CONTROL OF THE PUBLIC MILK SUPPLY 437 



chain streptococci in some of the milk, and in every instance where he 

 traced it back to the farm, found one or more infected udders in the herd, 

 but of 9,387 cows that he examined 309 or 3.3 per cent, were positive; 

 4 out of 40 herds supplying the milk were badly affected and but 8 out 

 of the 30 dairymen who furnished milk continuously for twenty months 

 did so without bringing any streptococcic milk. 



Garget due to streptococcal and to other bacterial invasions of the 

 udder is very common, but from our present knowledge of septic sore 

 throat there is no reason to believe that the disease is derived from the 

 ordinary run of garget cases. Practically always, it seems to be due to 

 the Smith streptococcus which may infect the cow's udder from human 

 sources. Outbreaks of diarrhea have been produced by streptococci 

 coming from the udders of cows suffering with mammitis. So there is 

 good reason for removing cows with udder trouble from the herd. When 

 such animals shall be returned to the herd is usually left to the judg- 

 ment of the herdsman, who makes his decision on the appearance of the 

 udder and of the milk. The wisdom of this procedure is doubtful for 

 the infective organisms often persist long after physical evidence of their 

 presence has disappeared. 



Biological Test for Heated Milk. Frost has proposed the folio wing 

 method for detecting pasteurized milk by direct microscopical examina- 

 tion. One part of a saturated aqueous solution of methylene blue is 

 slowly added from a pipette to 5 parts of milk, the milk being rotated in 

 a flask during the addition to prevent coagulation of the milk which 

 vitiates the test. After 15 to 30 min. the stained milk is centrifuged in 

 Slack tubes or ordinary urine tubes and the sediment is smeared on glass 

 slides and dried when the smear is ready for examination. 



The stain is made up by adding 7 grains of Gruebler's dry dye to 

 100 c.c. of distilled water and allowing it to stand several hours with 

 frequent shakings. Finally, it is filtered. 



The differentiation between pasteurized and unpasteurized milk 

 depends upon a twofold change effected by heat on the leukocytes, viz. : 

 (1) it alters the shape and size of the cells; and (2) it changes their staining 

 reactions. 



The smear from the unpasteurized milk under the low power of the 

 microscope gives a light blue field in which the depth of the stain depends 

 on the thickness of the film but which very uniformly is stained more 

 highly than any of the smaller objects in the field. The oil immersion 

 lens shows the mononuclear cells well-stained and they are not considered 

 in this test. 



The polymorphonuclear cells are distinguished with some difficulty 

 and appear colorless, of irregular outline and 12/x in diameter. If the 

 nuclear material is differentiated at all, it is not well-defined. 



The smear from pasteurized milk under low power appears less 



