Methicillin-Resistant <em>Staphylococcus aureus</em> (MRSA)

Despite the finding of MRSA in pigs and pork in Canada, the potential role on human health is unclear, according to Dr Scott Weese of the University of Guelph, speaking at the 28th Centralia Swine Research Update.
calendar icon 14 July 2009
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Methicillin-resistant Staphylococcus aureus (MRSA) is a multi-drug-resistant bacterium that is a tremendous problem in people. MRSA was originally a concern mainly in hospitalised individuals but there has been a dramatic increase in community-associated disease (infections in people in the general population) over the past 10 years. MRSA is now a leading cause of skin and soft tissue infections in people in the general population and can also cause severe diseases such as necrotising fasciitis ('flesh-eating disease') and severe pneumonia. This bacterium is emerging concern in a variety of animal species and a potential problem for the pork industry because of real or perceived public human health risks associated with contact with pigs or pork. While not an important cause of disease in pigs themselves, there is tremendous concern about MRSA because of its role as an important (and high-profile) cause of disease in people.

Concerns about MRSA in pigs came to the forefront a few years ago, with reports from Europe identifying high rates of MRSA carriage in the nose and intestinal tracts of a large percentage of healthy pigs in different countries. This was identified following diagnosis of 'unexpected' cases of MRSA infection in pig producers, swine veterinarians and their families. High rates of MRSA carriage were also found in people that work with pigs, and a Dutch study reported that pig farmers in that country were 760 times more likely to carry MRSA compared to the general population. One unique aspect was the finding that virtually all pigs (and people associated with pigs) that carried MRSA had one specific strain, called ST398. This strain has historically been very rare in people and it is suspected that it is actually of pig (or pig and cattle) origin.

Since the initial finding of MRSA in pigs, there have been various reports implicating pigs as a source of human infections in Europe, with increasing rates of ST398 MRSA infection. Contact with pigs (or cattle) has been identified as a significant risk factor for MRSA carriage or infection in multiple European studies. The association between pig contact and MRSA is so strong in some regions that pig farmers and veterinarians are automatically isolated if they are admitted to hospital based on the assumption that they are carrying MRSA.

Since the first reports of MRSA in pigs in Europe, there has been considerable interest in this organism internationally. Studies have been performed in various countries evaluating the presence of MRSA in pigs and the possible role of pigs in human MRSA infections. The first study of pigs in North America identified MRSA carriage in 20% of healthy pigs on a sample of Ontario farms. 25 per cent of producers were also carrying MRSA, and in every instance where MRSA was found in a producer, the same strain was found in one or more pigs on the farm. This was followed by a report of MRSA in pigs in Iowa and it is likely that MRSA is widely distributed in pigs throughout North America.

Currently, studies are ongoing to evaluate the prevalence of MRSA carriage by pigs across Canada. Results should be available in early to mid-2009.

Not surprising, the finding of MRSA in pigs led to concerns about food as a source of infection, either through ingestion of MRSA or from people inoculating themselves, i.e. their nose or wounds, with MRSA from meat. A preliminary European study found MRSA in a small percentage of retail pork samples. A Canadian study then found MRSA in a larger percentage of retail pork products, with a combination of ST398 and common human-origin strains. More information on the sources of MRSA contamination of pork and the human health implications is needed. There is currently no evidence that food is an important source of MRSA transmission but this is an area that has only been minimally investigated to date.

Despite the finding of MRSA in pigs and pork in Canada, the potential role on human health is unclear. Unlike the situation that is currently present in Europe, human infections caused by ST398 MRSA are very rare in North America. In comparison with some European countries where ST398 MRSA accounts for 20% of more of community-associated MRSA infections in people, there have only been a handful of ST398 MRSA infections diagnosed in Canada. However, this should be interpreted with caution since only a small percentage of MRSA isolates are typed in Canada, and that typing tends to focus on hospital outbreaks (where ST398 would be less likely to be involved).

Further, it is possible that the Canada is simply a few years behind Europe in the epidemiology of ST398 MRSA and that ST398 infections could be silently on the rise. Only time and ongoing surveillance will determine that. Another complicating factor is the diversity of MRSA types in pigs and pork in Canada. Unlike in Europe, when almost all pigs with MRSA carry ST398, other strains have been found in Canadian pigs, including common human strains. Similar results have been obtained with pork. This makes it more difficult to determine the role of pigs and pork in human disease.

Some of the more important points to remember about MRSA, based on our current knowledge are:

  • MRSA is present in pigs in Canada, and pig producers appear to be at much higher risk than the general public for carrying MRSA in their nose. It is likely that pig producers (and their families) are at higher risk for MRSA infections as well, and they should make sure that their physicians realize this.
  • MRSA infections are typically skin infections such as wound infections and boils. Any pig producer (or a family member) with infections like this should see a physician and make sure they are aware of the MRSA risk.
  • General hygiene practices might be important for reducing the risk of MRSA exposure, particularly avoiding touching your nose in the barn and good hand-washing.
  • MRSA rarely causes disease in pigs so farms can easily be infected without anyone knowing.
  • MRSA can be found on retail pork but there is currently no evidence that this poses a significant risk. Standard meat handling and cooking recommendations should greatly reduce any risks.
  • We currently have no means to control MRSA on pig farms, however further research will hopefully identify practical infection control and biosecurity measures.

Further Reading

- You can view other papers presented at the Centralia Swine Research Update 2009 by clicking here.

June 2009
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