Evidence Suggests ‘Livestock-Associated MRSA’ Not Widespread in U.S. Swine Industry

Staphylococcus aureus are versatile bacteria that can survive and multiply in many environments. Methicillin resistant S. aureus (MRSA) have been a growing problem in human medicine for 50 years, and one of the so called “superbugs” due to frequent resistance to (often many) antibiotics.

Animals were thought to have no role in MRSA spread to humans until the discovery in the Netherlands in 2004 that commercial pigs commonly harbor an unusual variant of MRSA (termed ST398 based on a genotyping method). This sparked much speculation and scaremongering about the health threats posed by “livestock-associated MRSA,” or LA-MRSA. Ten years later we have the benefit of both research and experience to give us a more-informed picture of the importance of LA-MRSA. Before looking at some of our ongoing work in the United States, it is worth looking at the “big picture” when it comes to LA-MRSA in animals and humans around the globe.

First, MRSA have now been reported in many animal species including food animals and pets. Also, ST398 MRSA appear to have negligible impact on pig health. Although you may often read that ST398 MRSA cause a large proportion of MRSA “cases” in countries like the Netherlands and Denmark, this is misleading, as it includes people who are merely carrying the bacteria (for example in the nose), but are not infected.

Analyses of actual infections show ST398 MRSA are much less prevalent, particularly among serious cases such as blood stream infections in those countries. Importantly for farm workers, medically significant ST398 MRSA infections in healthy livestock workers appear to be very rare despite frequent exposure. The small numbers of serious or fatal ST398 MRSA infections have generally been in medically compromised people without any livestock contact, and the picture is complicated by the fact that there are some ST398 variants that circulate in the human population without involvement of animals.

Not all MRSA are equal, and it has been shown that LA-MRSA is less likely to spread among people than MRSA of human origin. Also, ST398 MRSA from pigs appear to contain few of the known S. aureus virulence factors (genetic components that enable them to cause severe infection), which may explain the rarity of infections in swine workers worldwide. Although ST398 MRSA have been found in nasal swabs of livestock, farmers and veterinarians in the United States, there has yet to be a single case of infection with ST398 MRSA reported in the United States.

While ST398 has been the focus in Europe, broader research has shown that other MRSA variants can occur in pigs in other regions, particularly ST9 in Asia and ST5 in North America (United States and Canada). Thus far, ST9 appears to have negligible impact on human health, but the situation with ST5 variants is complex, as this “family” is common in human infections in many countries. A collaborative study (funded by the Pork Checkoff) of ST5 S. aureus from U.S. pigs and farm workers (including veterinarians) is being conducted by Tracy Nicholson at the U.S. Department of Agriculture National Animal Disease Center, Tim Frana at Iowa State University, and our group at the University of Minnesota, and should give us better understanding of the relationships between pig-associated ST5 and human disease isolates.

Over the last three years at the University of Minnesota, we have been studying S. aureus epidemiology in pigs (funded by the National Pork Board) and swine veterinarians (funded by NIOSH through the Upper Midwest Agricultural Safety Center) in the United States. Results of these studies point to two things. First, S. aureus are common in both pigs and swine veterinarians, and the ST398, ST9 and ST5 lineages make up the vast majority of isolates in both groups (suggesting most of what we find in veterinarians may be of pig origin). Second, MRSA variants have been relatively uncommon. Our veterinary study was similar to a study in Holland where about two-thirds of veterinarian samples were positive for S. aureus, including 44% with MRSA. While the same proportion (two-third) of U.S. veterinarian samples were positive for S. aureus, a much lower proportion (about 7%) were MRSA-positive.

On the pig side, some published MRSA studies in Canada and the United States already point to a relatively low proportion (< 10%) of MRSA-positive herds. To date in our ongoing study of S. aureus in pigs, as with the veterinarians, S. aureus is common and ST398, ST9 and ST5 variants account for almost all isolates. However, only one farm out of 35 tested to date was MRSA-positive (with all pigs testing positive). However, this was a farm that was previously known to be positive and was deliberately included when farms were repeatedly testing negative for MRSA.

This accumulating evidence points to much less MRSA being present in the United States than in European countries with large pig industries such as Holland and Denmark. The reasons for these apparent differences remain to be explained.

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