Transmission dynamics of intramammary infections with coagulase-negative staphylococci.

A field trial was conducted on 2 US dairy herds to evaluate the transmission dynamics of coagulase-negative staphylococci (CNS) during the lactation period. Quarter milk cultures positive for CNS were classified as intramammary infection (IMI) or incidental isolation (transient colonization). The average proportion of quarters with CNS IMI was 0.114 in farm 1 and 0.09 in farm 2, and corresponding estimates for quarters with transient colonization of CNS were 0.122 and 0.088 throughout the 13-mo study period. Transmission parameters were obtained both for IMI caused by CNS and for CNS-positive quarters classified as the combination of transient colonization and IMI. Transmission between cows and between quarters in the same cow was assessed. The transmission parameters (β) for IMI caused by CNS were 0.0066 (farm 1) and 0.0111 (farm 2). Corresponding estimates of β when IMI and transient colonization of CNS were assessed jointly were 0.0256 (farm 1) and 0.0253 (farm 2). On farm 1, risk of transmission for CNS IMI between quarters in the same cow was 2 times greater than that between quarters from cow to cow. Transmission of CNS was considered contagious in one farm but no distinction between contagious and environmental transmission could be made in the other. We hypothesize that between-farm variation may be related to diversity between prevailing CNS species or by differences in management. In the current study, estimates of the basic reproduction number (R(0)) at the farm level for CNS IMI were 0.59 and 0.84 in farms 1 and 2, respectively. This shows that the transmission of CNS from IMI during the lactation period would to be too low to maintain the observed herd prevalence of CNS IMI. The R(0) values for the combination of transient colonization and IMI by CNS were 1.13 and 1.17 in farms 1 and 2, respectively. This indicates potential for sustained endemic infection of CNS throughout lactation. However, prevailing CNS species may also differ between transient colonization and IMI. Therefore, not only CNS originating from IMI or incidental isolation events, but also CNS IMI occurring from the period outside the lactation pen are essential for maintenance of the observed herd prevalence of CNS IMI throughout lactation in this study. The effect of IMI originating outside the lactation pen was verified in simulations with reduced entry of infectious quarters to the lactation pens. Measures against CNS IMI would probably increase in efficiency if prevention of infection during the dry period and early lactation were further emphasized in herd health programs.