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Subclinical Mastitis -- What You Can’t See CAN Hurt You


By Dr Linda Tikofsky

Added February 3, 2011. Somatic cells are one of the indicators of milk quality and, in the healthy udder, are epithelial cells from the lining of the mammary gland and some types of white blood cells. A healthy gland should have cell counts less than 100,000 cells/ml and typically, counts will be much, much lower than that (<25,000 cells/ml).

When bacteria invade the gland, the few white blood cells that are normally present send out an ‘alarm’ to the rest of the body and recruit thousands of additional white blood cells from the blood stream and into the mammary gland to fight infection. Infected glands will have SCC’s greater than 250,000 cells/ml and often the counts will reach into the millions. If all goes well, these white blood cells will kill the invading bacteria and the SCC of the gland will decreased to less than 100,000 cells/ml within 30 days.

We can estimate the number of infected quarters in a herd and the impact on production from the bulk milk SCC. See table below.

Mastitis is commonly divided into two categories: clinical and subclinical mastitis. Clinical mastitis is those infections that are typically accompanied by the classic signs of inflammation: redness, swelling, heat and pain and abnormal milk. Subclinical mastitis are infections which call high somatic cell counts but the udder and the milk appear normal and can only be detected with methods that measure the number of somatic cells in milk

Annual losses per cow for subclinical mastitis are estimated at $200 year in conventional herds. For organic producers the losses are likely greater because of the higher pay price per hundredweight and the loss of larger premiums. For every clinical case of mastitis in a herd, there are likely 15-40 cases of subclinical mastitis and these cases may be responsible for up to 70% of production losses associated with mastitis.

There are various monitoring systems that detect SCC available and likely one of them is appropriate for your farm:

Electronic cell counting is performed by the Dairy Herd Improvement Associations and combines herd and cow data with SCC to provide more data for analysis. This information is especially useful for determining which cows contribute most to total bulk milk SCC. It is also easier to use the data to pinpoint potential management areas that may be influencing new mastitis cases. Are all high SCC cows fresh? This would indicate a management deficiency during the dry period. Are cows’ cell counts increasing over lactation? Milking time management or contagious mastitis may be the likely culprits in this scenario.

There are some electronic cell counters that are available for on farm use. The Direct Cell Counter (DCC) can return accurate results in a range of 10,000 to 4,000,000 cells/ml. A small cassette is filled a sample of milk and inserted in the DCC and results are available in <1 minute. The results from the DCC are in nearly perfect agreement with DHIA cell counting. Drawbacks to this unit are price of the units and individual cassettes but some organic farms are using this machine to fine tune their bulk milk SCC and achieve their SCC premiums monthly. For them the benefits outweigh the cost.

Recently the Protect has been introduced as another cowside SCC measurement device. A drop of milk is placed on a test strip which incubates at room temperature for 45 minutes and then is read in small handheld machine. Accuracy is similar to DHIA testing. The advantage to this unit is a lower cost than the DCC but time to result is lengthy. This test must be done on fresh milk; as milk ages, the test becomes less accurate.

The simplest cowside tests are the Wisconsin and California mastitis tests. Milk from each quarter is stripped into a special paddle and an equal amount of detergent reagent is added to the milk and then paddle is gently swirled. A thickening and color changed indicates mastitis in one or more quarters. A slight change in consistency and color (1+) is associated with cell counts of (400,000-1,500,000), more thickening is associated with SCC of 800,000-5,000,000 and very thick (egg white) means that quarter has an SCC>5,000,000. . These tests are quite good at detecting cows with very high SCC but may miss those cows with cell counts less than 500,000.

Once cows with subclinical mastitis are identified, action can be taken to improve milk quality. Cows with high SCC two months in a row should have milk samples cultured to determine what bacteria are responsible. Some of the most common bacteria causing subclinical mastitis are the contagious bacteria, Staphylococcus aureus and Streptococcus agalactiae. The longer these bacteria remain undetected in a herd the more likely they are to spread, causing bulk milk SCC to increase over time. Another major cause of subclinical mastitis, especially in fresh cows is environmental Streptococcus spp. These infections may remain subclinical for months but eventually will have clinical episodes. We know that the earlier we identify and Strep spp., the less likely they will become chronic.

The benefits of taking action on subclinical mastitis in the herd outweigh the costs of added labor and expense. Every dollar invested in a mastitis control program will return $15-20 in production, premiums, and reduced death and culling.

Dr Linda Tikofsky is an extension veterinarian with Quality Milk Production Services at Cornell University. She works with organic and conventional dairy farmers on milk quality and udder health. She can be reached at:, or 607-255-8202.