MRSA outbreak investigation in pig workers in a pig farm in NSW Australia

Methicillin-resistant Staphylococcus aureus (MRSA) strain of porcine origin has been well established as potential zoonotic pathogen in Europe and some other parts of the world. However, limited studies to date have assessed carriage of these bacteria among pigs and pig workers in Australia. A recurrent MRSA outbreak in workers in a piggery in NSW affecting a large proportion of employees over a two years period has been reported.
The main focus of this study was to conduct an investigation of this outbreak and to determine potential risk factors for MRSA carriage in piggery workers.Nasal swabs were collected from all volunteer pig workers at the outbreak sites after completion of a questionnaire that gathered information on their background, work and health history, antibiotic treatment, hygiene, and perception towards development of antibiotic resistance. At the same time samples were collected from pigs at all piggery sites. The piggery included three different sites and each site had five to six sheds, including dry sows, farrowing, weaners, growers and finishers. At each site a total of 240 samples were taken and pooled in pools of 10, which allowed detecting MRSA at a prevalence of 2%, with assumed test sensitivity of 90%. In addition, environmental samples were also collected from all sheds and other shared areas including kitchen, bathrooms and offices.
Samples were pre-enriched in Mueller-Hinton broth containing 6.5% sodium chloride followed by selective-enrichment in Tryptone Soya Broth containing 3.5 mg/L cefoxitin and 75 mg/L aztreonam. Later on a loop full of the selective enriched cultured were inoculated onto chromogenic agar and blood agar respectively. All isolates were cultured and screened for various antibiotic resistances in the laboratory using Clinical and Laboratory Standards Institute methods. Confirmed MRSA isolates were further characterized by staphylococcal protein A (spa) typing, followed by multi-locus sequence typing.
The outcomes of this study will not only help to investigate the current identified MRSA outbreak but also determine associations between farm-level and farmer-level factors of MRSA carriage and infection in piggery workers.