

PREVENTIVE MEDICINE 245 



in which 12 out of 65 animals were attacked. Deer and buffaloes 

 have also been attacked naturally. 



The period of incubation is very long. In experimental cases 

 in which large doses of infective material were given it has been 

 found to be about six months, but in natural cases it is longer, and 

 as a result the disease is only encountered in cattle at least a year 

 old. The organism is strictly obligatory, and in practice it has been 

 found that cases over a period of years always have some con- 

 nection with antecedent cases. The natural method of infection 

 is by ingestion. M'Fadyean and Sheather* found that cattle and 

 sheep appear to offer a marked natural resistance to experimental 

 infection. They also failed to transmit the disease to horses by 

 intravenous inoculation with large doses. Every case in which 

 symptoms appear is fatal, though fluctuations are often seen in the 

 course of the disease. Owing to the chronic diarrhoea which is 

 seen in practically every case, it is obvious that infection is spread 

 far and wide by diseased animals. Pastures become heavily con- 

 taminated, and the virus thus finds its way into food and water. 

 It is not known with certainty how long the bacilli can maintain 

 their vitality in contaminated pastures, &c., but like tubercle bacilli 

 they are fairly resistant to adverse conditions and consequently 

 are probably capable of producing infection some time after being 

 voided from the body. 



PREVENTIVE MEASURES. Although M'Fadyean states that 

 probably 90 per cent, of cases of chronic diarrhoea occurring all over 

 the world are cases of Johne's disease, the difficulties of prevention 

 are somewhat enhanced by the fact that there is no very certain 

 method of diagnosis in the case of infected animals which are not 

 showing well-marked symptoms. In this connection it must be 

 remembered that an animal may be distributing infection through 

 the medium of its faeces for some time before any marked wasting 

 or diarrhoea becomes evident. Preventive measures include 

 destruction of clinical cases and rigid isolation of suspected animals. 

 Those showing only slight symptoms should receive particularly 

 good feeding with a view to being handed over to the butcher at the 

 earliest moment. The animals should be kept stalled in order to 

 limit the spread of infection, and all faeces and litter should be 

 spread on arable land and ploughed in. If practicable pastures 

 known to have carried infected animals should be kept clear of 

 ruminants for at least a season. Sheep suffering from disease in 

 any respect simulating Johne's disease should be carefully examined 

 and if affected should be slaughtered. Sheep owners should be 

 * Journ. Cotnp. Path., 1916, Vol. XXIX., p. 62. 



