Evaluation of the risk of transmitting Staphylococcus aureus strains between replacement heifers through commingling at a heifer-rearing facility

Abstract

The objective of this study was to evaluate transmission of Staphylococcus aureus (S. aureus) strains between heifers that originated from either a high- or low-prevalence S. aureus herd after commingling at a heifer-rearing facility. Three to four-month-old heifers from the University of Pennsylvania (UPa) and the University of Delaware (UDel) were transported on a bimonthly basis and raised commingled at Eutgers University. Preliminary work established unique baseline strains of S. aureus at both universities prior to commingling, and a high prevalence of S. aureus at the UPa (22.4%) and a low prevalence of S. aureus at the UDel (2,7%). Composite milk, samples were obtained for culture of S. aureus from both universities three times in 2006 and four times in 2007. Strain typing of S. aureus was performed by pulsed-field gel electrophoresis. The predominant strain isolated at the UPa was not spread to the UDel when heifers were commingled. The prevalence of S. aureus at the first routine culture after parturition (3-124 days after parturition) in primiparous cows previously commingled was similar in both herds. Sixty percent of the S. aureus strains isolated from commingled primiparous cow from the UPa were strains that were also found in the lactating herd. The UPa had significantly more S. aureus infections acquired in later lactation or subsequent lactations in primiparous cows that had been previously commingled and negative at the first routine culture after parturition, and from cows previously not commingled, than the UDel. The predominant strain of S. aureus isolated in these cows was the same as the predominant strain isolated from the preliminary baseline cultures. In this study, three to four-month-old commingled heifers returned to their herd of origin two to seven months prior to parturition were more at risk of acquiring an endemic strain of S. aureus than strains they may have potentially been exposed to at a commercial heifer-rearing facility. In this study, low-prevalent strains not found in the lactating herd and isolated from primiparous cows at parturition were of minor importance, and the infected mammary gland of lactating cows was the major reservoir of S. aureus.