Identifying the Problem
Most mastitis problems are defined as either a series of undesirable bulk tank milk somatic cell count (SCC) or an unacceptable incidence of clinical mastitis. Both of these are consequences of the cows' responses to intramammary infections. The magnitude and duration of these responses are largely a function of the microorganisms infecting the glands. Therefore, to make permanent desired corrections in a herd, the infection rates and duration of infections must be controlled and not merely introduce procedures that eliminate the signs and symptoms manifested by the disease. The key to the process is to correctly identify the prevalent pathogens causing intramammary infections in the problem cows.
The most reliable procedure for identifying etiological agents of mastitis remains the bacteriological culturing of milk from individual mammary quarters. The sampling scheme most prudent when troubleshooting herds is simply to sample the quarters causing the problem. In high SCC herds, the quarters or cows to sample can be chosen from a recent DHI report or by measuring individual cow SCC on the herd. Herds that are not routinely measuring individual cow SCC and are hesitant to spend the time and money for such testing may use a California Mastitis Test (CMT) to determine the high SCC quarters. A concern with this approach is the number of cows or quarters that should be sampled to assure a true representation of the herd problem. A host of epidemiological models is available to calculate statistical probabilities and confidence limits, but a more pragmatic approach is to predetermine the lower SCC (or CMT) considered a problem and sample all cows greater than that limit.
Problem herds experiencing an unacceptable rate of clinical mastitis are more problematic than high SCC herds because clinical signs are often transitory and the affected quarters or cows treated with an antimicrobial drug. Sampling quarters that had clinical mastitis within the past week to a month will generally provide little useful information because most pathogens will have been eliminated by therapy or spontaneous cure. A sampling scheme should be implemented to secure milk samples from clinically mastitic quarters prior to any therapy. These milk samples can be frozen for up to a month or until enough of the clinical cases have sampled to ascribe a source of pathogens.
The primary reason for sampling problem cows when troubleshooting is to help determine the source of pathogens. A general idea of what pathogens are causing infections will allow for an educated deduction of what farm management practices may be responsible for the mastitis problem. Sampling schemes other than that described above tend not to be as efficient in securing the data needed to make the proper decisions. For example, taking random samples to find out which cows are infected can have some benefit in screening a herd, but a tremendous amount of error is associated with sampling a specific cow one time to determine her true intramammary infection status. Whole herd sampling schemes generally are expensive and sampling a cow that is not a problem will almost always tell you what you already know..... she is not a problem! Instituting a monitoring system such as sampling all fresh cows can have future long term reward in early detection of a problem. However, most herds in need of troubleshooting have shuffled resources to other endeavors and require more immediate response to stay legal or profitable.