Beyond the false linkage to the Type A H1N1 influenza virus, another seemingly dark cloud hangs on the pork industry’s horizon. It’s called MRSA — methicillin-resistant Staphylococcus aureus.
But why is MRSA a concern to you? Researchers in the Netherlands discovered a new strain of MRSA called ST398 in pig populations in 2005. Soon after, it also was found in Denmark. In 2007, Canadian researchers detected the same strain in herds there. Finally, ST398 was found in U.S. herds, and subsequent monitoring has discovered that some producers and veterinarians carry the infections.
Because MRSA is a bacterial challenge, it is too readily and freely linked to overuse of antibiotics in food animals. specifically hogs.
Confusion breeds threat
MRSA is often portrayed as an ominous threat for people involved in pork production. Yet closer inspection of the facts suggests that health risks for producers may be less than feared, and risks to other groups may be minimal.
According to the Centers for Disease Control and Prevention, MRSA can be found on the skin or in the nostrils of ap- proximately 1.5 percent of all people in the United States, while Staph aureus can be found in up to 30 percent of the population. MRSA is commonly sourced from hospitals, gymnasiums, day-care facilities, nursing homes and prisons, and is usually classified as either hospital-acquired or community-acquired MRSA, based on the likely exposure site.
MRSA-related infections can cause serious health threats ranging from deep-tissue wounds to pneumonia. Serious complications are most common in hospital-acquired infections but also can occur with community-acquired disease. Public fear rises due to the hard-to-treat nature of some infections and the name “flesh-eating” disease sometimes used in serious and rare MRSA cases.
There has been only one completed U.S. study on MRSA prevalence, looking at two farms. (A much larger study is nearing completion.) One farm had pigs with ST398 and one did not. This small sample makes it difficult to confidently extrapolate a nation-wide prevalence. However, a larger Canadian study found 24.9 percent of pigs tested positive for a MRSA strain; in the Netherlands, 39 percent showed the same strain. Pigs testing positive have been found in herds that use antibiotics and those that don’t, making the link to overuse of antibiotics ambiguous.
Another supposed swine-MRSA linkage came from an opinion article written earlier this year by a New York Times editor, in which the author tried to directly tie pigs with human illness and death. However, Paul Ebner, a microbiologist at Purdue University, says there is no evidence of this being true. “The overwhelming majority of human MRSA infections are obtained in hospitals or from direct contact with other infected people,” he points out.
To the uninitiated or those prone to quick conclusions, finding any type of MRSA in pigs is enough to lump it with hospital-acquired or community-acquired MRSA and all of those negative implications. However, those who have studied ST398 closely say not so fast.
“Unlike MRSA strains found in hospitals since the 1970s, the new strain in pig populations has only very rarely caused infections in pigs or humans,” Ebner says. “It’s too early to say definitively, but the ST398 strain may lack several of the virulence genes associated with other MRSA.”
Another key difference between human and livestock MRSA strains is the type of antibiotics routinely used. While human medicine used methicillin in the past, for which MRSA is named, Ebner notes that pork producers never used this drug in pigs. “However, it’s more complicated than that. Resistance to methicillin confers resistance to other antibiotics in the penicillin and cepha- losporin families. ST398 isolates are usually resistant to tetracycline as well, which may be because the two resistance genes are linked.”
Peter Davies, an epidemiologist at the University of Minnesota, continues to follow MRSA in pigs globally. He says, “Although ST398 is widespread in pig populations of Holland and Denmark, there is yet to be one reported community disease outbreak linked to ST398 in either country, despite the high density of pig populations.”
Davies also points out while there are valid concerns about occupational contact with pigs, MRSA can be found frequently in many places including everyday things like contaminated hard surfaces in the home or even pets. “Despite the fact that livestock producers in many countries may be at increased risk for carrying ST398, as yet there’s no evidence that they are at higher risk for clinical infections, and fortunately, there is yet to be a fatal case of ST398 reported.”
Davies explains that there is some evidence that ST398 may be less likely to cause infections in people than the more common human strains circulating in the general population (about 1.5 percent of people are carriers). Another possible explanation is that veterinarians and farmers do have some increased risk of infection, but data are not yet available to support that.
In addition, Ebner discounts the notion this new MRSA could be connected to infections resulting from pork consumption. “Foodborne illnesses can be caused by Staph aureus, but they are usually the result of ingesting toxins in the food due to improper storage and typically not by actual infection with the bacterium itself.”
CDC concludes that although MRSA has been found in some uncooked retail meats, transmission to humans, if it occurs, likely accounts for a very small proportion of infections. Of course, any health risk can be nearly eliminated by following proper food handling; as with most foodborne risks, MRSA is destroyed by proper cooking.
Ebner says it’s still too early to offer any MRSA-specific protocols for producers to mitigate the risk other than what they already do for other known disease pathogens. These include proper sanitization before and after working with pigs, stringent hand-washing, strict biosecurity and safeguarding barns from common disease vectors such as birds, wildlife and pets. Also, anyone with an open cut or wound should not work around livestock.
Davies says producers should take MRSA seriously, “but there’s no cause for undue concern.” He notes that even in Europe where producers face high risk of ST398 colonization, it doesn’t appear to translate into infection.
As with any health risk, on-farm first-aid procedures should always be immediately available and medical attention sought if an infection develops.
From a public education standpoint, Ebner tells producers to be ready to offer their neighbors key facts when discussing topics such as MRSA. “Remind them that the health and well-being of your animals, your workers and the community is of paramount importance and you’re taking the right steps to minimize any health risk.”
MRSA Fast Facts
MRSA stands for methicillin-resistant Staphylococcus aureus.
Roughly 30 percent of Americans carry Staph aureus in their noses, and 1.5 percent carry MRSA.
MRSA is typically found in hospitals and some community settings. Infections are designated as hospital-acquired or community-acquired strains.
A specific strain of MRSA (ST398) has been found in pigs in the United States, Canada and Europe.
MRSA ST398 has also been found in cattle, horses, poultry, companion animals and wildlife.
Producers and veterinarians have been found to carry MRSA ST398, but higher risk of infection (illness) than the general public has not been confirmed.
At this time, MRSA ST398 is not generally associated with pig or human illness.
The European Food Safety Authority recently concluded there is no evidence of increased risk of human colonization or infection following contact with or consumption of food exposed to ST398.