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Laboratory Methods for the Detection of Milk from Cows Infected with Mastitis

University of Idaho, Moscow, Idaho

ABSTRACT

1. Acute mastitis in cows usually is easily recognizable. The affected quarters are intensely inflamed and the cow suffers much discomfort. The milk from these quarters is characterized by producing abundant colonies on dextrose blood agar though the count on plain agar may appear normal. The milk usually contains clots and its consistency is watery—in extreme cases the solids completely separate into a spongy mass which floats in an amber-colored serum. Milk from these quarters contains millions of leucocytes per cubic centimeter; the catalase content is extremely high; usually producing 10 cubic centimeters or more of gas according to the technichere used; the hydrogen-ion concentration decreases until the milk is neutral or slightly alkaline. However, isolated cases are found where the H-ion concentration actually increases; the milk usually contains chlorine in excess of 0.14 per cent, and the curd tension according to Hill's method is almost entirely destroyed.
   
2. Chronic or sub-clinical mastitis is the most common form. This type of infection is commonly so mild that it passes without recognition. The udder appears superficially normal; flakes or clots may appear occasionally but the milk usually appears normal. This condition is very common in dairy herds. The laboratory detection of this milk is best accomplished by examining the milk from individual quarters of the udder. In this type of mastitis the bacterial count often appears normal on plain agar; dextrose blood agar, however, usually reveals an abnormally high count. The leucocyte count is in excess of 100,000 per cubic centimeter, and the catalase test usually produces 2.5 cubic centimeters or more of gas, according to the technic here used; the H-ion concentration is usually only slightly reduced; the chlorine content of the milk is commonly normal; and the curd tension according to Hill's method is usually reduced.
   
3. The addition of dear fresh blood serum from blood cells to milk increases the catalase content markedly. The addition of serum also causes a decrease in the curd tension. Blood serum has a much greater effect in reducing the curd tension than the same degree of dilution by water.
   
4. Chemical tests regularly employed for the detection of pus in urine have been negative when applied to milk known to originate from cases of sub-clinical mastitis.
   
5. The physical examination of the udder possesses merit for locating tissue change due to disease; however, chemical and bacteriological methods alone can actually establish the quality of milk which is produced.
   
6. Retail milk samples when subjected to these tests showed that 33 of the 54 samples tested contained catalase in sufficient quantities to produce 2.5 cubic centimeters or more of gas and 34 of the 54 samples contained more than 100,000 leucocytes per cubic centimeter. The discrepancy between the number of samples showing excessive leucocytes and catalase is due to the pasteurized samples in which the catalase has been destroyed. The H-ion concentration was normal in practically all samples. Only 8 of the 54 samples contained chlorine in excess of 0.14 per cent. The curd tension of herd samples apparently had no relation to the number of leucocytes or to the catalase test.
   
7. According to this study it appears safe to conclude the leucocytes in excess of 100,000 per cubic centimeter and catalase in sufficient quantity to produce 2.5 cubic centimeters of oxygen or more, according to the method reported by Orla-Jensen, are reliable indices of udder infection.