Intramammary infections in lactating Jersey cows: prevalence of pathogens and association of pathogen type with milk somatic cell count and persistence of infection
Authors:   , Haw Samantha Rose 
Published: February 2023

[EMBARGOED UNTIL 8/1/2023] There is limited data available regarding pathogens causing intramammary infections (IMI) in Jersey cows. The objectives of this study were to characterize the prevalence of IMI caused by different microorganisms in lactating Jersey cattle, evaluate pathogen associations with somatic cell count (SCC) and persistence of IMI, and determine if the first sample SCC in a sequence of 3 samples could be predictive of IMI persistence with specific pathogens. This prospective, observational, longitudinal study included lactating cows (n = 753) enrolled from 4 Jersey dairy farms within a 250-mile radius of Columbia, Missouri. Mammary quarter foremilk samples were aseptically collected once monthly for 3 consecutive months. Microorganisms were identified using aerobic milk culture and matrix-assisted laser desorption/ ionization-time-of-flight mass spectrometry. A commercial laboratory measured SCC using flow cytometry. Milk culture results were classified as single microorganism growth, mixed infection, contaminated, or no significant growth. Intramammary infection was defined based on isolation of [greater than] 1 colony for all microorganisms except Staphylococcus aureus and Streptococcus agalactiae, where IMI presence was defined by isolation of ≥ 1 colony, or for Bacillus spp. when ≥ 5 colonies were isolated using an [approx] 10μL inoculum. Persistent infection was characterized by identification of a single species-level pathogen at first sampling and at least 1 other visit. Non-persistent infections were those caused by a single species-level pathogen that was not subsequently encountered at another visit. Non-aureus staphylococci, particularly Staphylococcus chromogenes and Staphylococcus simulans, were the most commonly isolated microorganisms among the 7,370 quarter-level milk samples collected. Median prevalence (using all 3 samplings) of specific pathogens varied among farms; however, S. chromogenes was 1 of the top 2 isolates found at all farms. There were some nuances observed between herds, likely attributable to farm management differences, that appeared to correlate with preponderances of certain microorganisms, specifically S. simulans and S. aureus. A linear mixed model evaluated the association of IMI status and natural logarithm SCC. An interaction with days in milk was observed for some species (S. aureus, Streptococcus uberis, Serratia marcescens, and Corynebacterium spp.). The predicted SCC associated with Staphylococcus haemolyticus, Bacillus spp., and coliforms were not significantly different from quarters with no growth. The most common species that persisted were S. chromogenes, S. simulans, S. aureus, and Strep. uberis. Mixed logistic regression revealed an association of natural logarithm SCC with IMI persistence (P = 0.04); however after pairwise comparison, there were no detectable differences in odds of persistence among pathogens. First sample period SCC was associated with persistent infection, but pairwise comparisons failed to show an association at the pathogen level. Evaluating associations among commonly identified pathogens and SCC helped define how prevalent pathogens may associate with milk quality in Jersey cows. Overall findings shared some similarities with results of previously published studies, but overall SCC tended to be lower than other reports evaluating the same microbes among quarters. Some microbes were not significantly different from one another and showed similar alterations in SCC to uninfected quarters. Staphylococcus aureus and Strep. dysgalactiae were the most inflammatory causes of IMI. Dairy husbandry practices and milking hygiene are still critical control points for minimizing IMI and promoting udder health.