Clinical mastitis in ewes; bacteriology, epidemiology and clinical features

Clinical mastitis is an important disease in sheep. The objective of this work was to identify causal bacteria and study certain epidemiological and clinical features of clinical mastitis in ewes kept for meat and wool production. The study included 509 ewes with clinical mastitis from 353 flocks located in 14 of the 19 counties in Norway. Clinical examination and collection of udder secretions were carried out by veterinarians. Pulsed-field gel electrophoresis (PFGE) was performed on 92 Staphylococcus aureus isolates from 64 ewes. S. aureus was recovered from 65.3% of 547 clinically affected mammary glands, coagulase-negative staphylococci from 2.9%, enterobacteria, mainly Escherichia coli, from 7.3%, Streptococcus spp. from 4.6%, Mannheimia haemolytica from 1.8% and various other bacteria from 4.9%, while no bacteria were cultured from 13.2% of the samples. Forty percent of the ewes with unilateral clinical S. aureus mastitis also had a subclinical S. aureus infection in the other mammary gland. Twenty-four of 28 (86%) pairs of S. aureus isolates obtained from clinically and subclinically affected mammary glands of the same ewe were indistinguishable by PFGE. The number of identical pairs was significantly greater than expected, based on the distribution of different S. aureus types within the flocks. One-third of the cases occurred during the first week after lambing, while a second peak was observed in the third week of lactation. Gangrene was present in 8.8% of the clinically affected glands; S. aureus was recovered from 72.9%, Clostridium perfringens from 6.3% and E. coli from 6.3% of the secretions from such glands. This study shows that S. aureus predominates as a cause of clinical ovine mastitis in Norway, also in very severe cases. Results also indicate that S. aureus is frequently spread between udder halves of infected ewes.