Two recently published studies involving scientists from the USDA National Animal Disease Center (Dr. Tracy Nicholson), and the Colleges of Veterinary Medicine at Iowa State University (Dr. Tim Frana) and the University of Minnesota (Dr. Peter Davies) have brought new insights about the public health implications of ‘Livestock Associated’ MRSA in the USA. The fact that food animals could be a reservoir for the ‘superbug’ methicillin resistant Staphylococcus aureus (MRSA) shocked the medical and veterinary worlds when first discovered in Europe around 2004. The early studies identified a novel type of MRSA (ST398) in pigs and other livestock in Europe, but things soon became more complicated when other varieties (e.g., ST9 in Asia and ST5 in North America, among others), were also found to be harbored by pigs. It was also clear that the ‘livestock associated’ MRSA were often detectable in the noses of people working with livestock, and in some cases their family members. The ST398 variants have been intensively studied in Europe, and it is established that they are capable of causing actual clinical infections in people.
However, they appear to be less transmissible than standard human variants, and also less likely to cause severe infections. Interestingly, although people working with livestock are at high risk of exposure, to date there are very few reports of serious infection in this group. Also, detailed studies in Denmark indicate that there is negligible concern for foodborne transmission.
The paths of Nicholson, Frana and Davies, together with graduate students Samantha Hau (ISU) and Jisun Sun (UMN) converged around a unique situation in North America where ST5 variants had been found to be relatively common in pigs in Iowa [Frana’s work funded by the National Pork Board (NPB) checkoff program) and in a national study of swine veterinarians by Davies and Sun funded by the Upper Midwest Agriculture and Safety and Health Center (UMASH). Unlike ST398 and ST9, which to date appear to have had no significant role on public health in the USA, ST5 MRSA are among the most common variants causing human clinical infections. With further NPB funding, the groups pooled the isolates from that work to enable Nicholson and Hau to spearhead a molecular level comparison of ST5 MRSA from ‘swine related sources’ with human isolates sourced from clinical infections.1 They found that the swine associated ST5 MRSA isolates uniformly lacked a set of genes known as the immune-evasion cluster (IEC) genes. These genes are central to the ability of S. aureus to cause serious infections in people, and are transmitted by a specific bacteriophage (virus infecting bacteria). This phage was also uniformly absent from the swine related isolates but present, along with IEC genes, in over 90% of the human ST 5 isolates. This lack of IEC genes is also documented in ST398 MRSA and is thought to be related to the adaptation of the bacteria to animal hosts.
In parallel, with NPB funding Sun and Davies also leveraged the UMASH network of swine veterinarians to document the prevalence of S. aureus, including MRSA, in pigs on 38 swine farms in 11 states.2 There were two surprises. The first was that no herds were positive for MRSA other than one intentionally included positive control herd. This suggests the MRSA prevalence if US swine herds is probably no more than around 10%, which is substantially lower than in many European countries such as Denmark (estimated to be 70%). Secondly, by not solely focusing on MRSA, they established that methicillin susceptible variants of the 3 lineages that are most often identified among ‘livestock associated’ MRSA worldwide (ST398 in Europe, ST9 in Asia, ST5 in North America) are all common in healthy pigs in the USA. As usual, these findings have generated as many questions as answers, but important messages are the MRSA appear to be much less common in US pig herds than anticipated, and that those that have been found appear to have a diminished capability to cause human disease compared with variants circulating in the human population.