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Dry period hygiene is important to keeping infection rates low during lactation

Inflammatory response to intramammary infection (IMI) leads to high somatic cell counts. In fact, somatic cell counts (SCC) can be used to monitor or perhaps predict early infection. Although most of the inflammatory events such as high SCC or clinical signs of mastitis occur during lactation, many of the infections that result in mammary gland inflammation have their origins in the dry period.

Many mastitis infections occur during the dry period due to a very high exposure to pathogens. Clinical signs of mastitis will show at parturition or during the early weeks of lactation. We often try to improve the hygiene and housing situation of fresh cows to reduce the risk of mastitis. However, the focus area for improvement of hygiene, housing and nutrition should really be the late dry period.

Typically, cows calve with a somatic cell count that is higher than 100,000 cells per ml. However, in healthy cows the SCC drops very fast to levels below this threshold. In fact, in uninfected quarters, the third milking after calving is expected to show a low SCC. In quarters that are infected, SCC remains high and the drop in SCC does not occur at all or occurs at a much slower pace.

The usefulness of quarter milk SCC and California Mastitis Test (CMT) for screening was evaluated in a study by Drs. Sargeant, Leslie and co-workers at the University of Guelph in Canada. They calculated sensitivity and specificity for various threshold values and days post calving. A SCC threshold of 100,000 cells/ml for quarter samples evaluated on d 5 post-calving had the maximal sensitivity and specificity for detecting IMI. Evaluation of the CMT samples taken on d 3 post-calving using a threshold reaction of greater than zero had the highest sensitivity and specificity for detecting IMI. With this CMT sampling scheme, the sensitivities for detecting IMI with any pathogen, IMI with a major pathogen, and IMI with a minor pathogen were 56.7, 66.7, and 49.5, respectively.

Both early post-partum SCC and the use of the CMT could have a useful role in dairy herd monitoring programs as a screening test to detect fresh cows with IMI caused by major pathogens.