Longitudinal Studies of Contagious Mastitis in a High Somatic Cell Count Dairy Herd

The aim of this study was to investigate a sixty cow dairy herd with a persistently high bulk milk somatic cell count (BMSCC) and to find ways to control the problem. Investigations were initiated in November 1996 following notification to the farmer by the milk purchaser of possible termination of the milk contract if the BMSCC were not reduced to an acceptable level. The BMSCC had been greater than the EC standard of 400,000 cells/ml of milk since the winter of 1995. Investigation of the high BMSCC began by assessing the milking routine and by the measuring individual cow somatic cell counts (ICSCC). Cows were subsequently selected for California Mastitis Testing (CMT) on the basis of ICSCC > 400,000 cells/ml. Individual quarter samples, positive on CMT, were submitted for bacteriological culture. The results showed 67% of cows sampled to have infection with Streptococcus agalactiae and 43% to have infection with Staphylococcus aureus. The milking routine and general management on the farm were poor. Improvements in record keeping, general management and strict adherence to the mastitis control programme termed the 'Five Point Plan' were advised. Following uptake of these recommendations, whole herd 'blitz therapy' was implemented. The aim of the chosen treatment was to eliminate Streptococcus agalactiae infection from the herd and to produce a rapid reduction in the BMSCC, thereby allowing the farmer to continue to market the milk. The 'blitz therapy' comprised of intramammary treatment, with nafcillin, dihydrostreptomycin and benzyl penicillin (Nafpenzal MC; Intervet UK Limited), and parenteral treatment with penicillin (Depocillin; Intervet UK Limited), of all lactating cows; and parenteral treatment with amoxycillin and clavulanic acid (Synulox Ready-To-Use Injection; Pfizer Limited), of the dry cows and maiden heifers. The milk was discarded for the longest recommended withdrawal period for the drugs used and the milk purchaser tested the milk for antibiotic residues prior to collection for commercial sale. In order to assess the response to the management changes and to the whole herd 'blitz therapy', weekly BMSCC were monitored and bulk milk samples were collected for bacteriological culture weekly for 12 weeks, and then monthly for the next 9 months. In addition, bacteriological culture of individual quarter samples from all the lactating cows in the herd was carried out on two occasions following the 'blitz therapy', and from selected cows on two later occasions and from cows which calved within one month of the 'blitz therapy'. Individual quarter somatic cell counts (IQSCC) were carried out on the cows sampled during the herd screenings and were mostly within acceptable limits, i.e. < 250,000 cells/ml. The BMSCC was reduced to below 400,000 cells/ml for at least one year after the 'blitz therapy'. Streptococcus agalactiae was not isolated from any of the bulk milk samples or from any of the cows, with the exception of one cow which had been dry at the time of the herd treatment. This cow was isolated immediately and treated. Repeated sampling from this cow failed to detect Streptococcus agalactiae infection and to date, the herd has remained free of Streptococcus agalactiae infection for one year. The isolation of Staphylococcus aureus from 10/22 weekly or monthly bulk tank samples and also from 28% of the cows on one, or more, occasions during the herd screenings led to an attempt to reduce Staphylococcus aureus infection in the herd. Six cows from which Staphylococcus aureus was isolated from one or more milk samples taken during the herd screenings, were selected for antibiotic treatment. Ceftiofur (Excenel Sterile Powder; Pharmacia and Upjohn Limited) was administered parenterally to these cows once a day for 10 days. Following treatment, monitoring showed that despite the fact that fewer colonies of Staphylococcus aureus were cultured the bacteria were still present in the milk of these cows, indicating that this treatment failed to eradicate Staphylococcus aureus. In conclusion, despite failing to eliminate Staphylococcus aureus from treated cows, the combined effects of improved management and 'blitz therapy' were successful in bringing about a dramatic reduction in BMSCC, eliminating Streptococcus agalactiae and ensuring the farmer was able to market the milk. The reduction in the level of mastitis in this herd has meant greater financial returns for the farmer through increased yield and reduced penalties, in addition to a great improvement in welfare on this farm