374 DISEASES OF CATTLE. 



disease, we must study that disease and not rely upon any single 

 work to tell the whole story. 



Infectious diseases have, as a general rule, a period of incubation, 

 which comprises the time elapsing between the exposure to the infec- 

 tion and the actual appearance of the disease. This period varies 

 w^th the malady. The most common symptom of this class of diseases 

 IS fever. The severity of the fever is measured by the temperature 

 of the animal, and this is readily and accurately ascertainable by 

 the clinical thermometer. (See PI. Ill, fig. 1.) The other symptoms 

 are variable and depend upon the particular organ or organs most 

 implicated. Loss of appetite, cessation of rumination and milk secre- 

 tion, and general dullness are symptoms quite invariably present in 

 most infectious diseases. 



During the course of infectious diseases secondary diseases or com- 

 plications may arise which are largely due to bacteria other than those 

 producing the original malady. These complications are often so 

 severe as to become fatal. In general it may be stated that they are 

 due to filthy surroundings, and hence cleanliness may become an 

 important aid to recovery. 



The treatment of infectious diseases is given under each malady so 

 far as this is allowable or advisable. These diseases are not, as a rule, 

 amenable to treatment. When the symptoms have once appeared the 

 disease is apt to run its course in spite of treatment, and, if it is one 

 from which animals usually recover, all that can be done is to put 

 them into the most favorable surroundings. Many infectious diseases 

 lead sooner or later to death, and treatment is useless so far as the 

 sick are concerned, and it may be worse than useless for those not 

 yet infected. All animals suffering with infectious diseases are a 

 menace to all others more or less directly. They represent for the 

 time being manufactories of disease germs, and they are giving them 

 off more or less abundantly during the i^eriod of disease. They may 

 infect others directly or they may scatter the virus about, and the 

 surroundings may become a future source of infection for healthy 

 animals. This leads us to the subject of prevention as the most 

 important of all which claim our attention. In this place only a few 

 general remarks will suffice to bring the subject before the reader. 



The most important thing is to keep disease away from a herd or 

 farm. To do this all sick or suspicious animals should be avoided. 

 A grave form of disease may be introduced by apparently mild or 

 trivial cases brought in from without. It is generally conceded that 

 continual change and movement of animals are the most potent means 

 by which infectious diseases are disseminated. 



With some cattle diseases, such as anthrax, rinderpest, and pleuro- 

 pneumonia, preventive inoculation is resorted to in some countries. 

 This may be desirable when certain diseases have become stationary 



