360 DISEASES OF CATTLE. 



disease, we must study it 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 

 with the malady. The most common s3^mptom of this class of dis- 

 eases is fever. The severity of the fever is measured by the tempera- 

 ture of the animal ; this is readily and accurately ascertainable by 

 the clinical thermometer. (See PL III, fig. 1.) The other symp- 

 toms are variable and depend upon the particular organ or organs 

 most implicated. Loss of appetite, cessation of rumination and milk 

 secretion, and general dullness are symptoms quite invariably pres- 

 ent in most infectious diseases. 



During the course of infectious diseases secondary diseases or com- 

 plications may arise which are largely caused by 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 allovv^able or advisable. These diseases are not, as a rule, 

 amenable to treatment. When the symptoms have once appeared the 

 disease is liable 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 dis- 

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

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

 yet infected. All animals suffering with infectious diseases are more 

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

 being manufactories of disease germs, and they are giving them 

 off more or less abundantly during the period 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 im- 

 portant 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 established 



