The emergence of community associated MRSA (ST93) in piggery workers and associated risk factors

In Europe, livestock associated (LA) MRSA (ST398) carriage in pig enterprises has emerged as an occupational challenge where a high prevalence has been reported in pig farmers (Cuny et al. 2009) and pigs (Morcillo et al. 2015). Recently, a high prevalence of MRSA carriage has been found in pigs and people working in a pig farm in Australia, where 84% of total MRSA positive workers (n = 31) were carrying community associated (CA) MRSA (ST93) and 16% LA-MRSA (ST398) (Sahibzada et al. 2017). The objective of this study was to determine the potential risk factors associated with different MRSA clone carriage with regard to occupational pig exposure on this farm.

Information was collected from the participants via questionnaires. Associations between MRSA carriage and the presence of potential risk factors were investigated using the statistical package R v3.3.3 (R foundation, Vienna, Austria), fitting univariable generalised linear models (GLM) with binomial distribution logit link function. Significance was set at 0.05. Clone-specific carriage trend was compared separately for each clone with the overall MRSA non-carrier by excluding the counter-clone from the baseline model.

A total of 52 piggery workers participated in the study, 77% male and 23% female. No significant association was found for MRSA carriage on this farm with age, gender, ethnicity, number of years working with pigs, chronic disease, or history of hospitalisation. Pig contact and contact intensity (number of hours working in direct pig contact), the role of pig workers, and level of education were noted to be significantly (P < 0.01) associated with MRSA and MRSA clone carriages. The prevalence of MRSA carriage was 85.2% for persons working in pig sheds, 60% for those in maintenance roles and 50% amongst feedmill workers. No MRSA carriage was found in those with administrative or pastoral roles. The odds ratio (OR) for MRSA carriage in workers who had high school or lower levels of qualifications was 3.96 (CI = 1.25–13.84, P = 0.02) compared with those with tertiary education. Univariable logistic regression analysis showed that the odds of ST93 carriage increased by 7% (OR 1.07, CI = 1.02–1.14) for each hour increase in pig contact in a week. A similar higher odds ratio was noted for ST398 (OR 1.08, CI = 1.02–1.21).

ST93 has been frequently isolated from communities in Australia. However, this strain has never been reported as an occupational risk, unlike ST398 which has been studied thoroughly and linked with pig contact and contact intensity.

We describe for the first time the CA-MRSA clone carriage of ST93 as an occupational risk for piggery workers which is strongly associated with intensity of contact between workers and pigs. Given that it has been reported on a single farm, it is important to investigate MRSA carriage in humans and pigs on other pig farms.