20 JOHNFS DISEASE 



with Johne's disease) in providing for the needs of the 

 growing foetus, from the act of parturition, and from 

 the sudden onset of the full activity of the mammary 

 gland. 



The possibility that one cow will infect a number of 

 others may be greater when the animals are closely 

 housed and are lying in pairs in stalls than when they 

 are at pasture; for, as is well known, cattle usually 

 avoid feeding on or near the masses of rank grass 

 which grow where dung has been deposited. On the 

 other hand, the infective material must be fairly gener- 

 ally distributed by rain, etc., so that the greater 

 frequency of the disease in stall-fed animals is probably 

 due more to a lowered resistance than to greater 

 opportunities of infection. 



On the whole, it is probable that the prevalence of 

 the disease in a particular herd or race is determined 

 rather by the condition of life under which the animals 

 are kept and their feeding and general hygiene than 

 by any inherited tendency towards infection. The 

 cases seen by Le Sueur, occurring in succeeding 

 generations of Jersey cows, are probably the result 

 of the calves becoming infected from the faeces of the 

 cows, and not of an infection m utero which is exceed- 

 ingly unlikely to take place. 



The frequency with which pedigree and "milk 

 record " animals are sent about the country to shows, 

 etc., and bought and sold or hired for breeding purposes 

 makes it probable that there are very few districts free 

 from the disease. 



The etiological factors which determine the spread 

 of the disease in other susceptible animals have not 

 been accurately determined, but probably they are 

 much the same as for cattle. 



