From the National Mastitis Council Newsletter "Udder Topics", August 1998
Using somatic cell count (SCC) records on a monthly basis is a useful tool for monitoring udder health. Multiple SCC results on individual cows or at the herd level are most useful, whereas a single SCC test result can be relatively inconclusive. Interpretation of SCC records is particularly applicable to herds with contagious mastitis problems. Since infections by these pathogens tend to be of long duration, new infections in the herd may lead to increased prevalence of infection and is reflected in elevated SCC in bulk tank or herd average SCC scores.
Well-managed herds that have controlled mastitis due to contagious pathogens can experience clinical mastitis problems due to environmental pathogens, yet maintain herd average SCC below 300,000. In this case the long-term udder health status may not be clearly reflected in monthly herd SCC or BTSCC. Infections by environmental pathogens tend to be of shorter duration than those caused by contagious pathogens; 60 to 70% of environmental infections may last less than 30 days. The time period of elevated SCC in these cows is correspondingly shorter as well. The prevalence of infection by environmental pathogens at any time point also tends to be low (less than 10% of quarters in the herd). Thus, herds that have predominantly environmental mastitis may have BTSCC below 300,000 (some may be below 200,000), because the relatively small number of environmental infections in the herd is not having a major impact on the herd SCC. Exceptions may be during times of increased clinical incidence, when there are greater than usual numbers of clinical cases at once.