Overuse of antibiotics: A remote study (part 2)

The controversy about the overuse of antibiotics in raising livestock (see the last post) is background for an interesting scientific study that took place in the Galapagos. It looked at the spread of antibiotic resistant strains of bacteria among animals that were totally removed from antibiotics.

Would antibiotic resistance become widespread in the absence of antibiotics?

The immediate motivation for the research was two contradictory studies. In a wooded area of northwest England, researchers had found that wildlife developed antibiotic resistance even though they had not been exposed to antibiotics. This would argue against the idea that antibiotic use in animals should be restricted, since it suggests that antibiotic resistance would develop anyway.

Another study, however, found that wildlife in a remote area of Finland had an almost complete absence of antibiotic resistance. This would argue that resistance could be reduced by restricting antibiotics.

So which was it? How could you design an experiment that controlled possibly confounding factors, such as climate, animal interaction, and human interference?

Coprophagial iguanas

The researchers chose to study a species of iguanas (Conolophus pallidus) on an isolated island (Santa Fe) in the Galapagos . Unlike the English countryside, the island was uninhabited by humans, though tourists made daytime excursions to a restricted area. It offered an example of what life was like in a pre-antibiotic era. This tropical island, which was near the equator, was also unlike the remote area of Finland, where winters were long and cold, the population density of animals was low, and there was limited interaction among animal species.

The island was ideal for the spread of bacteria: It was warm, humid, with constant periods of daylight and high animal density. Plus, juvenile iguanas acquired their intestinal microorganisms through coprophagy (eating feces). This results in bacteria spreading readily throughout the community of iguanas.

This was the perfect setting to test the following: If there are no antibiotics present, and an antibiotic resistant strain of bacteria is present, will it spread throughout the community?

When there’s no chronic antibiotic exposure, resistance doesn’t spread

The researchers discovered that no, it would not spread. A small minority of animals did have bacteria that exhibited resistance – probably acquired from a visiting tourist — but the rest of the community was uncontaminated with these resistant bacteria.

Here’s the conclusion of the study (emphasis added). Let me restate the first sentence in non-academese: Will an antibiotic resistant strain of bacteria spread readily throughout a community if there are no antibiotics present? If so, then this was the ideal situation to observe that happening.

[B]acteria colonizing the gut can easily spread within the reptile community and, if an introduced resistant strain should not need the presence of antibiotics to become widespread, in Santa Fe it would find the optimal conditions for this to occur. In this scenario, the detection of two E. coli isolates with acquired resistance traits of likely human origin as non-dominant microbiota in a small minority of animals, reveals that even highly isolated ecosystems are susceptible to contamination by multiresistant strains. However, in the absence of a chronic antibiotic exposure sustaining resistance, these strains failed to disseminate despite the fact that environmental conditions and animal habits were highly favourable to inter-individual spread, and that contamination from humans to wildlife could recurrently occur at that site. … [L]imited human-driven contamination, in the absence of a chronic antibiotic exposure, is not sufficient for the diffusion of acquired antibiotic resistance in wildlife.

This is not a definitive answer to the question of whether antibiotic use should be reduced or banned in raising livestock. But it certainly does suggest that when antibiotics are not present, antibiotic resistant bacteria do not thrive and spread.

Update 7/3/10:

FDA recommends new limits on livestock drugs (Reuters)

[T]he U.S. Food and Drug Administration recommended on Monday [6/28/10] that livestock farmers use the drugs solely to cure or prevent disease in animals, phasing out their use to promote growth.

Antibiotics in Animals Need Limits, F.D.A. Says (The New York Times)

Federal food regulators took a tentative step Monday toward banning a common use of penicillin and tetracycline in the water and feed given cattle, chickens and pigs in hopes of slowing the growing scourge of killer bacteria.

But the Food and Drug Administration has tried without success for more than three decades to ban such uses. In the past, Congress has stepped in at the urging of agricultural interests and stopped the agency from acting.

Antibiotics and Agriculture (The New York Times editorial)

The Food and Drug Administration is taking some long overdue but still too timid steps to rein in excessive use of antibiotics in American agriculture. For years now industrial and many smaller-scale farmers have routinely fed antibiotics to their cattle, pigs and chickens to protect them from infectious diseases but also to spur growth and weight gain while using less feed. That may be good for agricultural production, but it is almost surely bad for the public’s health.

Jolley: Five Minutes With Dr. Scott Hurd & Agriculture’s Drug Problem (CattleNetwork)

To hear the FDA and numerous health experts, our very survival as a species is at stake as the future potency of antibiotics for humans begins to falter. The feds unequivocally state that drugs like tetracycline and penicillin transfer from farm animals to the people who consume their meat and overuse of such drugs create new ‘franken’-strains of antibiotic-resistant superbugs. …

Wow. Death is knocking at my door and laughing at penicillin because I gave a feverish pig a shot? Better let that porker die so that I may live.

Resources:

Photo source: Yves Vallier

Maria Cristina Thaller et al., Tracking Acquired Antibiotic Resistance in Commensal Bacteria of Galápagos Land Iguanas: No Man, No Resistance, PLoS One, February 1, 2010