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Mastitis Case Studies

Milk Composition and Quality

This resource describes milk quality as it relates to mastitis.

Changes in Milk Composition and Quality: Mastitis results in several changes in milk composition which can be used in mastitis detection (see Detection of Mastitis), but which also affect the quality and value of milk.

  • lactose - synthesis is decreased.
  • casein - proteolysis (plasmin from blood, proteolytic enzymes from leukocytes and bacteria) leads to poor curdling, lowered cheese yield, altered texture of cultured products, production of peptides, bitter taste. This is particularly important if the producer is being paid a premium for milk quality.
  • milk fat - susceptibility of milk fat globule membranes to the action of lipases, resulting in breakdown of triglycerides, oxidation of fatty acids, and off-flavors.
  • Na+, Cl-, K+ - electrical potential across apical membrane disrupted. This is the basis of the electrical conductivity methods of detecting mastitis.
  • Polymorphonuclear neutrophils (PMNs) - also called somatic cells (SC) in the milk. Mastitis causes chemotaxis of the cells into the tissue and disruption of epithelial tight junctions. Associated logarithmically with the decrease in milk production. This is the basis of many mastitis detection methods.
  • albumin, immunoglobulins, pH (bicarbonate) - enter the milk via disrupted tight junctional complexes. These have also been used as the basis for some mastitis detection methods.

Grade A Milk: Producers are issued a permit by their state to ship Grade A milk. To meet the standards of Grade A milk, the milk shipped should not exceed 40 F and should have a lowbacterial count. Repeated violation of these standards can lead to revocation of the producer's permit. Some other standards included in designation of Grade A milk include: prohibition of addition of water (see Freezing point of milk in the Milk Composition module), absence of antibiotics (see resource on Mastitis Treatment & Control), exclusion of milk from diseased animals and of colostrum, proper construciton and maintenance of facilities, safe water supply, sanitary waste disposal, milking equipment properly constructed, cleaned and sanitized, and cooling milk within the prescribed period of time to a temperature that restricts microbial growth. Non-Grade A milk would either be considered as manufacturing grade or reject milk.

Bulk Tank Bacterial Counts: Herds should strive for bacterial counts from the bulk tank milk of less than 10,000 per mililiter. Having an elevated bulk tank bacterial count may mean that the milk is becoming contaminated after the milking process (post-harvest), or it may mean that there are a number of cows in the herd infected and sheading large numbers of bacteria. This is particularly true of there are cows infected with contagious pathogens. A high proportion of Staph. aureus probably indicates several chronically infected cows. The presence of any Strep. agalactiae would indicate that that organism is causing a mastitis problem (remember that S. agalactiae only survives in the udder, therefore it could only come from an infected cows' udder).

Somatic Cell Count: The concentration of leukocytes in a cow's milk, also called somatic cell count (SCC) is the best indication that a cow has mastitis. When a quarter becomes infected, the bacteria and the tissue may send out chemoattractant substances that attract neutrophilic leukocytes to leave the blood and enter the tissue and eventually make their way into the milk in the alveolar lumen. This is part of the inflammatory process that occurs in the tissue during infection. The degree of infection is somewhat proportional to the number of leukocytes that enter the gland. The SCC can be determined crudely in the cow-side test as used on-site by veterinarians or other herd health consultants (for example, the California Mastitis Test) or by analyses on milk samples sent to an authorized laboratory, such as in the case of the monthly Dairy Herd Improvement (DHI) testing.

Antibiotic Residues: These are residual antibiotic in the milk and tissue after treatment with antibiotic for mastitis. Antibiotics presist in the tissue for a period after treatment. Each antibiotic has a prescribed period where the milk from that cow should not be put into the bulk tank. See above for Grade A milk and see Mastitis Treatment & Control for Antibiotic Treatment.

Premium Payments: Producers of milk are paid for the volume that they ship, but also for the nutrient value of that milk (% fat, % protein or % solids-not-fat) and for the "quality" of the milk as indicated by Somatic Cell Count. Some producers who primarily milk Holsteins (highest milk volume producing breed, but lowest milk fat and milk protein percentage of the breeds) also will have several Jersey cows to add a little higher milk fat percentage to the bulk tank milk. Somatic cells in the milk secrete proteolytic enzymes that partially degrade the casein, thereby decresing the yield of cheese obtained from the milk. Cheese manufacturers of course want milk with very low SCC. Also, bacteria and the SCC can generate oxygen radicals that partially hydrolyze lipids (milk fatty acids) and can change the taste and smell of the milk. Milk samples are taken each time the milk truck picks up milk from a farm. That sample is taken back to the processing plant and tested for these components, including the presence of antibiotics in the milk.

Mastitis Case Studies
Mastitis Resources