MRSA in Cattle - An Investigation into Selected Properties of Isolates Recovered from Clinical Veterinary Diagnostic Samples

The main objective of this research is to determine whether or not methicillin-resistant Staphylococcus aureus (MRSA) is present in dairy cattle in England and Wales and to study the population biology of the S. aureus isolates recovered. The population of organisms studied will comprise Staphylococcus aureus recovered mainly from clinical mastitis samples submitted to VLA regional laboratories strategically located throughout England and Wales. From this sample population, the following prevalence figures will be calculated: i) Overall prevalence of MRSA in bovine clinical mastitis samples. ii) An estimate of the herd-level prevalence of MRSA in herds submitting samples to VLA Regional Laboratories. The prevalence will be based on the assumption that an infected herd will submit at least one infected sample. iii) An attempt will be made to relate the number of MRSA isolates and the number of methicillin-susceptible S. aureus isolates to the total number of cattle eligible for sampling in herds submitting samples to VLA Regional Laboratories and from which S. aureus was isolated.
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Staphylococcus aureus isolates available from other species will also be examined (sheep, goats and poultry) for methicillin resistance. This approach will complement the examination of samples from companion animals in other Defra-funded studies. In addition to determination of the minimum inhibitory concentration (MIC) of methicillin, the MIC of vancomycin will also be determined for all S. aureus isolates in view of the recent emergence of vancomycin resistant S. aureus in human clinical isolates.
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A further principal objective is to develop laboratory procedures suitable for use in discriminating between Staphylococcus aureus strains of bovine or human origin. These procedures will be used to determine the likely origin of any MRSA isolates that are recovered. Pulsed field gel electrophoresis (PFGE) is now established as the gold standard for strain discrimination and investigation of the molecular epidemiology of methicillin and other resistant S. aureus. Fully-harmonised methods will be applied here by the medical and veterinary partners. Statistical analyses will be performed to determine any association between hosts and PFGE patterns. Representatives of all PFGE subtypes will be examined according to the methods used in the medical field to determine their toxin gene complement [PFGE pattern will be used to ensure that a genetically diverse range of strains are examined for toxins]. Staphylococcus aureus strains representative of different PFGE patterns will also be examined using a microarray for detection of over 40 resistance, toxin and other genes. The data will be analysed to establish any gene associations with strain/host.
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Genetic analyses to determine staphylococcal cassette chromosome mec (SCCmec) subtype and epidemic strain type will be performed on any MRSA isolates that are recovered. MLST (Multi-locus Sequence Typing) in addition to Single Locus Sequence Typing of the spa gene will be used on all MRSA isolates recovered to further identify them to the clonal complex level. (If large numbers of MRSA isolates are recovered then these techniques will be used to examine a representative sub-sample of isolates).
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Policy Relevance.
The epidemiology of MRSA in GB has shown dramatic changes recently, with a number of clinical infections being reported in companion animals. There is an urgent need to clarify whether or not there have been any similar changes in food-producing animals and to determine the likely origin of any MRSA that are isolated from these animals.
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Intended Use of Results.
Results will be used to determine whether current public health safeguards regarding food-producing animals are adequate or need to be revised and whether food-producing animals are a significant reservoir of MRSA. They will provide a baseline against which future trends may be measured.