Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Animal Husbandry

Major Professor

W.W. Overcast


Since the advent of organized dairying mastitis has been a constant problem to the dairy industry. Modern developments in pro duction such as confinement of cattle, increased production, and management practices, have aggravated this problem, especially since farm labor has been hard to obtain and train. In the United States mastitis reduces the milk yield and shortens the productive life of affected cows, causing an estimated loss of 225,504,000 dollars annually (1,15). This amounts to an average annual loss of 10 dollars per cow for each dairy cow in the United States, with the loss for some indi-vidual cows as high as 100 dollars, and total loss in case of death. Mastitis is, without question, one of the dairyman's largest economic losses. Research conducted on many phases of mastitis has produced volumes of literature on the predisposing factors, etiology, diagnosis, prevention, control, and treatment. Study of these research data have resulted in the initiation of many educational programs, only to have more complexing problems evolve. This has been the case with the causative organisms of mastitis. In former years streptococci were almost the sole species responsible for mastitis. However, in recent years staphylococci have emerged as a more dangerous threat to the dairy man since they are one of the most difficult organisms to control. McCoy (54), presenting figures to the Mastitis Conference at Chicago in 1960, indicated that; one cow in four was shedding staphylococci from at least one quarter; this was a twenty times higher incidence of mastitis than the herd owner was aware of or than the veterinarian could detect on routine examination, and there was no correlation between the shedding of staphylococci and total plate count or methylene blue reduction time. Therefore, there was no way for the owner to detect the presence of staphylococci at an early stage. The rise in staphylococci and fall in streptococci as the etiological agent of mastitis has been explained on the basis that antibiotics are highly effective against streptococci and the way has been opened for the staphylococci to invade the udder without considerable competition. Jezeski (36) observed that an increased number of isolated organisms showed characteristics similar to those associated with human disease. The possibility exists that cows have become infected with the so-called "human strains" of staphylococci. The udder thus may become a reservoir for staphylococci capable of causing disease in humans. Many times the bovine udder harbors various species of bacteria without noticeable symptoms of infection. These mild or chronic cases may be quite dangerous as they may be carriers and spreaders of the organisms to other cows in the herd. Likewise, these same organisms may be responsible for acute infections when favorable conditions develop in the udder. Therefore, a good management program should include some method for diagnosis at regular intervals. Some barn tests that have been used to determine the presence and extent of mastitis in a herd are: physical examinations of milk and the milked out udder for abnormalities, strip-cup teat for clots, bromthymol blue test for altered pH, Whiteside Mastitis Test, Negretti Mastitis Test, and the California Mastitis Test, referred to hereafter as GMT. Of these tests the GMT has been found to be the most effective diagnostic test at the time of milking. The GMT, developed by Schalm (77), estimates the degree of udder irritation. This test has been broadly related to the number of leucocytes present in the milk. However, the GMT does not necessarily detect infection, nor can it distinguish between infectious and noninfectious mastitis; it is merely an estimate of udder irritation. The GMT has been found to be a good screening test for dairymen to use. However, the GMT should not be the sole criteria for establishing a mastitis treatment program. Because of the increased use of the GMT as a field test for detecting udder irritation, and the observed increase in the incidence of staphylococcal mastitis, and because few studies have been made correlating the incidence of staphylococci with the GMT in raw milk, the present investigation was undertaken.

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