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Is the way we raise our food giving us MRSA?
The antibiotics fed to the farm animals we eat may have helped to create superbugs like the drug-resistant staph bacteria known as MRSA.
By Alex Koppelman
News
Nov. 7, 2007 | You may want to put down your BLT before reading this, because there's a chance that the most delicious part of your sandwich -- the bacon, of course -- may be playing a role in the latest national health scare.
Methicillin-resistant Staphylococcus aureus, also known as MRSA -- or, in the parlance of New York tabloids, "super staph" -- is an antibiotic-resistant version of one of the bacteria collectively known as staph. Staph, which can cause everything from skin infections to more life-threatening diseases, usually attacks older hospital patients who develop infections after surgery. The newer, often more virulent strains collectively known as CA-MRSA (community-acquired MRSA) have been all over the news in the past few weeks, as they affect people younger and healthier than the usual targets. A recent study suggested that MRSA infection was responsible for almost 19,000 deaths in the United States last year -- more than AIDS -- including the very public deaths of children and adolescents in Virginia, New York and elsewhere. Public health officials have tried to quiet fears, but the problem could get worse. MRSA remains treatable with a number of different antibiotics, but there are already signs that resistance to some of those drugs might be just around the corner.
Bacteria typically become resistant to antibiotics through exposure to them. The finger of blame for the emergence and spread of antibiotic-resistant bacteria -- a spread that apparently began in the early 1990s -- is generally pointed at human overuse of the drugs, especially in hospitals, where until recently MRSA was most often seen and transmitted. But while MRSA may be most easily transmitted in a hospital, that doesn't mean the bug developed its resistance there. When it comes to the overuse of antibiotics, even the most profligate of hospitals can't touch the sheer amount thrown around down on the farm.
Today, by most estimates, farming consumes many more antibiotics than human medicine does. No one, including government agencies, has definitive numbers, but in 2001, the Union of Concerned Scientists released a now widely accepted estimate suggesting that up to 84 percent of all antimicrobials (a slightly broader category that includes antibiotics) were being used in agriculture. Studies conducted in Europe -- and one just released in Canada, the leading exporter of pork to the United States -- suggest that farm animals are at the very least reservoirs for heretofore-unseen strains and that the animals are passing those strains on to their human caretakers. Here in the United States, however, scientists have yet to study the possibility that agriculture may be playing a role in the changing nature of MRSA -- even though the way we raise the food we eat may be making us sick.
"If you really step back from the whole problem in a realistic kind of a fashion and say, Where is this coming from? Where is this being generated?" says Ellen Silbergeld, a professor of environmental health sciences at Johns Hopkins and editor in chief of the journal Environmental Research, "then your mind really has to turn to agriculture because of the overwhelming amount of antimicrobials that are used in agriculture as opposed to clinical use."
Antibiotics aren't aspirin. The same dose of aspirin you take today for a headache will probably work on the headache you get tomorrow. Antibiotics, by contrast, are meant to attack bacteria, living things that can adapt and evolve to become resistant to the drugs meant to wipe them out. As far back as 1945, in the lecture he gave on the occasion of his Nobel Prize, Alexander Fleming, the man generally credited with discovering penicillin, warned of the potential for bacterial resistance to his discovery. Within years, he was proved right -- Staphylococcus aureus, in fact, was the first known bacteria to develop penicillin resistance.
But Fleming talked about resistance developing because of improper use by humans; what he didn't foresee was the use of antibiotics in farming, which became popular in the 1950s. Farmers put antibiotics in livestock feed because modern farming practices, with their cramped, dirty spaces and unnatural food, make animals sick. But an unexpected benefit of antibiotic use was also discovered: For some still unknown reason, they seem to make animals grow faster.
Bacteria can mutate and develop resistant strains in animals just as they do in humans. That question was settled in the 1990s, when studies showed that a farm's prior use of an antibiotic called avoparcin, which was once used for growth promotion in animals in Europe, was strongly associated with the presence on those same farms of bacteria resistant to avoparcin and related antibiotics. There has been continuing debate over the question of whether resistant bacteria can make the jump from animals to humans. But that argument, too, is all but over. In the United States, where avoparcin was never approved for use in animals, people in the community generally do not carry the resulting resistant bacteria; they pick it up instead in hospitals, where the human variant of avoparcin is used. But in Europe, where avoparcin was used on farms, the resistant bacteria was caught in the community and rarely in hospitals. This was but one of the factors that made the European Union decide to ban the practice of feeding antibiotics to animals for the purpose of growth promotion.
Recently, something about MRSA -- and its epidemiology -- has been changing in ways that suggest that those changes could be taking place among livestock. Traditionally considered a disease picked up in hospitals, MRSA is now being seen more and more often in the community. And it doesn't appear that the hospital-acquired strains have just left the hospital and gone feral. The community-acquired strains of MRSA are genetically different. They're new. And though there is as of yet no definitive proof identifying livestock as the source of the major new MRSA strains, there is a growing body of evidence that suggests animals are, at minimum, reservoirs for other new strains now infecting humans.
Those studies done to date in Europe and Canada on MRSA give some credence to the involvement of livestock in MRSA's mutation. Hospitals in the Netherlands, for example, have had fantastic success at controlling MRSA. They employ a "search and destroy" policy, using aggressive screening, strict infection-control procedures, and severe restrictions on the quantity of antibiotics dispensed. They have managed to keep MRSA rates far below those in the rest of Europe. Dutch rates are so low, in fact, that Dutch hospitals list a previous visit to a foreign hospital as an MRSA risk factor.
Recently, though, Dutch researchers have proposed the addition of two new groups to those being screened as risks -- pig farmers and veterinarians. One group of researchers found that pig farmers in one area of the country were more than 760 times as likely as the general population to be carrying MRSA. They concluded that if their observation held true elsewhere, then "pig farming poses a significant risk factor for MRSA carriage in humans that warrants screening wherever pig farmers or their family members are admitted to a hospital." Other studies have recommended isolating the farmers until tests come back negative -- guilty until proven innocent. A separate study found that veterinary professionals students were carrying MRSA at a rate similar to that of another risky group, people who'd been hospitalized in a foreign country.
Dutch scientists have also traced the spread of one relatively new strain of MRSA from pig farms out into the community, specifically to a nurse who had treated one infected patient, the son of a veterinarian who worked mostly with pigs.
The newest study, the first evidence of MRSA in food animals in North America, concerns the discovery of pigs in Canada, the single largest exporter of pork to the United States and the sole country from which the U.S. imported live pigs last year. "Unfortunately there aren't good surveys for resistance in food animals in the United States," Marcus Zervos, head of infectious diseases at Detroit's Henry Ford Hospital, told Salon. "But it's thought that since the MRSA strain is in pigs in Canada, it is likely in pigs in the United States also, because there's international movement of pigs from Canada to the United States."
The study's authors surveyed 285 pigs of three different age groups from 20 pig farms in southwest Ontario. Twenty-five percent of the pigs, they found, were colonized -- that is, carrying the bacteria, but not necessarily infected by it -- with MRSA. In all, 45 percent of the farms had at least one colonized pig, and 20 percent of the farmers themselves were colonized. There was no non-pig-farmer control group in this study, but that's much higher than previously reported colonization rates for the general population, and there was a significant association between the presence of MRSA-colonized pigs on a given farm and MRSA colonization in the farmers who worked there. "The role of antibiotics in agriculture on the emergence of MRSA is completely unknown at this point. It will be hard to objectively evaluate as well," Scott Weese, one of the study's coauthors, told Salon via e-mail. "It is clear that antibiotic use is an important factor in the epidemiology of MRSA in humans and some animal species, and it is reasonable to assume the same in pigs, but we don't have enough information yet to say anything definitive."
MRSA has also been found in pigs in Denmark, the second-largest exporter of pork to the United States. And MRSA has been found not just in living animals but in small concentrations in the food chain as well: in pig meat in the Netherlands as well as in milk in South Korea, mozzarella in Italy, and chicken in Japan.
There is as yet no smoking gun to link animals to the strains implicated in the current MRSA scare, which are different from the antibiotic-resistant MRSA strains attributed to pigs so far. To be fair, any number of factors could have triggered the changes in MRSA's epidemiology, especially because MRSA (like the normal Staph aureus) is very easily spread, most often without ever making its hosts sick. Dr. Fred Angulo, chief medical epidemiologist for the Centers for Disease Control's Foodborne Disease Active Surveillance Network, says it could be that livestock are not the source of the so-called super staph. But it's also possible, he says, that they are. "It's possible that MRSA could be present in pigs and then occasionally be transmitted to humans and then occasionally get into a hospital and create these new strains." Angulo says the matter needs to be explored. The failure thus far to find a smoking gun could also simply be because there is currently so little data on the subject.
Scientists in the United States are just beginning to consider studying the relationship between MRSA and agriculture. Observers with whom Salon spoke -- even those who tend toward activism -- largely blamed this on institutional problems within the United States' food-safety structure, not on Bush administration policy. Simply put, there isn't money available to study MRSA, and if money were to be put aside for that purpose, it would have to be taken away from some other important area of interest.
"There are billions of dollars at stake here, and there's an entrenched way of growing animals that ensures that they will get sick and therefore need to be treated, so it's not at all hard to imagine why folks who want to pursue the public interest are going to run into resistance," says Margaret Mellon, a molecular biologist who specializes in agriculture issues for the Union of Concerned Scientists. "The public health infrastructure in this country, regardless of this issue, is really quite badly off ... They don't have much political clout, whereas something like the NIH, which leads directly to the production of drugs, which leads directly to billions of dollars for the private sector, could not be better funded."
Beyond a lack of resources to study the problem, there's the problem of cooperation. If you're a scientist who wants to determine whether farms in the United States could be reservoirs of MRSA, the farm has to let you collect the proof. As Peter Thorne, the director of the Environmental Health Sciences Research Center at the University of Iowa, points out, that's sometimes easier said than done. "One of the problems has been, in the U.S. anyway, that many of the studies we would like to do to look at the role of industrialized livestock production on people's health require the cooperation of the industry. And that's been a challenge for academicians," says Thorne, who has done research on MRSA in the Netherlands. "We can still go to family farms -- those that remain -- although very few now are raising livestock, because they can't compete."
The scientific method has, in effect, been turned against itself. Scientists are loath to make definitive pronouncements on anything until every possible controlled study has been conducted. In the vacuum left by that failure to truly know, scientists' hedging can be exploited by anyone with a desire to do so. In this case, both big agriculture and big pharma have a profound interest in doing just that.
"Unfortunately, it remains one of the most controversial areas in medicine," Henry Ford's Zervos says. "There are many of us who believe that the scientific evidence is very clear, that it shows the risk of giving the antibiotic to the animal, and the resulting resistance that can make its way into people ... But many people, to be completely open about it, have self-motivating interest in this. It's a big business for the pharmaceutical industry. People don't want to change. Scientists, even, don't want to influence grants that they have from pharmaceutical industry sponsors."