THE REACTION OF COW'S MILK TO BLOOD SERUM PRECIPITIN

It seems well established that during normal lactation there passes a slight but well defined quantity of blood protein into the udder. When a specific serum precipitin is applied to diluted milk the reaction occurs at a relatively uniform level of dilution. This line may well be considered the normal level of blood protein elimination. In the main the serum protein level is the same when mastitis streptococci are present as when they are absent, provided they are not too numerous. On the other hand it has been shown that when streptococci are present in large numbers the milk while apparently normal reacts at dilutions considerably higher than the normal. In the non-lactating udder, particularly during the latter end of gestation, serum proteins accumulate. Little and Orcutt have shown that certain antibodies tend to become greatly concentrated within the udder. Howe and others have shown that the colostrum of cows is rich in blood proteins. It is to be noted, however, that colostrum while rich in serum proteins usually reacts with serum precipitin to about one-half the titer of the blood serum itself. It may well be that certain proteins of the blood passing into the acini are reabsorbed, leaving a portion of the antigen within the udder. Thus a gradual accumulation of antigen may occur. Shortly after parturition the udder may be drained with a consequent marked fall in serum content. There, however, apparently exists for the next few days considerable permeability of the capillaries for serum proteins. This is borne out by the reaction of the milk to the serum precipitin, since it may react above the normal level for as long as 10 days or 2 weeks after parturition. During the course of inflammation there may occur a considerable outpouring of blood protein. In the severe cases the exudate may give a titer at as great a dilution as the blood serum. In other milder cases where the inflammation may be confined only to the mucosa of the milk cystern, there is usually little that is abnormal in the appearance of the milk. The precipitin test, however, indicates that serum proteins are present in definitely greater concentration than in normal milk. Of practical interest is the effect of heating the milk on the reaction of serum precipitin. Milk heated to a temperature as high as 66°C. for 20 minutes will react at the same dilutions as the raw milk. Milk pasteurized at 68–69°C. for 20 minutes will no longer react. The precipitin test, then, affords a ready means of detecting milk heated above 68–69°C. It is assumed that the precipitin test may have some further practical application. The evidence presented indicates a well defined serum proportion in the milk which may be increased during inflammation of the udder. The experiments in which mastitis exudate was mixed with varying quantities of market milk indicate that such mixtures can readily be detected with serum precipitin. It seems not improbable that some such procedure might be applied as a presumptive test to the product of small herds or to the mixed milk of a number of cows.