Incubation. 39 



Period of Incubation. Latency. 



The time that elapses between the receiving of the 

 germs into the system and the manifestation of the 

 earliest symptoms of the disease, varies greatly. Dela- 

 fond sets it at from six to sixty days, Verheyen from ten 

 to sixty days, the French Commission extends the period 

 to sixty-seven days, Keynal has seen it exceed ninety 

 days, and Roll and Gamgee quote from eight days to one 

 hundred and twelve. It is true that Gamgee qualifies 

 this by the statement that when an animal sickens four 

 months after purchase, two or three latent instances of 

 the diseases have preceded the obvious one. Australia, 

 South Africa and Norway were each infected by cattle 

 that had shown a period of incubation of three months. 

 I have frequently seen cases in which cattle have passed 

 three or four months after the purchase in poor health, 

 yet without cough or any other obvioiis diagnostic symp- 

 tom, and at the end of that time have shown all the 

 symptoms of the lung plague. But, as such cows are 

 considered by the ordinary observer to be well, and as 

 many of them will convey to the mind of the veterinarian 

 nothing more than unthriftiness, we must, as a working 

 rule, accept as possible an incubation of three or even 

 four months. All quarantine regulations for this dis- 

 ease must be based on this occasionally long period of 

 latency. 



As regards the real or regular period, we may deduce 

 something from the exudation and swelling in the tail in 

 inoculated cases. The average period is on the ninth 

 diiy, though it may appear as early as the fifth, or it may 

 he delayed till the thirtieth or fortieth day. In the ex- 

 perimental transmission of the disease by cohabitation, 

 under the French Commission, a cough — the earliest 

 symptom— appeared from the sixth to the thirty-second 

 day, and sometimes continued for months, though no acute 

 disease supervened. 



