Short communication: Relationship between herd intramammary infection incidence and elimination rate during the dry period

The objective of the study was to explore the relationship between herd dry period (DP) intramammary infection (IMI) incidence and elimination rates. A cohort of 91 Canadian dairy herds was recruited and followed in 2007 and 2008. Universal dry cow therapy was widely adopted among the participating herds. At the beginning of years 2007 and 2008, a sample of 15 cows was selected in each herd. A series of quarter-milk samples consisting of 2 predry and 2 postcalving samples were collected on each quarter of these 30 cows. Milk samples were analyzed using routine bacteriological milk culture, and predry and postcalving IMI statuses of these quarters were established using parallel interpretation of the 2 predry and 2 postcalving tests, respectively. Intramammary infection status was defined as IMI by Staphylococcus aureus, streptococci other than Streptococcus agalactiae, coagulase-negative staphylococci (CNS), or Corynebacterium spp. Incidence and elimination rates of DP IMI were computed for each herd. Lowess curves and linear regression were used to investigate the association between DP IMI incidence and elimination rates. Significant negative associations were found between incidence and elimination rates of Staph. aureus and CNS. The relationship between incidence and elimination rates was nonlinear for CNS, with a relatively strong negative relationship between DP IMI incidence and elimination rates for herds with relatively low DP IMI incidence (<0.10 new IMI/quarter-month). For herds with higher DP IMI incidence (>= 0.10 new IMI/quarter-month), a weaker negative relationship was observed between rates. No significant associations could be seen between DP incidence and elimination rates of streptococci other than Strep. agalactiae and of Corynebacterium spp. These results suggest that, conversely to the general belief, acquisition of new IMI and elimination of existing IMI during the DP may be driven, at least for staphylococci, by common mechanisms. The use of management strategies that would lead to the selection of pathogens less well adapted to the host could explain the associations observed between acquisition and elimination of IMI over the DP.