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The Effect of Incomplete Milking on Chronic Mastitis Caused by Streptococcus Agalactiae

Divisions of Veterinary Science and Animal Husbandry, University of California

ABSTRACT

The effect of incomplete milking on chronic mastitis caused by Str. agalactiae was studied with infected cows that were producing a visibly normal secretion at the time they were placed on experiment.

Five cows, having 8 quarters infected with Str. agalactiae, 5 quarters shedding staphylococci, and 7 quarters free of any infection were selected for the first trial on incomplete milking. An attempt was made to leave about 2 pounds of milk in the udder of each cow at every milking over a 13-week period. Under this system of milking, the noninfected quarters continued to produce a normal milk, 4 of the 5 quarters shedding staphylococci infrequently showed visible particles in their foremilk, while every quarter harboring Str. agalactiae developed readily visible symptoms of mastitis. The strip-cup test became positive with 2 Str. agalactiae-infected quarters during the first week of incomplete milking, with 4 such quarters during the second week, with 1 infected quarter during the third week, and with another during the fifth week. With 7 of the 8 infected quarters, flakes, clots, shreds, and sometimes thick pus, were found in the foremilk with great regularity as long as incomplete milking was continued. Two quarters developed acute mastitis; and their parenchyma, as well as that of another quarter, increased in firmness. The chlorine content and the cell count of the Str. agalactiae-infected milk increased significantly as soon as the incomplete milking was started, whereas no appreciable changes were observed in the secretion of the normal quarters. The total bacterial count of both normal and infected milk was not affected to any great extent. When thorough milking was resumed, there was a definite tendency toward a return to a visibly normal secretion, although with 3 quarters production was considerably reduced and the milk was somewhat watery.

Leaving 2 pounds of milk in the udder represents an extreme procedure. Some dairymen, however, do not strip their cows after machine-milking. The effect of this form of incomplete milking on normal and infected quarters was studied on 2 cows with noninfected udders and 4 cows having a total of 6 quarters infected with Str. agalactiae, 6 quarters shedding staphylococci, and 4 quarters free of any infection. These animals were not stripped after normal machine-milking over a period of 17 weeks, except after 2 evening milkings each week to ascertain the quantity of milk left in the udders. The average amount of strippings per udder varied from 0.52 to 1.1 pounds per milking and the average quantity retained by the individual quarters per milking varied from 0.03 to 0.4 of a pound. The non-infected quarters continued to produce a normal secretion, 4 of the 6 quarters shedding staphylococci infrequently contained visible particles in their foremilk, and the quarters harboring Str. agalactiae reacted in a variable manner to nonstripping. A surprising result was that, among the Str. agalactiae-infected quarters, the two retaining the smallest average quantity of strippings, 0.05 and 0.10 of a pound of milk per milking, developed the most pronounced clinical symptoms of mastitis, while quarters retaining from 0.2 to 0.4 of a pound of milk per milking only infrequently showed mild evidence of mastitis. In every case, however, the inflammatory process in the Str. agalactiae quarters was aggravated to some degree, as indicated by a significant rise in cell count when nonstripping was practiced. The chlorine content and the bacterial count of both normal and infected milks were not affected materially.

The number of cows studied was small. The results, though not conclusive, especially those of the second experiment, indicate that incomplete milking may lead to increased severity of the clinical manifestations of Str. agalactiae infections. Conversely, thorough milking of cows affected with chronic mastitis seemed to reduce the severity of the disease. It is hoped that the results reported here will motivate further study of this important, aspect of the mastitis problem.