Abstract
Intramammary infusion of antimicrobials at the end of lactation (dry cow therapy) has been a cornerstone of mastitis management for many years. However, as only a proportion of cows are infected at this time, treating only those cows likely to be infected is an important strategy to reduce antimicrobial usage and minimize risk of emergence of antimicrobial resistance. Such an approach requires the ability to discriminate between cows and quarters likely to be infected and uninfected. This study compared assignment of cows or quarters to antimicrobial treatment at the end of lactation based on cow composite somatic cell count (SCC; i.e., all quarters of cows with a maximum SCC across lactation >200,000 cells/mL received an antimicrobial; n = 891 cows, SCC-group) or assignment to quarter-level treatment based on a quarter level California Mastitis Test (CMT) score ≥ trace (n = 884 cows; CMT-group) performed immediately before drying off. All quarters of all cows also received an infusion of a bismuth-based internal teat sealant. Milk samples were collected for microbiology following the last milking, and again within 4 d of calving. Clinical mastitis records from dry off to 30 d into the subsequent lactation were collected. Multilevel, multivariable models were used to assess the effect of assignment to antimicrobial treatment. At drying off, a total of 575 (8.8%) and 147 (2.3%) of the 6,528 quarters had a minor, and a major intramammary infection (IMI), respectively. At drying off, 2089/3270 (63.9%) and 883/3311 (26.7%) of quarters were treated with dry cow therapy in the CMT and SCC-groups, respectively. Apparent bacteriological cure proportion for any IMI was higher in quarters assigned to the CMT than the SCC-group (349/368 (0.95, 95% CI 0.92-0.97) versus 313/346 (0.90, 95% CI 0.87-0.93)). New IMI proportion was lower among quarters assigned to the CMT than SCC-group [101/3,212 (0.032, 95% CI 0.025-0.038) versus 142/3,232 (0.044, 95% CI 0.036-0.051)]. The prevalence of any IMI postcalving was lower in quarters assigned to the CMT than SCC-group [119/3,243 (0.037, 95% CI: 0.030-0.044) versus 173/3,265 (0.054, 95% CI: 0.045-0.062)]. There was no difference in incidence of clinical mastitis between treatment groups. The total mass of antimicrobials used was 63% higher in the CMT-group than in the SCC-group (3.47 versus 2.12 mg/kg of liveweight). Selection of quarters for antimicrobial treatment at the end of lactation based on CMT resulted in greater proportion undergoing bacteriological cure, reduced risk of any new IMI and reduced post calving prevalence of any IMI compared with selection of cows based on SCC. However, CMT-based selection resulted in higher antimicrobial use compared with SCC-based selection, and further research is required to analyze the cost benefit and impact on risk of antimicrobial resistance of these 2 strategies.
Highlights
Due to concerns about antimicrobial use and the risk of selection for resistant pathogens, dairy farmers around the world are under pressure to reduce antimicrobial usage
Selection based on California Mastitis Test (CMT) resulted in a higher bacteriological cure proportion, a lower new IMI proportion over the dry period, a lower prevalence of IMI postcalving, and a lower proportion of cows with an SCC >200,000 cells/mL at first production recording in the subsequent lactation, but there was no difference in the clinical mastitis incidence over the dry period or the first 30 d of subsequent lactation
Selection of quarters for DCT treatment at the end of lactation based on a quarter level CMT >0 results in greater overall antimicrobial usage compared with selection based on a maximum production recording SCC >200,000 cells/mL and treatment allocation at cow level
Summary
Due to concerns about antimicrobial use and the risk of selection for resistant pathogens, dairy farmers around the world are under pressure to reduce antimicrobial usage. For cows or quarters likely to be uninfected, infusion of only an internal teat sealant (ITS) containing bismuth subnitrate offers a nonantibiotic approach to reducing the rate of new IMI over the nonlactating period. A combination of an ITS and DCT reduces the risk of new IMI over the dry period and reduces the SCC and clinical mastitis incidence in the subsequent lactation compared with DCT alone (Bradley et al, 2011; Golder et al, 2016). Categorizing cows or quarters as likely to be infected or uninfected, and treating likely infected cows or quarters with DCT, and using ITS in likely uninfected cows (selective DCT) provides an approach that may reduce overall antimicrobial usage but minimize new infection rates over the dry period, and SCC and clinical mastitis incidence in the subsequent lactation. Implementation of selective DCT requires that infected and uninfected cows or quarters can be differentiated
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
