In my last post I promised to catch up on some of the other research that has been published on the flow of MRSA (and other resistant organisms) between farm animals and farm workers as a result of farm antibiotic use.

Before I do that, though, I want to nod toward two other great pieces published on this. First, Mark Bittman examined this issue closely at the New York Times. And Clare Leschin-Hoar also covered the new research at Take Part. (Bonus: Don't miss her dissection of the news that a National Geographic photographer was arrested in Kansas after taking pictures of a feedlot — from the air.)

Next, I promised I'd revisit the paper by Tara Smith and group on antibiotic-resistant bacteria on farms and in farm personnel. This research was published in May, but it has been a long time coming. Now that it is out, it is an important addition to the still-sparse literature on livestock MRSA and the influence of farm antibiotic use on antibiotic resistance. (If that idea is new to you, start here.)

As a reminder, Smith and her colleagues were the first to identify livestock MRSA, or MRSA ST398 for short, in pigs and pig-farm workers in the United States, after it had already been identified broadly in the European Union and also in Canada. That first study in 2009 was small and they wanted to do a larger one. Here from their abstract is what they looked for and found:

We collected nasal swabs from pigs and farm workers at 45 swine herds (21 antibiotic-free herds; 24 conventional herds) in Illinois, Iowa, Minnesota, North Carolina and Ohio. MRSA was isolated from 50 of 1085 pigs (4.6%) and 31 of 148 (20.9%) of farm workers. MRSA-positive pigs and people were clustered in four conventional swine farms in Iowa and Illinois. Based on genotyping, spa type t034, a common livestock associated variant, was predominant among both human and swine isolates. These results confirm the presence of LA-MRSA in pigs and swine farm workers in the USA, but the prevalence found is relatively low compared with European studies.

As I mentioned in my last post, that staph or MRSA would be found in farm workers is not unusual: roughly one-third of the population carries staph in their nostrils at any time, without experiencing an infection, and about 4 percent of the population carries MRSA, drug-resistant staph. The question has been, how common is MRSA ST398, a strain of drug-resistant staph that can be linked back to farm drug use by its specific resistance signature? The answer is important, not just because it reveals a specific potential human health risk, but also because it helps to fill in the still-disputed question of how much farm-drug use contributes to the increase in antibiotic resistance overall. The table to the right, from Smith's paper, answers the question for the group of farm workers they took samples from: Out of 31 people in the group who carried some strain of MRSA, 21 were carrying the livestock strain.

Those workers were in Iowa and Illinois, and one was in Ohio; and the previous work Smith and colleagues did found livestock MRSA in Iowa as well. That makes North Carolina, discussed in my last post, the fourth state in which livestock MRSA has been found in farm workers. Livestock MRSA has now been found in pigs in those states, and Minnesota, South Dakota and Connecticut.

As I mentioned, livestock MRSA has a specific genetic signature – resistance to tetracycline – that has been interpreted to be the result of tetracycline use in swine-raising. (Tetracycline resistance is unusual in human MRSA because tetracycline isn't commonly prescribed for the infection. When it appears, it may be because the gene for tetracycline resistance has traveled into the bacterium as part of a set of genes conferring resistance to several antibiotics at once.) There is an interesting discussion in Smith's paper of whether on-farm antibiotic use is in fact responsible, or whether other factors are influencing the staph strains; but it is complex enough to leave for another post.

Meanwhile: that antibiotic use on farms causes the emergence of antibiotic resistance is supposed to be settled science at this point. On the basis of that assumption, governments in Europe have banned the use of certain drug types and formulas in farming, and for decades, researchers in the US have been pressing this government to do the same. (Unsuccessfully, so far – which is why Rep. Louise Slaughter of New York keeps introducing PAMTA, the Preservation of Antibiotics for Medical Treatment Act.)

But a piece in the new BMJ captures that for many people, notably in agriculture and veterinary pharma, the question isn't settled at all. The colloquy between physician David Wallinga and veterinarian David G.S. Burch, titled "Does adding routine antibiotics to animal feed pose a serious risk to human health?" does a great job of capturing how far apart the two sides remain. The colloquy is open access and I urge you to read it. Quick excerpts that capture the two authors' points of view:

Wallinga: "Routine antibiotics are not necessary for animal health. Pasture based production was the norm before antibiotics. Industrial style meat production, in which animals are confined in close quarters and fattened on soy and maize based feeds, also is possible without routine antibiotics... Almost every European and North American public health authority agrees: routine antibiotic use in animal food production likely worsens the epidemic of resistance." Burch: "Given that the critical antimicrobials in human medicine are not used in animal feed, that regulatory authorities conduct thorough assessments of the risk of resistance from use of antimicrobial substances, and that the environmental effect and the effects of residues in edible tissues are also assessed, it is highly unlikely that adding antibiotics to feed poses a serious risk to humans, especially compared with the extensive use of antibiotics directly in humans."

I urge you to read the whole thing.