DISEASES OP THE OX AND SHEEP. 171 



a definite micrococcus, and there can be little doubt that the 

 virus is inhaled by the animal, and so enters the lungs. It 

 seems probable that the lining membrane of the air-tubes (the 

 bronchi) is first affected. 



Course of the Disease — Period of Incubation. — It is 

 said that an animal may be infected with the germs of con- 

 tagious pleuro-pneumonia long before any marked signs of its 

 presence are indicated. This is called the "period of incuba- 

 tion," and it has been said to vary in length from two weeks to 

 as much as six months. Generally speaking, signs of the disease 

 are apparent after about thirty-seven days. Owing to this long 

 period of incubation the disease is especially insidious, since it 

 may spread among large numbers of cattle before it can even be 

 detected. Animals, apparently healthy, may convey the disease 

 to others by exhaling the germs. Very generally the first signs 

 of the disease are not noticed. 



General Characters of the Symptoms. — The disease is 

 more rapid in young and vigorous animals than in the old, weak, 

 or sickly. Contagious pleuro-pneumonia may be acute, sub- 

 acute, or chronic in character. In the first case, large portions 

 of the lungs may be rapidly invaded. In sub-acute or mild 

 attacks, the patients may get well in what is called the first stage. 

 The normal condition of the lungs, however, is rarely, if ever, 

 regained. In chronic or prolonged cases, too, the animals may 

 recover, though this is seldom seen. The cough persists for a 

 long time, and it is accompanied by the expectoration of a great 

 deal of muco-purulent matter. The recovery is often, we might 

 say generally, incomplete, signs of the disease remaining one, 

 two, or three, months, during which the animal may infect others. 

 The immediate cause of death maybe general weakness, brought 

 on partly by the absorption of the products of inflammation, partly 

 from other causes. If the febrile symptoms of the second stage 

 come on rather late, the disease may last for several months. 

 We proceed to consider the first or developmental stage, and 

 shall then go on to the second or febrile stage, in which the 

 disease comes to a crisis. 



First Stage. — This stage may continue only a few days, or it 

 may be prolonged for from two to six weeks. An elevation of 

 temperature, or, according to some, a slight cough, is the first 

 sign of disturbance. Of the members of a herd supposed to be 



