Herd Health: How concerned should we be about MRSA 'super bugs' on pig farms?
Wednesday, April 2, 2008
The discovery of methicillin-resistant Staphylocccus aureus (MRSA) in Ontario pigs and pig workers is raising alarm bells in the medical and public health communities that pig-associated MRSAs might extend out into the general population.
by S ERNEST SANFORD
Recently, a mini-media frenzy erupted over a publication by researchers at the Ontario Veterinary College (OVC), University of Guelph, who had reported the presence of methicillin-resistant Staphylocccus aureus (MRSA) in a high percentage of pigs and pig farmers in a survey of Ontario swine herds.
MRSA is a well recognized "superbug" causing "flesh-eating disease" in humans and the anti-pig, anti-intensive farming, anti-antibiotics-in-livestock lobby groups immediately jumped on the report, claiming it was the wanton use of antibiotics by "factory farming" that created the resistant "superbug" in the pigs. They then projected that the resistant bacteria would spread to people.
The report was published in the scientific journal Veterinary Microbiology by a research team consisting of graduate student Taruna Khanna, well known swine researchers Drs. Bob Friendship and Cate Dewey, and Dr. Scott Weese, a pathobiologist at OVC who has done extensive research on MRSA in domestic animals. They collected nasal and rectal swabs from 285 pigs from three different age groups from 20 swine herds. The researchers also took nasal swabs from farm personnel working in the 20 herds. They found MRSA in 45 per cent, 25 per cent and 20 per cent of herds, pigs and pig farmers, respectively.
This whole business about MRSA in pigs, pig farmers and people closely associated with pigs got started in 2004 when Dr. Andreas Voss, a medical microbiologist in the Netherlands, cultured MRSA from a six-month-old girl during routine preoperative bacteriological screening prior to thoracic surgery. On further checking, the girl's parents, who were pig farmers, also had MRSA.
As he investigated further, Voss found a cluster of 23 MRSA cases among 76 pig farmers attending a pig meeting at the farm of the girl's parents. His publication of this unusual cluster of MRSA cases appeared in a medical journal in 2005 and precipitated a cascade of research efforts in several European countries, which resulted in similar identifications of MRSA in pigs and pig workers in Denmark, Germany, Austria, Britain and Spain.
The publication also attracted considerable media and internet blog coverage in the Netherlands and other European countries. The researchers at the OVC in Guelph are now the first to identify and report similar findings in the North American pig and pig farmer populations.
I started getting phone calls and questions from pork producers soon after Dr. Voss' publication in 2005. I'm getting more questions again since the publication by the OVC researchers.
Staphylococcus aureus is a normal inhabitant and common bacterium found in virtually all animals, including humans. Colonization by S. aureus occurs at birth or soon thereafter in most people. At various times in our lives, it can be cultured from our skin, hands, nasal cavities and other mucous membranes.
Although a normal inhabitant, under the right conditions S. aureus is also a pathogen capable of causing various ailments ranging from mild skin infections to severe life-threatening systemic disease. When antibiotics were first developed in the 1940s, S. aureus infections were easily treated with penicillin. In a few short years, however, S. aureus developed resistance to penicillin.
In 1959, semi-synthetic penicillins were developed by the pharmaceutical industry. These semi-synthetic penicillins overcame the penicillin resistance that S. aureus had acquired. Methicillin was a leading example of the new semi-synthetic penicillins. Within one year of its development, however, S. aureus was resistant to methicillin. This methicillin-resistant variant of S. aureus became known as MRSA.
MRSA has for decades been intimately linked to infections acquired in hospital environments and especially Intensive Care Units, where extensive antibiotic use is routine. In recent years, the range of MRSA has expanded out into the general community at large, so much so that community-acquired infections are at least as common and important as the traditional hospital derived variety.
Is MRSA new in pigs?
Let's first look at what we know about MRSA in other animals. MRSA has been reported in many animals including cattle, horses, poultry, dogs, cats and even sea mammals. Until Dr. Voss reported the infection in pigs in 2005, nearly all previous reports of MRSA in domestic animals had been in dogs, cats and horses. Interestingly, infection in these animals has nearly all been attributed to their human owners having infected them first.
It must be emphasized, however, that most people and animals infected with MRSA are asymptomatic carriers who never develop disease. U.S. data suggest that about 2.5 million healthy Americans are asymptomatic carriers of MRSA. There is always the potential, however, for a clinical disease to break out in anyone carrying MRSA, which would then create difficulty in curing them because of the antibiotic resistance.
Furthermore, MRSA infection in domestic animals is not innocuous. Infected animals can become carriers, which then pass the infection on to other people, or, worse yet, reinfect their owners who may have been previously treated and cured of the infection, thereby creating an ongoing infection-reinfection cycle.
In addition, the resistant bacteria in pets could potentially transfer the antibiotic resistance from their bacteria to non-MRSA S. aureus bacteria in people. These possibilities have raised concerns and alerted medical and veterinary personnel to check in-contact pets or animals whenever humans are being treated to eliminate MRSA. This way, the entire family group, including the household pets, can all be treated at the same time.
So now we come back to MRSA in pigs and pig workers, or those in close contact with pigs. The circumstances described above cause flags to be raised now that MRSAs are being identified in pig populations. Even though the major MRSAs are not the types which normally infect people, the mere presence of any type of MRSA in pigs poses a potential threat that they can be transmitted to humans, as exemplified in the European and Guelph studies, and this could then spread to the population at large.
Of particular concern is what appears to be the emergence of MRSA identified as sequence type (ST) 398. ST398 represents a clonal congregation of MRSA which is now considered to be a livestock-associated type of MRSA having been isolated primarily from farm animals. ST398 represented 59.2 per cent of isolates from pigs and pig farmers in the OVC study.
This clonal congregation of ST398 MRSA has caught the attention of regulators, who are starting to take action to determine just what the true prevalence of these MRSAs is. In November 2007, the European Task Force on Zoonoses Data Collection was asked by the European Food Safety Authority (EFSA) to launch a survey on the occurrence of MRSA in animals across the European Union. The National Pork Board in the United States has recently funded research to determine the prevalence of MRSA in swine veterinarians and in pork.
So is it new in pigs? Since it's only recently that anyone has been looking for MRSA in pigs, we just don't know the answer to that. Best guess? Very likely the MRSAs have been present in pigs for decades. It must also be pointed out that, however long they have been present in pigs - and presumably in people working with pigs - there are no reports of pig farmers becoming ill from these bugs over the last several decades.
The link between the use of antibiotics and the presence of MRSA is also in the realm of speculation at this time. Now that tests are being done for MRSA in pigs, the researchers are indeed finding it in pigs. However, MRSA is found in animals which have had little or no exposure to any antibiotics at all - for example horses and sea mammals.
Finally, it must be stated emphatically that there is no food safety concern between MRSA in pigs and contamination in pork. It is remotely possible that food handlers, who happen to be carriers of MRSA, might contaminate pork, or anything else for that matter. Routine cooking of pork would destroy any MRSAs, if indeed they did manage to get on to pork.
What should producers do?
At this time, there is no need for panic or any immediate reaction. The various agencies and regulatory authorities in different countries are gearing up to determine just what the prevalence of MRSA is in pigs and in livestock in general. When these prevalence levels are known, or if a direct link is found between MRSA in livestock and infection in people, we'll very likely see even greater pressure brought to bear for a complete ban of certain or perhaps a wide range of in-feed antimicrobials. Stay tuned, this issue is unlikely to go away.
Addendum
The CDC (Centers for Disease Control and Prevention) in a letter to the U.S. Congress on Feb. 28, 2008, categorized claims that food animals, including pigs, were increasingly the source of MRSAs in humans as "greatly exaggerated." In a direct quote, the letter said "If transmission of MRSA from food animals to people occurs, it likely accounts for a very small proportion of human infections in the United States". BP
S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: esanford@bur.boehringer-ingelheim.com