MILK-BORNE INFECTIONS 463 



decrease; heat-coagulable albumin may increase remarkably, 

 while casein decreases. The reaction is decidedly alkaline and 

 alcohol coagulates mastitis milk without the presence of acid. 

 Some enzyms, especially catalase, increase in quantity. Coagula- 

 tion due to acid or rennet action is usually delayed. 



Mastitis may be caused by several kinds of bacteria, but 

 streptococci are by far the most common cause. Other organ- 

 isms are Bacillus coli and staphylococci, and occasionally mem- 

 bers of the paratyphoid group of bacilli. 



Streptococcus mastitis is extraordinarily common. Once it 

 has gained a foothold in a herd, it is difficult to eradicate, espe- 

 cially if the pernicious habit of spilling the first few streams of 

 milk on the floor is practised. By this means mastitis streptococci 

 are disseminated through the air and by direct contact. With 

 increasing care and cleanliness dissemination becomes less general. 



The intensity of the disease depends upon several factors, the 

 most important of which is perhaps the relative virulence of the 

 organism, which, as is well known, may vary within wide limits. 

 The affected part or parts of the udder, as a rule, degenerate as a 

 consequence of the disease. In many cases the animal never 

 recovers. The individual susceptibility is an important factor, 

 and accidental lesions of the udder may afford opportunity for 

 the streptococci to gain a foothold. 



It is of importance to recognize cases of mastitis at an early 

 period of the disease. Clinical examination will frequently fail 

 if the disease is in its initial stage or is of the slowly developing, 

 chronic type. Certain laboratory tests have been devised for 

 the discovery of the disease. Some of the tests are based on the 

 presence of body cells in large numbers accompanied by long 

 chained streptococci; other tests are the catalase test (see page 237) 

 and the alcohol test (see page 208). 



Much importance has been attached by some sanitarians in 

 the past to the number of leukocytes in milk. Methods for enu- 

 merating leukocytes have been devised by several authors. The 

 commonest one is the smeared sediment test, which consists in 

 smearing the sediment of centrifugalized milk on a slide, fixing 

 and staining with methylene-blue, and counting the cells in a 

 field of the immersion lens. The first method was devised by 

 Stokes and Wegefarth. Ten c.c. of the milk were placed in a 

 centrifuge and whirled for five minutes. After the supernatant 

 fluid was poured off a loopful of the sediment was spread on a 

 slide, the fat dissolved with ether, and the film then stained with 

 methylene-blue. Ten fields of the microscope were counted and 

 the standard fixed at five cells per field. Above this number, 

 pus was supposed to be indicated. From five cells per field the 



