Mastitis and Somatic Cells

This page briefly defines and describes the relationship between Mastitis infection in cows, and the presence of Somatic Cells in milk. For more information, see the website of the National Mastitis Council, as several of the articles on their website discuss Mastitis and its Control, A Somatic Cell Primer, and Somatic Cell Count, Mastitis and Dairy Product Quality.


Mastitis is an inflammation of the udder, typically caused by a microbiological infection. Mastitis can occur in all mammals, including humans. Many types of microbes can cause infection and they can be transmitted from both environmental sources (for example, contaminated water, soil, bedding) and from contagious sources (from other infected cows). The microbes can enter the udder and multiply. The microbes can enter milk as it passes through the udder during the milking process. Mastitis in cows (or other dairy animals) can be a source of disease causing bacteria (pathogens) and spoilage organisms in milk.

Milk from cows infected with mastitis generally have higher total bacteria counts and somatic cell counts than milk from uninfected cows. Therefore, bacterial counts and somatic cells counts are used by dairy farmers and processors as indicators of milk quality. In general, the higher the counts, the lower the milk quality. Milk from mastitic cows may have off-flavors and may undergo deterioration of the milk fat and protein more quickly than milk from healthy cows.

There are regulatory standards for microbial numbers (total bacteria count) as well as quality control and human health parameters (somatic cell count, and antibiotic drug residues) in milk, as specified by the Grade A Pasteurized Milk Ordinance (2005). Cows with mastitis are usually separated from the herd to help control the spread of infection and to ensure the quality of the milk produced on that farm. On some farms, cows with mastitis are treated for the infection with antibiotics. The milk from the treated cows is either discarded or diverted to a separate tank to prevent contamination of milk collected from healthy cows. Milk from cows treated with antibiotics is not used for human consumption.

Somatic Cells

Somatic cells are cells from the cow (predominantly white blood cells, otherwise known as leukocytes) that are normally present in milk. During most mastitis infections, the number of somatic cells present in the udder increases to help the cow fight the infection. There are several types of somatic cells that have different functions in fighting infection. Somatic cells can contain lipolytic and proteolytic enzymes, which degrade fats and proteins, respectively. An increase in somatic cells count during a mastitis infection increases the amount of destructive enzymes present in the milk, which increases the rate of deterioration of the milk fat and protein.