Randomized controlled field trial comparing quarter and cow level selective dry cow treatment using the California Mastitis Test

摘要

Selective use of antibiotic dry cow treatment can beimplemented at the cow or quarter level, with the latterhaving the potential to further reduce antibiotic use.Our objective was to compare these 2 approaches in 6herds in the United Kingdom in which environmentalmastitis predominated. Eight hundred seven cows wereenrolled and categorized as having a high cell count(n = 401) or low cell count (n = 406) in the last 3 moof lactation and clinical mastitis history. All quartersof all enrolled cows received an internal teat sealant.Within each category, cows were randomly allocatedto 1 of 3 groups; in one group antibiotic treatmentwas allocated at cow level (i.e., all 4 quarters receivedantibiotic), whereas in the 2 remaining groups antibiotictreatment was allocated at quarter level, based onCalifornia Mastitis Test (CMT) findings. Two differentthresholds, score 1 and 2, were used to determine likelyinfection status. Quarter milk samples were collectedat dry off and postcalving for bacteriological cultureand somatic cell count (SCC). Cows were monitored forclinical mastitis from dry off until 100 d in milk. Cowlevel SCC and milk yield data were collated from farmrecords. Within each category, the 2 quarter level treatmentgroups were compared with cow level treatment atdry off. Leaving quarters untreated with intramammaryantibiotic in cows in the high cell count group, with aCMT <2 or <1, reduced antibiotic use by 55% and 31%,respectively, and resulted in no difference in the oddsof being infected with any pathogen postcalving, butwas associated with a higher SCC at the first test day.Intramammary antibiotic treatment of quarters with aCMT ≥1 in cows in the low cell count category at dryoff was not associated with any reduction in the oddsof being infected with a major pathogen postcalvingbut was associated with a decrease in the odds of beinginfected with a minor mastitis pathogen postcalving.The use of antibiotics in quarters of cows categorizedas low cell count at dry off, increased the proportion ofquarters treated with antibiotic from 0% at cow level to31% (CMT ≥ 1) and 12% (CMT ≥ 2) at quarter level,only resulting in a reduction in SCC of around 20,000cells/mL at the first test day, if all quarters with CMTscore ≥1 were treated with antibiotic. No differences inclinical mastitis incidence and milk yield in the first 100d in milk were detected between any of the treatmentgroups. These study findings support selective quarterlevel dry off treatment only in cows with cow level SCC>200,000 cells/mL at dry off.