From the NMC newsletter "Udder Topics", December 1997

Demands by consumers and processors for safe, high quality dairy products together with pressure from international markets for products with documented high quality, are the major factors driving somatic cell count (SCC) standards.

Milk SCC is used to monitor inflammation of the mammary gland. There is considerable agreement among research workers that the primary cause of inflammation is bacterial infection. Good udder health is essential for quality milk production. SCC is the most widely accepted criteria for measuring udder health and milk quality in all major milk producing countries throughout the world.

At the cow level, SCC is used to determine if an individual cow or quarter is mastitic and the likelihood that the cow or quarter is infected. Bulk tank milk somatic cell count (BTSCC) is a measure of the amount of mammary disease or mastitis in the dairy herd. In addition, BTSCC is related to a number of product quality issues such as cheese yields, shelf life, appearance, flavor, and odor.

Physiological Standards

Several studies over the past 20 years have addressed issues surrounding SCC, their variation, and the potential use of SCC for monitoring milk quality. Clearly the major factor affecting SCC is a mammary gland infection -- this holds true at the quarter, cow, or bulk tank level. Other factors such as lactation number, stage of lactation, and season of the year have only minor influences by comparison.

Most studies suggest that cows with SCC less than 200,000 cells/ml are not likely to be infected with major mastitis pathogens, while cows with SCC of 300,000 or greater are very likely to be infected. Depending upon the study, the threshold SCC indicative of inflammation or mastitis has been set at 200,000, 250,000 or 283,000 cells/ml. A SCC reference value of 250,000 appears to be a useful standard indicating that milk from individual cows is either mastitic (250,000 or above) or physiologically normal (less than 250,000). While statistically this value is useful as an indicator of the presence or absence of intramammary infection, a small percentage of cows truly infected with major pathogens will have cell counts less than 250,000. In addition, some cows infected with the minor pathogens and particularly the coagulase negative staphylococci, will have SCC exceeding 250,000. Also, cell numbers in milk are not static but part of a dynamic process regardless of the infection status. Given the dynamic nature of SCC in mammary glands, interpreting SCC data based on a single sample is subject to considerable misdiagnosis or misclassification.

Bulk milk SCCs have three broad uses. They are used to monitor mastitis prevalence in dairy herds, as an indicator of raw milk quality to processors, and as a more general indicator of the hygienic conditions of milk production on farms.

Bulk tank SCC is a function of the percentage of quarters infected by major pathogens in a dairy herd. Eberhart et al. (J. Food Prot. 45:1125) reported a linear relationship between BTSCC and the percent quarters infected with major pathogens. Using this relationship, the percent quarters infected at BTSCC levels of 200,000, 400,000, 750,000, and 1,000,000 cells/ml is 6.2, 12.8, 24.3, and 32.6, respectively.

BTSCC primarily measures subclinical mastitis, and is a poor monitor of clinical mastitis in dairy herds. Given the fundamental differences in epidemiology of mastitis pathogens, BTSCC does a better job monitoring mastitis caused by contagious pathogens such as Staphylococcus aureus and Streptococcus agalactiae than mastitis caused by the environmental pathogens such as Streptococcus uberis and Escherichia coli. The latter frequently cause clinical mastitis, and high SCC milk from clinical cows generally is withheld from the bulk tank. The experience in the U.S. is that many herds consistently producing low BTSCC milk (<250,000) may still experience undesirable incidence of clinical mastitis. However, no evidence is available indicating that clinical mastitis will automatically increase if a herd produces low SCC milk.

Research over the past 10 years has established a clear relationship between BTSCC and milk quality for processing purposes. Much of the reduced quality associated with mastitic milk is due to the enzyme plasmin. Plasmin is found in both blood and milk, and increases in milk during inflammation of the mammary gland. Plasmin can cause extensive damage to milk casein in the udder prior to milk removal, consequently reducing dairy product quality and yield.

Bulk tank SCC is also used as an indicator of hygienic conditions of milk production. In general, the hygienic conditions on farms producing low SCC milk are more desirable than conditions on farms producing high SCC milk. The relationship between BTSCC and percent quarters infected would suggest a reduced probability of finding potential human pathogens in low SCC milk. Such pathogens include: Staphylococcus aureus, Escherichia coli, Campylobacter jejuni, Yersinia enterocolitica, Listeria monocytogenes, Salmonella spp. and Clostridia spp.

Regulatory Standards

As a result of the relationships described above, BTSCC is used: 1) to evaluate mastitis control in dairy herds; 2) in payment schemes for milk; and 3) by regulatory agencies as a monitor of hygienic production and milk safety. Regulatory agencies in the various countries have established upper SCC limits for raw milk. The European Union has imposed a 400,000 cells/ml SCC limit, while the U.S. limit is 750,000 cells/ml. The argument for the higher upper limit in the U.S. has centered around the concept that no direct human health risk can be attributed to SCC levels less than 750,000 cells/ml. The lower limit in the European Union may be a function of greater use of raw milk and raw milk products than occurs in the U.S., and greater emphasis on milk quality for processing purposes.

Various milk cooperatives and processors in the U.S. have encouraged low SCC milk production by offering producers incentives for low SCC milk. Regardless of SCC limits imposed by regulatory agencies or monetary incentives for low SCC milk, the goal of a small percentage of producers will always be to remain legal rather than to produce low SCC, high quality milk. Lower standards may be necessary to persuade such producers to produce high quality milk. Lower standards may be more difficult for small herds to consistently achieve as a single cow in a small herd can have a major impact on BTSCC. Conversely, there is considerable dilution of the single cow in large herds.

As a result of increasing world trade in milk and milk products, there is need to establish a world standard for SCC. Mastitis can not be eradicated from dairy herds and zero mastitis in dairy herds is not a realistic goal. The relationship between BTSCC and percent quarters infected suggests that 12.8% of quarters would be infected at a BTSCC of 400,000 and 24.3% of quarters infected at a BTSCC of 750,000. Obviously, considerable mastitis exists even at a SCC of 400,000; while a herd at 750,000 has a clear and serious mastitis problem. The technology to consistently and economically produce milk with a BTSCC of less than 400,000 is available in all major dairy countries of the world. These data argue in favor of the European Union SCC standard of 400,000 as the basis of international trade in safe, high quality, milk and milk products.

Editor's Note: This article was excerpted from a paper presented by K. Larry Smith, Ohio State University, at the International Dairy Federation (IDF) meeting in Vienna, Austria, September 1995. The entire article was published in the IDF Mastitis Newsletter, September 1996.