#09-161

Complete

Date Full Report Received

06/18/2010

Date Abstract Report Received

06/18/2010

Investigation

Institution:
Primary Investigator:

Recent news stories, and many web sites and blogs, have suggested that routine use of antibiotics on pig farms may be threatening public and worker health. A widely expressed fear is that tetracycline and other antibiotics may create selection pressures that favor the spread of antibiotic-resistant “superbugs,” such as methicillin-resistant Staphylococcus aureus (MRSA).  The scariest stories discuss the rapid increase in antibiotic-resistant illnesses in recent years, and the large death toll – as many as 70,000 deaths per year in the United States – that resistant infections are causing.  The implication is clear:  farming practices that threaten human lives must be changed, perhaps through Congressional action.

Conspicuously absent from these stories has been a quantitative discussion of how many excess deaths, treatment failures, or days of illness each year are caused in the United States by MRSA arising from use of antibiotics in pig production.  In other words, how big is the risk?  It is impossible to make wise policy decisions without first understanding how large the human health risks are now, and how much they would be changed by proposed interventions.  The objectives of this report are, therefore, to quantify risks to the public and to pig farm workers, by using available facts and data to estimate the annual number of pig-associated MRSA deaths and illnesses in the United States, and the fraction that might be prevented by halting antibiotic use.

We examined two major exposure pathways by which the specific type of MRSA that is associated with pigs (Clonal Complex 398, CC398 MRSA) might reach and infect humans:  via the food chain, and via direct  contact on farms.  (Other occupational risks, e.g., in slaughter houses, are much smaller, and would not greatly change the total risk estimate.)  A review of health statistics and scientific literature on CC398 MRSA in the United States, Canada, and Europe showed that pig-associated MRSA risks are minimal.  In contrast to the large risks (e.g., 70,000 excess deaths per year) mentioned in some news stories, the actual number of deaths that have ever been found to have been caused by pig-associated (CC398) MRSA in the United States is zero.  In fact no human deaths and no serious infections have been found to have been caused by pig-associated MRSA.  (The large numbers of excess deaths per year referred to in some stories about public health risks from antibiotic use and MRSA on pig farms are actually caused by something else:  hospital-associated MRSA and other resistant bacteria, arising from hospital antibiotic use and inadequate hand washing and infection control in hospitals, and mainly affecting patients with severely compromised immune systems.  Attributing these risks to antibiotic use on pig farms is simply incorrect.)

That the risk to human health from pig-associated MRSA in the United States has so far been undetectably small does not mean that it is necessarily zero.   We therefore quantitatively estimated colonization rates among pig farmers, and infection rates per colonized individual.  From these, we calculated that the true risk of serious MRSA infection or deaths in the United States due to pig-associated (CC398) MRSA from pig farms, while uncertain, is probably less than 1 excess infection case per year in the entire United States population.  The true risk could be much smaller (possibly zero), but more years of data showing no adverse consequences among colonized people are needed to reduce the plausible upper bound on quantitative risk much below 1 case per year.  Modeling the dynamics of MRSA in hospitals shows that CC398 MRSA risks are unlikely to increase dramatically in future, e.g., via rapid spread through hospitals, as the CC398 type has a relatively low potential for spread (basic reproductive rate).

Finally, even if the risk to humans from CC398 MRSA, although vanishingly small, eventually proves to be greater than zero, it is unlikely that ceasing use of antibiotics in pig production would affect the risk significantly.  Indeed, current knowledge suggests that the main selection pressure favoring MRSA on pig farms may not come from antibiotics at all, but from zinc compounds fed to recently weaned pigs.  Thus, although MRSA from pigs poses minimal health risks to humans, efforts to reduce the perceived risk (which can be much larger than the real risk) by banning animal antibiotic use is unlikely to create any human health benefit.