Abstract
Simple SummaryMastitis is the most prevalent disease of dairy cattle that causes significant economic losses. Different agents cause mastitis which leads to increased somatic cell count (SCC) and low milk quality. Treating mastitis with antimicrobials is essential to reduce SCC and improve milk quality. Excessive use or misuse of antimicrobials in dairy farms leads to the development of antimicrobial resistant bacteria. The objectives of this study were (1) to isolate and identify the causative agent of mastitis and (2) determine antimicrobial resistance profiles of bacterial isolates. A total of 174 quarter milk samples from 151 cows with high SCC and clinical mastitis from 34 dairy farms in Tennessee, Kentucky, and Mississippi were collected. Bacterial causative agents were determined by bacteriological and biochemical tests. Antimicrobial resistance of bacterial isolates against 10 commonly used antimicrobials was tested. A total of 193 bacteria consisting of six bacterial species, which include Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, Escherichia coli, Klebsiella oxytoca and Klebsiella pneumoniae were isolated. Staphylococcus aureus was the predominant isolate. The proportion of resistant isolates was relatively higher in Gram-negatives than Gram-positives. Continuous antimicrobial resistance testing and identification of reservoirs of resistance traits in dairy farms are essential to implement proper mitigation measures.Mastitis is the most prevalent and economically important disease caused by different etiological agents, which leads to increased somatic cell count (SCC) and low milk quality. Treating mastitis cases with antimicrobials is essential to reduce SCC and improve milk quality. Non-prudent use of antimicrobials in dairy farms increased the development of antimicrobial resistant bacteria. This study’s objectives were (1) to isolate and identify etiological agents of mastitis and (2) to determine antimicrobial resistance profiles of bacterial isolates. A total of 174 quarter milk samples from 151 cows with high SCC and clinical mastitis from 34 dairy farms in Tennessee, Kentucky, and Mississippi were collected. Bacterial causative agents were determined by bacteriological and biochemical tests. The antimicrobial resistance of bacterial isolates against 10 commonly used antimicrobials was tested. A total of 193 bacteria consisting of six bacterial species, which include Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, Escherichia coli, Klebsiella oxytoca and Klebsiella pneumoniae were isolated. Staphylococcus aureus was the predominant isolate followed by Strep. spp., E. coli, and Klebsiella spp. Results of this study showed that Gram-negatives (E. coli and Klebsiella spp.) were more resistant than Gram-positives (Staph. aureus and Streptococcus spp.). Continuous antimicrobial resistance testing and identification of reservoirs of resistance traits in dairy farms are essential to implement proper mitigation measures.
Highlights
Bovine mastitis causes significant economic losses to dairy farms and usually results in a loss of sustainability of dairy farming [1,2]
Subclinical mastitis was defined as an infection of one or more udder quarters of a cow without manifesting visible inflammatory changes in the mammary gland tissue or milk but with high somatic cell count (SCC) (>200,000 cells/mL of milk)
The majority of E. coli isolates were associated with clinical mastitis, whereas most Staph. aureus isolates were from subclinical mastitis, other studies reported inconsistent findings concerning pathogens that were uniquely linked to subclinical and clinical mastitis
Summary
Bovine mastitis causes significant economic losses to dairy farms and usually results in a loss of sustainability of dairy farming [1,2]. Dairy cows are susceptible to intramammary infection (IMI) during early non-lactating (dry period) and transition periods [3,4]. The incidence of IMI is high during the early dry period because of the cessation of hygienic milking practices such as pre-milking teat washing and drying [5] and pre-and post-milking teat dipping in antiseptic solutions [6,7], which are known to reduce teat end colonization by bacteria and infection. Udder infected during the early dry period usually persists and manifests clinical mastitis during the transition period [8].
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