Staphylococcus aureus is a bacterial species that is a normal “resident” of the nose and skin of people and other mammals.
It is also an important “opportunistic” pathogen of people that usually causes localized skin infections but can sometimes cause severe and fatal infections, most often in people suffering other health challenges.
Particular clones (or strains) of antibiotic resistant SA, particularly those resistant to a drug called methicillin and related compounds (thus “MRSA”), are among the most important causes of hospital-acquired infections across the world. Until recent years, this problem was largely limited to hospitals but the epidemiology of MRSA has changed globally in two aspects.
Firstly, in many countries there has been a marked increase in the occurrence of MRSA infections occurring outside of hospitals (known as community-acquired MRSA).
This has attracted a lot of media attention in the United States over the last six months, particularly when cases have affected healthy adolescents and some fatal cases have been reported. Most of this increase has been associated with a small number of MRSA clones that are distinct from the major clones involved in hospital-acquired MRSA infections.
Secondly, while animal reservoirs are not thought to play any role in hospital-acquired MRSA, questions are now being asked about a possible role of animals in community-acquired infections.
In some countries it has been found that people exposed to livestock — including pigs, cattle and horses — are at higher risk of carrying MRSA in their noses, and therefore are likely to be at elevated risk of developing MRSA infections.
MRSA has been detected in many animal species. Although MRSA isolated from companion animals tends to be similar to common human clones, the most common isolates from livestock are relatively uncommon among human cases. Recent reports from the Netherlands and Canada found a surprisingly high prevalence of MRSA in pigs, and also people working with pigs.
Surprisingly, all the MRSA found in pigs in Holland, and 75 percent of those in Canada, were of an unusual clone (now called livestock-associated MRSA by some in Holland).
In collaboration with colleagues at the University of Iowa and Ohio State University, we are now undertaking studies to understand the situation in the United States. The U of I’s Tara Smith recently reported on a pilot study on a single system where 70 percent of pigs were positive, and all isolates examined were of a single clone with strong similarities to the common Dutch-Canada clone. We do not adequately understand the situation in pigs in the United States, and the potential health risks of livestock-associated MRSA to swine workers are not yet clear. However, the following points need to be kept in mind.
• Given the links between the United States and Canadian industries, we should expect that the most likely outcome is that the U.S. situation will be similar to that of Canada (and Holland). That is, it will not be surprising that MRSA may be fairly prevalent in pigs and also people exposed to pigs.
• The pig-associated clone is distinct from the major clones causing human MRSA infections in the United States and Canada. The CDC has determined that these clones are not implicated in the increase in community-acquired MRSA in the United States over recent years.
• The pig-associated clone can certainly cause human infections; a small number of cases, including some severe infections, have been reported from Holland. However, there is no report yet of fatal disease. Given that Holland (which has a rigorous surveillance system for MRSA) is an important pig-producing country and has been aware of high prevalence of exposure of workers for several years, this indicates we are not likely facing any imminent crisis for occupational health in the industry.
• Farms are places where minor injuries (e.g. skin cuts) are frequent. Recognition that our workers may have an above-average risk of MRSA exposure should make us ensure that proper procedures are in place for rapid treatment of workplace injuries. It is important to practice good hygiene: regular hand washing with soap and water; pay attention to existing and new skin wounds (clean and cover with bandages until healed); do not share personal items. Also, seek medical attention if concerned about any infections that develop.
For more information on MRSA see the CDC website at www.cdc.gov/ncidod/dhqp/ar_mrsa_ca_public.html#8.
“The Pork Professor” is a monthly column created by members of the University of Minnesota Swine Extension team. This column was written by Peter Davies, a swine disease researcher and Leman chair in the University of Minnesota College of Veterinary Medicine in St. Paul.