Farm antibiotics, human illness and what connects them. (It has legs.)

When it comes to the impact of farm antibiotics on human health, there’s a data gap.

That the use of antibiotics on conventional/confinement farms provokes the emergence of drug-resistant bacteria really isn’t in dispute; it’s been proven, over and over again, for about 30 years now. (Here’s a long bibliography from the Pew Charitable Trusts that lists the major pieces of research.) And there’s good research as well that those bacteria move off farms via animals, farm workers, groundwater and air currents. (Another long bibliography here, from the Center for a Livable Future.)

But proving the links between resistant farm bacteria and human illness is trickier. Among the reasons: When an individual person who is sick with a foodborne illness goes to the doctor, that doctor does only enough testing to figure out how to treat them. The kind of subtyping you would need to do on a foodborne organism to prove its farm-drug link isn’t useful to a primary-care physician, and the equipment isn’t accessible either; it’s found in academic medical centers and state public health labs. But the public health system isn’t filling the data gap either. The CDC’s main foodborne outbreak-tracking program, FoodNet, monitors the prevalence of 10 illness-causing organisms, but doesn’t test for antibiotic resistance. And the joint federal program that does monitor resistance, NARMS (for National Antimicrobial Resistance Monitoring System, shared by the CDC here, USDA here and FDA here) uses randomized anonymized samples from humans, animals and retail meat, so it can’t illuminate whether resistant bacteria are causing outbreaks.

So are resistant bacteria from farms causing outbreaks of human illness? The Center for Science in the Public Interest says yes. In a white paper published this week, the group documents 35 outbreaks between 1973 and 2009 for which epidemiologic and microbiological links are clear. Quoting from the report:

  • Reporting of outbreaks due to antibiotic-resistant bacteria has increased in each decade since the 1970s, with 40% (14 out of 35) occurring in the last decade … Outbreaks were most common in dairy products (34%) and ground beef (26%). Two outbreaks each were linked to poultry, pork, produce, and seafood, and one outbreak each was linked to eggs and multi-ingredient foods. The food vehicle was unknown in four of the outbreaks.
  • A total of 19,897 people were sickened from these 35 outbreaks, resulting in 3,061 hospitalizations and 26 deaths.
  • For the 31 outbreaks for which antibiotic-resistance patterns were determined, the responsible bacteria displayed resistance to a total of 14 different antibiotics … Of those antibiotics, seven are classified by the World Health Organization (WHO) as ‘critically important’ to human medicine and eight as ‘highly important’ to human medicine. Bacteria showed resistance to tetracycline in 30 outbreaks. Resistances to streptomycin and ampicillin, both classified as critically important antibiotics, were the next most common. Bacteria associated with 19 outbreaks were resistant to at least five antibiotics. Fifteen of those occurred between 1990 and 2009.

There’s still a data gap, of course: Exactly how are the organisms getting from the animals or their manure into the guts of humans? Via meat, or milk, is the logical assumption. But an article also published this week suggested the organisms might have help — from cockroaches and flies.

Researchers from Kansas State University and North Carolina State University scooped up house flies and German (common) cockroaches on conventional confinement farms in both states, and also scooped up poop from the pigs being grown on the farms. They tested all three for the presence of resistant forms of the common gut bacteria Enterococci. Almost all — 89 percent of the pig-manure samples, 94 percent of the cockroach guts and 98 percent of the flies’ guts — contained Enterococci. Of the Enterococci, at least 90  percent of those found in each species were resistant to tetracycline; from 50 percent to 70 percent were resistant to erythromycin; and from 10 percent to 40 percent were resistant to ciprofloxacin and streptomycin — NB, all drugs used in essentially identical forms in humans as well as livestock. PFGE analysis of the Enterococci from the pigs and the insects showed they were carrying the same bacterial clones.

The researchers write:

Organic wastes in and around animal production
facilities including swine farms provide excellent habitats for house flies and German cockroaches. Several features of house flies and cockroaches, including their dependence on live microbial communities, active dispersal ability and human-mediated transport, attraction to places where food is prepared and stored, developmental sites, and mode of feeding/digestion make these insects an important “delivery vehicle” for transport of bacteria including antibiotic resistant enterococci from reservoirs (animal manure), where they pose minimal hazard to people, to places where they pose substantial risk (food)…

High frequency of resistance to tetracycline, erythromycin, streptomycin, kanamycin, and
ciprofloxacin in our study likely reflects use of tetracyclines, macrolides, aminoglycosides and
fluoroquinolones as feed additives for swine in the USA… The source of antibiotic resistant
enterococci in house flies and cockroaches in this study was the swine manure due to very high
prevalence of antibiotic resistant enterococci in all three sources.

Cite: Ahmad, A et al. Insects in confined swine operations carry a large antibiotic resistant and potentially virulent enterococcal community. BMC Microbiology, 26 January 2011, 11:23doi:10.1186/1471-2180-11-23

Food bin Flickr/j_bongio/CC; Fly face Flickr/e_monk/CC


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