Winning the Battle of Subclinical Mastitis Detection

Mastitis can occur in both clinical and subclinical form. Clinical mastitis can easily be detected by abnormalities in the milk or udder. The detection of subclinical mastitis is more difficult because clinical signs are not present and indirect tests are required. When diagnosing subclinical mastitis producers must observe the immune response of the cow. Subclinical mastitis often goes undetected and has the greatest economic impact because of long term effects on milk yield. The presence of cows with subclinical mastitis within the herd can have potentially negative consequences, leading to high overall herd somatic cell count (SCC), which results in the producer being paid less for milk sold. Failure to treat subclinical mastitis may result in chronic inflammation that is unlikely to respond to antibiotic treatment. Monitoring the subclinical mastitis status in a herd should be done at both the herd and individual cow level. At the herd level simply reviewing the bulk tank milk somatic cell count score received monthly or more frequently is the first step in knowing how much subclinical infection is in the herd.

Milk from subclinically infected quarters appears normal, even when millions of somatic cells are present. A quarter with a SCC level of >200,000 cells/mL is considered to be evidence of subclinical mastitis. Individual cow monitoring for subclinical mastitis infection should be done routinely. There are several ways of getting SCC or inflammation data on each cow. Having somatic cell counts run monthly on each cow through the Dairy Herd Improvement (DHIA) program is one approach. Doing cow-side SCC monitoring monthly or more frequently is another approach. The long-time used California Mastitis Test (CMT) kit is an acceptable first line method to use. Several cow-side electronic testing devices are another method that can be used effectively. Some milking systems have the capability to measure electrical conductivity of milk as a way of detecting subclinical infections.

The CMT is a valuable tool that is inexpensive and easy to use. It is important to identify new infections early to achieve optimum cure. It is a good practice to implement the use of the CMT on fresh cows the first few days after calving. This practice will help to identify new infections early and catch subclinical cases before they are chronic. Finding cases of mastitis early in lactation will help to determine where the infection is occurring and allow you to prevent future cases of mastitis.  

As mentioned, individual cow SCC results are available through DHIA test day data. This valuable tool gives you the data you need to assess the status of individual cows. On the individual cow “hot sheet" you will find information such as; individual cow SCC, stage of lactation, lactation number, pounds of milk and percent of bulk tank contribution. The hot sheet ranks the cows with the highest contribution to the bulk talk on the top of the list. Producers can utilize this information to investigate each cow further to detect potential cases of subclinical mastitis. Cows with a SCC of 200,000 or greater should have a CMT performed to detect inflamed quarters. To fully benefit from the DHIA hot sheet, you may want to consider collecting milk samples for bacteriological culture for all cows with SCC greater than 200,000 or at least the top 10 or 20 cows on the list. This practice would provide valuable information for developing prevention strategies and making individual cow treatment or culling decisions.

To be successful in detecting subclinical mastitis, producers must utilize many tools available to them. Without individual cow records or cow-side SCC testing, the battle is almost impossible to win.