Survival of LA-MRSA in Dust from Swine Farms

Abstract

Dust is suspected to be an important factor in transmission of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) between pigs and pig farmers and their families. The aim of this study was to determine the rate of decay for Staphylococcus aureus and LA-MRSA in dust from swine farms. Electrostatic dust fall collectors (EDCs) were used for passive sampling of settling airborne dust in 11 stable sections from six swine farms. Extraction, plating, identification, and enumeration of cultivable S. aureus and LA-MRSA from the EDCs were performed after storage for 0-30 days postsampling. The survival of S. aureus was measured in 196 dust samples from all farms, and data were used to estimate the decay constant λ according to a model for exponential decay: N(t) = N0 × e-λt. The number of S. aureus colonies was up to 600-fold higher than the number of LA-MRSA colonies on MRSA selective agar. The data showed a good fit to the model (λ = 0.13, r2 = 0.86) even with a large difference in initial concentrations of S. aureus between stables. The loads of S. aureus and LA-MRSA in the dust were significantly reduced by storage time, and the half-life was 5 days for both S. aureus and LA-MRSA. In dust samples with high initial concentrations, LA-MRSA and S. aureus could still be cultivated 30 days after sampling. On all farms MRSA isolates belonged to the clonal complex (CC) 398, and at one farm some isolates also belonged to CC30. A screening for other Staphylococcus species in the farm dust revealed 13 different species numerically dominated by Staphylococcus equorum. Based on the exponential decay model, S. equorum had a half-life of 4 days. In conclusion, the presence of MRSA in airborne dust from five of six farms indicates that dust might be an important vehicle for transmission of LA-MRSA. LA-MRSA and S. aureus was found to survive well in farm dust with half-lives of 5 days, and dependent on the initial concentration they could be found in farm dust for weeks. The 99.9% die-off rate was 66 days for LA-MRSA. Thus, farm dust can pose an exposure risk for humans in the farm environment, but also when transported to other environments. On the other hand, the risk will decrease by time. These results provide important knowledge to diminish spread from farm environments to other environments on, e.g., tools or clothing, and in relation to cleaning of emptied LA-MRSA-positive stables.

Measurement of dust‐borneMRSAin pig farms using different approaches

“…) and for up to 7 days in pig farms (Feld et al . ). In farmers’ homes, overloading of EDCs has been found when sampling of endotoxin for 28 days (Kilburg‐Basnyat et al .…”

“…Staphylococcus aureus has been described as a robust bacterium when present in farm dust (Feld et al . ). However, in order to properly evaluate risk of an exposure, the sampled micro‐organisms should be undamaged and the number of sampled micro‐organisms should be representative of the airborne concentration (Cox ).…”

“…aureus is the electrostatic dust collector (EDC) (Feld et al . ). Since the introduction of the EDC in 2008 (Noss et al .…”

“…) and up to 7 days (Feld et al . ), impingers for 30 min (Friese et al . ), IOM for 150 min (Friese et al .…”

“…In the risk evaluation of occupational exposure to airborne S. aureus, it is important to consider the living bacteria which are typically measured as colony forming units (CFU) of S. aureus or MRSA. Staphylococcus aureus has been described as a robust bacterium when present in farm dust (Feld et al 2018). However, in order to properly evaluate risk of an exposure, the sampled microorganisms should be undamaged and the number of sampled micro-organisms should be representative of the airborne concentration (Cox 1991).…”