Microbial Aetiology, Antibiotic Susceptibility and Pathogen-Specific Risk Factors for Udder Pathogens from Clinical Mastitis in Dairy Cows.

Mastitis is one of the most important infectious diseases and one of the diseases that causes the greatest use of antibiotics in dairy cows. Therefore, updated information on the bacteria that cause mastitis and their antibiotic susceptibility properties is important. Here, for the first time in over 10 years, we updated the bacterial findings in clinical mastitis in Swedish dairy cows together with their antibiotic resistance patterns and risk factors for each bacterial species. During the period 2013-2018, samples from clinical mastitis were collected, together with information on the cows and herds of origin. The samples were cultured, and a total of 664 recovered bacterial isolates were subjected to susceptibility testing. () was the most common pathogen and accounted for 27.8% of diagnoses, followed by () (15.8%), () (15.1%), () (11.4%), () (7.7%), non- staphylococci (NAS) (2.8%), spp. (2.7%), spp. (1.3%), and () (1.2%). Various other bacteria accounted for 2.6%. Staphylococci were, in general, susceptible to most antibiotics, but 2.6% of and 30.4% of NAS were resistant to penicillin. No methicillin-resistant staphylococci were found. All were susceptible to penicillin. Bimodal and trimodal MIC distributions for penicillin in and , respectively, indicate acquired reduced susceptibility in some isolates. The mostly unimodal MIC distributions of indicate that acquired resistance does usually not occur in this species. Among , 14.7% were resistant to at least one antibiotic, most often ampicillin (8.7%), streptomycin (7.8%), or sulphamethoxazole (6.9%). spp. had low resistance to tetracycline (9.1%) but is considered intrinsically resistant to ampicillin. Pathogen-specific risk factors were investigated using multivariable models. , , and were more common, while was less common in quarters with more than one pathogen. and were mostly seen in early lactation, while was more common in peak to mid lactation and in early to peak lactation. and spp. were associated with a previous case of clinical mastitis in the current lactation. was associated with tie stalls and with loose housing. All pathogens except and had a seasonal distribution. In conclusion, the aetiological agents for clinical bovine mastitis have remained relatively stable over the last 10-15 years, , , and being the most important. Resistance to penicillin among Gram-positive agents was low, and in general, antibiotic resistance to other compounds was low among both Gram-positive and Gram-negative agents.