244 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



the bacilli are destroyed, the infection checked. Often they fail in 

 their object and are themselves destroyed and the multiplication of 

 the germs continues. 



The second line of defense is formed by the cells of the tissue 

 invaded by the germs. These cells arrange themselves in a circle 

 around the germs and try to form a living wall between them and 

 the rest of the body. This barrier gradually becomes thicker and 

 thicker and forms a little hard lump or tubercle from which the dis- 

 ease gets its name. If this wall is complete and successfully impris- 

 ons the bacilli, these gradually die and the disease in that particular 

 spot is arrested. 



Frequently, however, both these safeguards are overcome. The 

 germs break through the barriers and are carried in the blood stream 

 or lymph channels to other parts of the body. ^ New points of attack 

 are selected and the process begins again but with less chance on the 

 side of the animal. As the tubercles increase in number the power of 

 the body to grapple with them becomes less and less, and gradually 

 the animal falls a prey to the disease. 



The tubercle bacillus does not multiply outside the body of an 

 animal. It can live for a long time in favorable surroundings, such 

 as dark and dirty stables. Sunlight soon destroys it. Freezing does 

 not hurt it, but it can only stand a moderate amount of heat. Ex- 

 posure to 149 F. for 20 minutes kills it. Protected by a layer of 

 dried mucus, such as is coughed up from the lungs, it withstands 

 drying, light, and ordinary disinfectants, but is readily killed by 

 steam or boiling water. 



How the Disease Spreads. Sooner or later the tuberculous cow 

 begins to give off the germs of the disease. The germs escape by the 

 mouth and nose, the bowels, in the milk, and in discharges from the 

 genital organs. When the germs are being given off in any of these 

 ways, the disease is known as open tuberculosis. 



Germs discharged from the mouth and nose are coughed up from 

 the lungs and are sprayed over the food in front of the cow or are 

 carried in the air for a time until they fall to the ground. Cows in 

 adjoining stalls may take in these germs in the air they breathe or 

 in the food they eat and so contract the disease. 



Germs discharged from the bowels are mixed with the manure, 

 and may infect cattle and hogs that are allowed to pick over the dung 

 heap. The practice of having hogs and cattle together in the same 

 yard is sure to result in the infection of the hogs if any of the cattle 

 are affected. The germs in the manure come from matter that is 

 coughed up and swallowed, and in some cases from tuberculosis in 

 the bowels themselves. Manure containing tubercle germs may easily 

 infect the milk. Particles of dried manure may fall into the milk 

 pail from the skin of a dirty cow or be accidentally flicked off from 

 the tail and fall into the milk. Straining the milk afterwards only 

 removes the larger particles. The smaller ones, including the germs, 

 remain in the milk. 



When the udder is tuberculous the milk contains the germs in 

 vast numbers. Such milk may look and taste perfectly good, but 



