Investigation of risk factors for methicillin-resistance in staphylococci on swine farms

Staphylococci are a major disease burden for animals and humans. In swine, Staphylococcus hyicus is the causative agent of exudative epidermitis (greasy pig disease) which is characterized by skin lesions and a greasy exudate. Outbreaks of this disease can result in significant morbidity and mortality. Swine are also common carriers of Staphylococcus aureus and, although this agent poses little threat to swine, its zoonotic potential is concerning for public health. Both of these staphylococci species have been found to carry the methicillin-resistance gene (mecA) which commonly colocates with the zinc-resistance gene (czrC) within a mobile genetic element (SCCmec). Given the co-location of these genes, it is predicted that exposure to zinc may co-select for the mecA gene in the absence of antibiotics.

A cohort study of 390 pigs at 26 farms was completed. Farms were surveyed for demographics, management practices, biosecurity, and antimicrobial usage. Nasal cultures for methicillin-resistant S. aureus (MRSA) were completed at weaning and again at 3 weeks postweaning. Isolates of MRSA were screened for mecA, mecC, czrC, PVL, and phenotypic zinc resistance. Multivariate random-effect logistic regression models were created to analyze the data.

Ninety (23.3%) nursery pigs and 10 (38%) farms tested positive for MRSA. Univariate analysis revealed that MRSA in nursery herds was positively associated with greater stocking densities (P=0.048), disinfection of pens for incoming pigs every time (P=0.022), and use of in-feed zinc ≥2,000 ppm (P=0.022). Multivariate analysis predicted that the odds of MRSA carriage in pigs on a 3,000 ppm zinc diet was 12.4 times greater than the odds of MRSA carriage in pigs on a 250 ppm zinc diet (95% CI: 3.04-50.25; P< 0.001). Phenotypic zinc resistance and the czrC gene were detected in 36 (90%) and 25 (63%) of the MRSA isolates, respectively. High zinc diets (≥2,000 ppm) were used on 72.7% of the nursery farms. The presence of MRSA was not associated with any particular antibiotic, the number of antibiotics used, or the route of antibiotic administration.

The presence of MRSA in nursery pig herds was epidemiologically associated with the use of high levels of in-feed zinc and this was consistent with the molecular evidence. This evidence supports the theory that zinc can co-select for antibiotic resistance due to the genetic linkage of antibiotic- and zinc-resistance genes within the same genetic element.

Disclosure of Interest: None Declared.