17 September 2011

5 minute read

Mastitis is a universal problem that all dairy producers struggle with to varying degrees at one time or another. Management strategies to reduce the risk of mastitis usually focus on decreasing exposure of cows to the bacteria that infect the mammary gland.

These strategies include practicing proper milking protocols, paying close attention to hygiene, maintaining clean free stall beds and corrals, and limiting introduction of infected animals to the herd. Tools have been developed to help dairy producers detect mastitis, and these have evolved over many decades from simple cow side tests to specialised DNA analyses.

Few tools were available to monitor mastitis in the early days of dairy production. For individual cows, one relied on observation to find cows with abnormal milk or swollen quarters. By the time these clinical signs were obvious, infection was already well established. Early detection is valuable to identify infected animals so that effective treatment decisions can be made.

The Standard Plate Count (SPC) was one of the first milk quality tests applied to measure total bacteria in milk. The SPC of commingled milk reflects hygienic conditions of milk harvest and storage, and also gives an indication of udder health status. In 1915, the SPC upper limit in California for raw milk was 200,000 cells/cc. This was reduced in the 1920’s and 30’s, and finally dropped to the current SPC limit of 50,000 cells/cc in 1970 [1]. In the late 1950’s, a simple and inexpensive cow-side test called the California Mastitis Test (CMT) was developed by veterinarians at UC Davis.

The CMT indirectly measures immune cells that travel to the mammary gland in response to an infection. With this test, dairymen could find cows with subclinical mastitis before visual signs developed. DHIA laboratories used the test to provide members with milk quality information in addition to monthly milk weights. The CMT ushered in a whole new era of milk quality management, and it continues to be widely used around the world.

In the 1970’s, technology for directly counting immune cells (also known as somatic cells) in milk became available. DHIA and other milk testing laboratories embraced this technology and replaced CMT reporting with more precise SCC information. In California, a regulatory threshold of 750,000 cell/ml SCC in bulk tank milk was adopted in 1970 (this was later reduced to 600,000 in 1990).

In recent years, other on-farm tests for measuring SCC in milk have become available, including the Porta-SCC and the Delaval Cell Counter. Both the CMT and the SCC are measures of the cow’s own cells in milk that are present to fight an infection. Other tests are needed to determine the types of bacteria causing the infection.

Bacteriological culturing of milk samples from individual cows or from bulk tanks is a way to distinguish specific mastitis pathogens. A small amount of milk is spread onto a plate of culture medium – usually bovine blood agar (BBA). The plate is incubated for 48 hours allowing bacterial colonies to grow on the plate if bacteria are present in the milk sample.

Different bacteria can be distinguished by trained technicians based on the appearance of the colonies that grow – Strep ag, Staph aureus or Coliform for example. Knowledge about the type of bacteria that are present helps to focus mastitis control efforts.

In California, routine programs for monitoring mastitis pathogens in bulk tank milk were initiated by UC Cooperative Extension Farm Advisors. One of the pioneers was Richard N. Eide, who worked with Danish Creamery, Fresno County DHIA and Sunnyside Veterinary Clinic to organise a monthly testing and reporting system for dairy producers. This was no small feat in the 1970’s, before personal computers and cell phones. Tracking monthly SCC and mastitis pathogens helped dairymen, farm advisors, veterinarians and field staff to implement programs to improve milk quality.

Routine culturing of bulk tank milk has been common for over 30 years. Culturing milk from individual cows is also common and is another way to keep tabs on infectious bacteria in the herd. Some dairies culture milk samples from cows when they freshen, or before dry treating. Others culture milk samples from cows with high SCC or clinical signs of mastitis to help guide treatment decisions. Several commercial laboratories offer bacteriological culturing services. The UC Veterinary Medicine Teaching and Research Centrw in Tulare has a world-renowned Milk Quality Laboratory. A few dairies have even established on-farm milk culturing capabilities.

Development of molecular-based testing for identification of pathogens in milk is the most recent innovative management tool. The analysis is based on a genetic “fingerprint” contained in the DNA of every living organism. A process called PCR (polymerase chain reaction) amplifies unique sections of DNA, and these are used to identify the presence of specific mastitis pathogens. PCR was developed in the 1990’s, and it has seen extensive use for clinical diagnostics in many fields. Commercial applications of automated PCR tests recently became available for milk. Lancaster DHIA in Pennsylvania was the first milk-testing laboratory in the US to introduce it in 2010. Benefits of PCR compared to conventional culturing include speed, accuracy and convenience.

Identification of all major mastitis-causing bacteria is possible in only four hours. Samples collected on DHIA test day can be used. On the down side, cost and interpretation of the results are barriers to widespread use. At about $20 per sample compared to about $5 for culture, only those with an urgent need for immediate results are likely to use the PCR option. Questions remain about what the results mean and how they should be applied to dairy management strategies. UC Cooperative Extension field studies will address these questions. I am excited to participate in the evolution of this incredible new tool for mastitis control on dairies!

From SPC, CMT, SCC, and BBA to PCR - what is next? I recently met with molecular biologists in the Animal Science Department at UC Davis. They described novel DNA based tests that are useful in human medicine, and these will make even the new PCR test for mastitis pathogens obsolete! Scientific advances will continue to provide new tools for use in dairy production. Application of these tools can enable us to efficiently produce safe and nutritious food for a hungry world.