MICROBIAL DISEASES OF MAN AND ANIMALS. 589 



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 produced 

 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 especially 

 the mucous membrane lining milk ducts and produce a catarrhal disease 

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

 tends 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 formation 

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

 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. 



These infections are indicated by dullness, lack of appetite, fever, 

 inflammation of the udder, and by small nodules or cord-like swelling 

 within and lengthwise of the teat. 



It must be borne in mind that the infecting microorganism is the 

 thing to be controlled. Outbreaks of this disease frequently have origin 

 in infected cows added to the herd. Some cows are unsuspected "car- 



