MICROBIAL DISEASES OF MAN AND DOMESTIC ANIMALS 739 



milk; but there is little information on this point, based on careful 

 work. 



This is to be considered as an infectious, enzootic disease and prob- 

 ably not specific. There is good reason to suppose that different out- 

 breaks have been due to several different pyogenic or pus-piroducing 

 organisms. 



We cannot consider any one species of bacteria as the specific cause. 

 Various micrococci, streptococci, and staphylococci have been found 

 acting as causal agents. 



Recent evidence indicates that udders of apparently healthy cows 

 may contain a variety of bacteria and that the infections may remain 

 more or less permanent. This is in part the explanation of recurrent 

 cases of mastitis. 



In the animal body this infection is practically limited to the udder. 

 Its discharge is either through the teat or rarely by external rupture 

 of abscess. Transmission from cow to cow is indirect, and frequently 

 on milkers' hands. 



Entrance is usually effected by way of the milk ducts; thence into 

 the milk cistern and to more remote parts of the gland. The infection 

 may also come by way of the blood or lymph channels to the glands. 

 A given case may thus be due to bacteria previously in the udder, the 

 attack being determined by an area of lessened tissue resistance pro- 

 duced by injury. 



In one class of cases, the gland structures are first involved; in other 

 cases the connective tissue frame-work is first involved. In one type of 

 this disease caused by streptococci these microorganisms attack espe- 

 cially the mucous membrane lining milk ducts and produce a catarrhal 

 disease of that membrane. This is indicated by a cord-like swelling 

 which extends along the milk canal through the teat to the milk cistern. 

 This infection frequently leads to "blind quarter; " i.e., to closure of the 

 teat canal and loss of the quarter; or this infection may lead to the for- 

 mation of one or more pea-like nodules along the teat canal and conse- 

 quent obstruction. 



In many cases the lactose is decomposed by the invading organisms, 

 leading to the formation of organic acids. These acids produce coagu- 

 lation. The coagula soon obstruct the milk ducts and alveoli and the 

 secreting cells degenerate. The invaded tissues may suppurate or even 

 become gangrenous. 



