Natural Infection. 393 



sponds to contagious pleuro-pneumonia. Tlie possibility of 

 infection by means of inlialed air is proven to some extent by 

 the successful experiments of Nocard & Roux, as well as by an 

 older experiment of Chauveau, in which a cow that inhaled air 

 from an affected animal through a sack passed over the heads 

 of both animals, became affected with contagious pleuro-pneu- 

 monia. 



As was shown by experience, the disease spreads by direct 

 contact of healthy with atfected animals, while the transmission 

 of the virus by intermediate hosts has not yet been positively 

 established. The contagion appears to remain virulent in in- 

 fected stables for a long time, and infects animals which are 

 subsequently placed in such stables. On the occasion of out- 

 breaks careful investigation usually reveals the fact that at 

 some time previously one or more affected animals had been 

 introduced into the affected herd. At the same time the disease 

 spreads preferably among cattle which are stabled closely 

 for a long time, but sometimes a rapid spread may also be 

 observed during the pasturing season. 



Affected animals may transmit the disease in all stages to 

 healthy individuals, the transmission being however easiest and 

 most effective during the acute sjanptoms. The virus may re- 

 main effective in the affected lungs for years, and may even 

 retain its virulence in the sequestrated and later encapsulated, 

 necrotic parts of the lungs. If an entirely closed connective 

 tissue capsule forms around such a sequestrum and other parts 

 of the lungs remain healthy, the infectiousness of the animal 

 is suppressed for a time ; however as soon as a communication 

 opens between the cavity and a bronchus, some parts of the 

 sequestrum and wdtli it micro-organisms enter the air passages, 

 whereupon the animal may again infect others. The disappear- 

 ance of the acute symptoms and improvement or apparent 

 recovery does not therefore mean that the animal is no longer 

 infectious. 



According to Walley the virus may remain active in the encapsulated cavities 

 for 15 months; according to other authors the infectiousness may last even con- 

 siderably longer. Minette traced outbreaks in herds to cows purchased two and 

 three years previously. 



The susceptibility of cattle to the disease varies greatly 

 in accordance with the breed and method of living, and also 

 with the individuals. The more hardened range cattle resist 

 natural infection far more than the more highly bred stable 

 cattle. The less frequent occurrence of the disease among range 

 cattle may also be explained by the fact that such animals live 

 more out of doors, and are therefore less exposed to an inten- 

 sive infection than milk cows or fattening cattle kept standing 

 closely in stables. Sometimes in herds of one and the same 

 breed some animals may withstand the disease in spite of favor- 

 able conditions for infection. 



