may take place. 
Duration. In animals, as in man, most of the infectious diseases 
are self limiting, but, as a rule, the percentage of fatal cases is larger 
in epizodtics than in epidemics. 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, becomes 
chronic or subchronic in nature and eventually terminates in death. 
The lateness in the development of the modified lesions often causes 
the nature of the terminal disease to go unrecognized. 
Transmission by inoculation. Finally, it is necessary in making 
a positive diagnosis to find the specific organism, or to prove the 
transmissibility of the malady from the sick or dead animals to healthy 
ones. The extent of the spread of the virus through the available 
channels for its dissemination will also aid in determining the infec- 
tious or noninfectious nature of the malady in an outbreak among 
animals. 
In diagnosing an epizodtic disease, investigations have shown 
that too much reliance can not be placed on the period of incubation, 
or the morbid anatomy. There are many possibilities, therefore, 
that an erroneous diagnosis may be made when the clinical and 
post-mortem evidences of the disease are alone considered. It has 
also been determined that certain non-infectious disorders often 
assimilate, in their more general manifestations, the character of 
infectious maladies. This necessitates care in the differentiation 
of outbreaks of animal diseases. 
The dietary and other non-infectious disorders do not exhibit 
definite, uniform differential characters excepting perhaps in cases 
of those caused by a few mineral poisons or by eating certain plants. 
As examples of these, lead poisoning and the Pictou or Winton 
disease of horses and cattle caused by eating a ragwort (Senecio 
