Dynamics of intramammary infections in suckler ewes during early lactation

Mastitis is an important problem in meat-producing sheep, but few studies have investigated the transmission dynamics of mastitis pathogens in these animals. The objective of this study was to describe the pathogens causing intramammary infections (IMI) in suckler ewes, their effect on somatic cell count, and the dynamics of these IMI in early lactation. We enrolled 15 sheep flocks early after lambing and selected ewes in each flock that were sampled twice with a 3-wk interval. Milk samples from both glands of each ewe were bacteriologically cultured, and somatic cell count was measured. Non-aureus Staphylococcus spp. were the most prevalent culture results. Somatic cell counts were most strongly increased in ewes infected with Mannheimia haemolytica, whereas staphylococci, including Staphylococcus aureus, were associated with a moderate increase in somatic cell count. The proportion of udder halves that remained culture-positive with Staphylococcus spp. during the 3-wk sampling interval was moderate, but M. haemolytica infections were stable during this time period. A substantial number of new infections were seen in the early lactation study period for non-aureus Staphylococcus spp., Staph. aureus, and Corynebacterium spp., but not for M. haemolytica or Streptococcus spp. The number of new IMI of Staph. aureus was associated with the number of Staph. aureus-infected udder halves in the flock at the first sampling moment, indicative of contagious transmission. Altogether, we show that substantial transmission happens in early lactation in suckler ewes, but that the dynamics differ between pathogen species. More research is needed to further describe transmission in different stages of lactation and to identify transmission routes, to develop effective interventions to control mastitis.