Detection of methicillin-resistant and methicillin-susceptible Staphylococcus aureus among pigs in different stages of production

Several European studies have found different levels of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) prevalence in pig farms ranging from 3 to 60% (EFSA 2009). A number of studies have found an association between the presence of S. aureus strains (MRSA and MSSA) on farm and carriage of these organisms by humans on those farms (van Cleef et al. 2014). The risk of acquisition of MRSA in human is considered to be directly related to frequency and type of contact with animals that are carrying the pathogen (Graveland et al. 2011). However, limited studies have been conducted regarding carriage of these pathogens in Australian pigs. The aim of this study was to detect and identify potential risk factors for MRSA and MSSA carriage among pigs at various stages of production in farms where persistent outbreaks of human MRSA has been reported among piggery employees.

A cross-sectional study was performed at a commercial pig farm in NSW. Swabs were collected from the internal nares and ear skin of individual animals. A questionnaire was also completed by the piggery manager. The questionnaire collected information on various aspects of farm practices, animal health, hygiene and biosecurity. The piggery had seven sheds that included two dry sow sheds, two grower sheds and a single shed for farrowing, weaners, and finishers. The number of animals varied between 1000 and 3000 pigs per shed. Each shed was divided into 8–10 rooms. From each shed 60 animals were randomly chosen resulting in a total of 420 animals being swabbed. Ear and nose swabs were taken from individual animals. Ear and nose swabs of 10 animals were pooled into one to give a total of six pool samples per shed. In addition, five environmental samples from shed walk ways, pen floors, feeders, fences and walls were also collected from each shed and pooled into one sample. All samples were processed within a week of collection commencing with pre-enrichment in Mueller-Hinton (MH) broth containing 6.5% sodium chloride for 18 h at 37°C. After the pre-enrichment stage, two separate procedures were used for MRSA and MSSA screening. For MRSA detection, a selective enrichment was performed in Tryptone Soya Broth (TSB) containing 3.5 mg/L cefoxitin and 75 mg/L aztreonam. Subsequently, a loop of the selective enriched cultured was inoculated onto chromogenic MRSA agar and blood agar. For MSSA screening, inoculum was directly streaked on mannitol salt agar and blood agar after the pre-enrichment stage. Presumptive colonies of MRSA and MSSA were subjected to further confirmatory tests including staining by Gram’s method, catalase testing, S. aureus Protein A latex agglutination testing, and tube coagulase tests. The susceptibility of all MRSA and MSSA isolates were tested to 28 different antimicrobial agents using the disc diffusion method following the Clinical and Laboratory Standards Institute protocols (CLSI 2014). The detection of S. aureus including MSSA and MRSA among pigs of different age groups was compared. The associations between farm practices, considered potential risk factors, and the presence of MRSA in pigs at different production stages were considered.

MRSA was found in pooled samples in every stage of production in this piggery (n = 40). Forty of the 42 pooled samples returned positive results. The MRSA was also identified in the environment of this piggery. However, no disease in pigs related to MRSA was identified in this piggery. MSSA was also found among dry sows, growers, and finishers. The environmental samples were also positive for these sheds at the same time. A total 10 out of 42 pooled samples were positive for MSSA. Weaner and farrowing sheds were negative for MSSA. Antimicrobial susceptibility panel testing was performed on all MRSA and MSSA pig as well as environmental isolates. A diverse antibiogram pattern was found amongst the isolates. MSSA isolates were resistance to fewer antibiotics compared to MRSA.