Detection of the Inflammation : The detection of the inflammation is based upon the response of the animal to the infection. Several significant changes occur in the tissue and in the milk in response to infection. These include infiltration of leukocytes (referred to as somatic cells) and increased vascular permeability, resulting in alterations in the chemistry of the milk resulting from hydrolysis of milk proteins by hydrolytic enzymes and oxidative substances released from phagocytes, alterations in milk pH and ionic solutes, and ingestion of milk components by phagocytes.

Approaches to Detection of Mastitis

Visualization and palpation of the udder : In clinical mastitis the udder may turn hard, red, and hot to the touch. Palpation of the udder may be painful to the cow. These symptoms arise from the changes in vascularity and blood flow of the gland when inflamed.

Visualization of the milk : Gross changes in the milk may be observed at the time of milking such as the presence of flakes, clots or seruous milk. This is the most common means of detection of clinical mastitis. Stripping the first few squirts of milk from each quarter into a strip cup at the beginning of milking is a preferred method of detecting flakes or clots in the milk.

A key response of the cow to infection by pathogens is localized entry of leukocytes (white blood cells) from the blood vessels in the infected tissue into the tissue near the site of infection. This movement of leukocytes from blood vessels to tissue is called diapedesis. The major leukocyte cell type that enters the gland during inflammation is the polymorphoneuclear neutrophile (PMN). Remember that mastitis is defined as inflammation of the mammary gland. And while most often that occurs from infection with bacteria, any trauma to the tissue that causes inflammation, even in the absence of an infecting pathogenic organism, will result in leukocytes entering the tissue.

The leukocytes that enter the milk in the alveoli are called somatic cells and the concentration of somatic cells in milk is called somatic cell count (SCC). The greater the SCC, the higher the level of inflammation in the tissue. So, SCC in milk is an important indicator of the inflammation status of the udder. Because diapedesis of leukocytes is localized, then only the udder quarter that is infected will have a significant increase in concentration of leukocytes (SCC). Tests such as the California Mastitis Test offer a cow-side very rough estimate of the SCC for each quarter and allow for focusing treatment efforts on that quarter. Cow-level SCC indicate the SCC of the mixture of milk from each quarter, such as obtained from DHI records. Ideally, uninfected mammary glands should have somatic cell counts of 50,000 cells/ml or less. In practice, composite milk samples (from all four quarters) of less than 200,000 cells/ml are taken as indicating the absence of infection. As cell counts increase so does the chance that mastitis is present.

Somatic cell counts can be run on milk from the bulk tank as an indicator of herd mastitis status. Bulk tank SCC indicates the overall level of mammary inflammation in the milking herd at each milking. It is the bulk tank SCC that is used by the milk processor plant to determine milk quality premiums to the producer. A bulk tank somatic cell count exceeding 750,000 cells/ml will result in loss of Grade A milk status (the producer can not continue selling the milk until the problem is taken care of). A single cow with high SCC probably will not increase the bulk tank SCC by very much, however if the herd has many chronically infected cows (often as in herds with a Staph. aureus problem or other contagious mastitis problem), then the bulk tank SCC may increase significantly.

Even if the cow is not infected, the sterile milk coming from the cow will become contaminated by bacteria as it moves from the cow to the bulk tank. These contaminating bacteria will include coliform bacteria and other species that ultimately affect the very perishable milk. The bulk tank will always have a level of contaminating bacteria. Normal ranges should be less than 10,000 per milliliter of milk. Herds should try to keep the bulk tank bacterial counts below 3,000 per milliliter. If the bulk tank contains Strep. agalactiae which only survives in the mammary gland, then that means the herd has cows infected with that organism. If the bulk tank has elevated Staph. aureus, then that is an indicator that Staph aureus may be a problem in the herd. However, there are always contaminating coliforms and other environmental bacteria in the bulk tank milk, so bulk tank bacterial counts and speciation will not help diagnosis environmental mastitis problems. Most bacteria should be killed upon pasteurization.

Detection of Somatic Cells : Several methods for detection of mastitis are available for detecting somatic cells in milk, including the California Mastitis Test (CMT; a cow-side test; this is the one we use in class), the Wisconsin Mastitis Test (WMT; on-farm test), Microscopic Somatic Cell Count (usually requires a laboratory), Electronic Somatic Cell Counting (requires sophisticated equipment). The CMT and WMT detect formation of a gel when DNA in somatic cells react with a detergent. The reaction occurs on a paddle (CMT) and is graded subjectively (neg, trace, 1,2,3), or in a tube (WMT) and is measured in millimeters. CMT or WMT results can be used as rough estimates of the number of somatic cells in milk.

Electrical Conductivity : Electrical conductivity of milk increases during mastitis due to increases in Na+ and Cl- and decreases in K+ and lactose. Changes in conductivity can be detected by hand-held or in-milkline instrumentation. The latter is the basis for the computerized milking systems that track electrical conductivity measurements on milk of cows at each milking. This data can be analyzed by comptuer programs to flag cows that have milk electrical conductivity that is altered from normal.