DIFFERENTIATION I3 



the first cases are chronic in nature and the later ones acute. 



4. Duration. In animals, as in man, most of the infec- 

 tious 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 con- 

 valescence, becomes chronic or subchronic in nature and 

 eventually terminates in death. The lateness in the develop- 

 ment of the modified lesions often causes the nature of the ter- 

 minal disease to go unrecognized. 



5. Trayismission by ijioctilatioyi. 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 to healthy animals. The extent of the spread of the 

 virus of the di.sease through the available channels for its dis- 

 semination will also aid in determining the infectious or non- 

 infectious nature of the malady in an outbreak among 

 animals. 



In diagnosing an epizootic disease investigations have 

 shown that too much reliance can not be placed on the period 

 of incubation, or the morbid anatom}'. There are many pos- 

 sibilities, 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 characters of infectious maladies. This 

 fact necessitates much care in the differentiation of outbreaks 

 of animal diseases. 



The dietary and other non-infectious disorders do not 

 exhibit definite, uniform differential characters excepting per- 

 haps in case 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 {Saiecw jacoboea) may be mentioned. 

 The non-infectious disorders are differentated from the infec- 



