380 DISEASES OF CATTLE. 



order to produce the disease. Some experimental attempts to infect 

 animals by placing them in stables where diseased animals have been, 

 and by placing the diseased lungs of slaughtered animals in their feed- 

 ing troughs have failed, and, consequently, apparently confirm this 

 view. 



On the other hand, it is known that the serum from affected lungs 

 retains its virulence and maybe used successfully for inoculation weeks 

 or months after the death of the animal from which it was taken. This 

 is particularly the case when this liquid is hermetically sealed in glass 

 tubes. Other investigators state that they have successfully infected 

 cattle by placing in the nostrils sponges or pledgets of cotton saturated 

 with such serum. Cattle have also, according to the best evidence 

 attainable, been infected from the clothing of attendants, from horns 

 used in drenching, and from smelling about wagons which have been 

 used to transport carcasses of animals affected with this disease. In 

 the work of eradicating pleuro-pneumonia from the United States many 

 stables have been found in which the disease would appear and reappear 

 after the slaughter of affected herds, and in spite of any precautions 

 which could be adopted. These were always old stables, with wood- 

 work in a decaying condition and with floors underlaid with filth which 

 could not be thoroughly removed or disinfected. In everyone of these 

 cases the destruction of the stable, the burning of the lumber of which 

 it was constructed, the removal of the accumulations beneath the floors, 

 and the thorough disinfection prevented the recurrence of the plague in 

 new stables built upon the same premises. This experience conclusively 

 shows that under certain conditions, at least, stables may retain the 

 infection for a considerable time, and that when restocked the disease 

 may break out again from such infection. 



As a rule, however, the disease is acquired by a healthy animal being 

 near to an affected one and receiving the contagion direct. Affected 

 animals may give off the contagion in the early stages of the disease 

 before the symptoms are apparent to the observer, and they may retain 

 this infectious character, if they survive the attack, for six months and 

 probably for a year after all symptoms of the disease have disappeared. 



Incubation. The time which elapses between exposure to the conta- 

 gion of pleuro-pneumonia and the first appearance of the symptoms of 

 this disease varies greatly with different individuals and with different 

 outbreaks of the disease. Ordinarily the symptoms of disease make 

 their appearance within from three to six weeks after exposure ; but 

 they may be observed within two weeks or they may not become ap- 

 parent until nearly or quite three months. It is this long period of 

 incubatfon, and the great length of time that an animal may dissemi- 

 nate the contagion after apparent recovery, which give the plague that 

 insidious character so often spoken of, and which greatly increases the 

 difficulties of eradication. 



Symptoms. The symptoms are such as would be expected with inflani- 



