Influences of season, parity, lactation, udder area, milk yield, and clinical symptoms on intramammary infection in dairy cows.

The aim of this study was to evaluate the influences of season, parity, lactation, udder area, milk yield, and clinical symptoms on bacterial intramammary infection (IMI) in dairy cows. A total of 2,106 mastitis pathogens in 12 species were isolated from 125 dairy farms distributed in 30 different cities in China, and the information about these factors was recorded at the same time. Mastitis pathogens were isolated from 63.43% of the milk samples, whereas Streptococcus agalactiae accounted for 38.61% of all pathogens, followed by Str. dysgalactiae (28.16%), Staphylococcus aureus (19.10%), Escherichia coli (6.90%), and other pathogens (7.23%). According to our investigation, IMI was more common in spring with the isolation rate of pathogens at 81.04%, and lowest in winter (52.34%). Cows were more likely to be infected by environmental pathogens (E. coli or Str. uberis) in summer, in rear quarters and in cows with higher daily milk yield or lower somatic cell count. In addition, Str. dysgalactiae exhibited a higher prevalence with increased parity. Different clinical symptoms of quarters with bacterial IMI were seen in this study, and mastitis pathogens were isolated from healthy quarters.