Molecular epidemiology of an outbreak of clinical mastitis in sheep caused by Mannheimia haemolytica

• Mastitis due to Mannheimia species can be a significant cause of mortality in lamb production systems during adverse climatic periods.
• While there was some evidence of clonality amongst the Mannheimia haemoltyica isolates in this study, the diversity between the clusters indicates that distantly related strains have the capacity to cause mastitis in the same flock.
• This trial demonstrated limited antimicrobial resistance in Mannheimia isolates obtained from an extensive sheep property.
• If climate change results in more extremes in climate, this type of disease outbreak may become more prevalent due to environmental and nutritional pressures on animals.

The aetiology and epidemiology of outbreaks of clinical mastitis in sheep under extensive pastoral conditions are incompletely understood. The objective of this study was to conduct a detailed investigation of a clinical mastitis outbreak that affected more than 10% of 230 at-risk ewes on a sheep and grain producing property in south east Australia during drought conditions in 2009. Milk samples were collected aseptically from all affected ewes and plated on sheep blood agar for bacterial identification. M. haemolytica was isolated from 80% of the samples that yielded cultivable microorganisms and thus was the main microorganism responsible for the outbreak. Analysis of the restriction endonuclease cleavage patterns of the isolates using pulsed field gel electrophoresis revealed some evidence of clonality, suggesting the possibility of horizontal transmission, but there was also considerable diversity between the clusters of closely related isolates. Multilocus sequence typing of the M. haemolytica isolates revealed most of the isolates belonged to ST1 with no association between the PFGE and MLST fingerprints of the isolates. Resistance to neomycin, streptomycin and sulphafurazole was detected in some of the isolates, but they were all susceptible to penicillin, ampicillin, ceftiofur, amoxycillin/clavulanic acid, ciprofloxacin, tetracycline, erythromycin and trimethoprim. This is the first published record of a comparison of the strains of M. haemolytica involved in a clinical mastitis outbreak in sheep and demonstrates the importance of this pathogen in sheep production systems, particularly during adverse climatic conditions and increased stocking rate.