New York Agricultural Experiment Station. 177 



The reason or reasons for the presence of the leucocytes are not 

 so clear. Many men feel that they are attracted into the milk 

 by the presence of bacteria in the udder, especially by the par- 

 ticular kind of bacteria known as the pus-forming streptococci. 

 This belief has arisen because of a large body of evidence which 

 indicates that the milk of cows suffering from mastitis caused by a 

 streptococcic infection, ordinarily, if not invariably, contains large 

 numbers of cells. But this is not the whole story. It has also been 

 shown that streptococcic infections of the udder occur where 

 there are no pathological symptoms. Are the streptococci 

 present in these cases pathogenic? Does this type of infection like- 

 wise cause an increased cell count? How many cows are infected 

 in this way? 



It is likewise known that the commonest bacteria present in the 

 udder are micrococci which are so closely related to the pus-forming 

 micrococci (staphylococci) that there are no known cultural tests 

 by which the two can be distinguished. Yet no one has ever 

 attempted to discover whether there is a relationship between the 

 presence of these bacteria and the number of cells discharged. Since 

 such bacteria are present in practically all cows' udders, it is evident 

 that if such a relationship does hold it would be without sanitary 

 significance. 



No comprehensive study of the relationship between the number 

 and kinds of cells discharged and the bacterial content of the 

 udder has ever been made which takes these facts into considera- 

 tion and so it is impossible to make even a reasonable guess as to 

 the probable correctness or incorrectness of the commonly current 

 statement that it is possible to detect infection of the udder by patho- 

 genic streptococci by means of high cell counts. No conclusive 

 results bearing on this point have been secured in the course of the 

 present investigation, although a considerable amount of work was 

 done in an attempt to secure such results. 



Enough data were secured however to make it probable that there 

 are other reasons for the discharge of leucocytes in the milk than the 

 presence of bacteria in the udder. These other reasons undoubtedly 

 have to do with the physiological conditions surrounding the process 

 of milk secretion. 



No differential counting of these cells has ever been attempted 

 where a satisfactory technique has been used. It is therefore not 



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