346 VETERINARY PATHOLOGY. 



side by side. They do not form spores, but thejr may contain 

 granules and vacuoles, and tbey may have a beaded appearance 

 because of fragmentation of their cytoplasm. 



The Bacterium tuberculosis is extremely resistant to external 

 injurious influences, probably because of a wax-like substance 

 that constitutes about one-third of the body weight and forms 

 the principal part of the external covering or capsule. (These 

 bacteria arc stained with difficulty but when once stained retain 

 their stain even thinigh subjected to the action of alcohol and 

 acids.) The staining peculiarities are probably due tn a fatty 

 substance the)^ contain. 



Source of infection. — The bacterium tuberculosis may be 

 transmitted direct from tubercular to healthy animals, Intt infec- 

 tion is more frequently L>btained from foodstults, or barns, feed 

 racks, watering troughs, posts, soil, etc. Tubercular animals 

 are almost constantly eliminating the bacterium which contam- 

 inates ever}-thing that the tulaercular discharges contact. The 

 cadavers of tuberculous animals are usually deposited in the siiil, 

 and, in many instances, the proper precautions are not taken to 

 destrov the infecting micro-organism. Infected manure is 

 spread upnn the soil and thus it becomes infected. The various 

 crops, including hav, gr(.iwn upon a tubercular infected soil, may 

 be ciintaminated with the Piacterium tuberculosis and infect sus- 

 ceptible animals that consume such food. Sometimes the car- 

 casses of animals dead of tuberculosis are thrown into rivers or 

 creeks, thus infecting the water. The waste products of many 

 small slaughter houses are fed to hogs and this affords oppor- 

 tunity for them to become infected. Skimmed milk and whey 

 from creameries and cheese facTi.iries arc also sources of tuber- 

 cular infection. 



Channel or avenue of entrance of the infection. — The Bacter- 

 ium tuberculosis may gain entrance into the tissues of a healthy, 

 susceptible animal through the mucous membranes or through 

 abrasions of the skin, though the latter mode of infection is not 

 of common occurrence in domestic animals. Cutaneous infec- 

 tion is occasionally observed in the mammae of sows and in the 

 castration wounds of barrows. 



From clinical and experimental evidence and autopsy lesions 

 observed in abattoirs, it seems evident that the digestive tract 

 is the principal channel of entrance of the Bacterium tubercu- 

 losis in hogs, cattle and fowls. It was originally erroneously 

 concluded that the presence of pulmonary tubercular lesions was 

 positive evidence that the infection had gained entrance through 

 the respiratory tract. Tubercular free experimental animals fed 



