30 April 2019- Many jurisdictions, even those that are quite strict when it comes to the use of in-feed antibiotics, make an exception for the animal-only antimicrobials known as ionophores.
In poultry, ionophores are primarily used to control coccidia, parasites which are protozoans, part of an entirely different branch on the tree of life from bacteria. Yet ionophores also have some antibacterial effects, namely against gram-positive bacteria. Importantly for the poultry industry, prophylactic ionophore treatment to control coccidiosis has also helped keep necrotic enteritis (caused by the gram-positive bacteria Clostridium perfringens) in check.
Nonetheless, ionophores—which are themselves not used in human medicine—also have a unique mode of action compared to other classes of antibiotics (i.e. RNA/DNA inhibitors, cell wall inhibitors, biochemical pathway inhibitors, or efflux pump inhibitors)—they transport cations across a cell’s membrane, disrupting its electrochemical balance and causing cell deaths. In the European Union, they are classified and regulated not as therapeutic antibiotics, but as a feed additive.
For some people, this separate classification is a positive thing; avoiding coccidial parasites and secondary gut problems such as necrotic enteritis in poultry is essential for maintaining animal welfare, and avoiding the need to treat later conditions using antibiotics which are important in human medicine. For others, it is a negative thing; they consider the separate classification of ionophores as something other than a veterinary antibiotic nonsensical, and some even consider that use of ionophores should be considered akin to antibiotic growth promotion.
The issue divides lobbyists and campaigners. But what does the science say about whether ionophore use contributes to antimicrobial resistance?
According to Dr. Live Nesse, lead author of a report “The risk of development of antimicrobial resistance with the use of coccidiostats in poultry diets” prepared for the Norwegian Scientific Committee for Food Safety in 20151, this should in fact be two questions.
The first of these questions is: does ionophore use promote resistance against ionophores themselves?
On this front, Dr. Nesse is not especially worried. In coccidia, the parasites against which these ionophores are being deployed, she claims that no significant resistance has been developed. Nor, she says, are there signs that Clostridium perfringens, the gram-positive bacteria behind necrotic enteritis which are also susceptible to ionophores, is developing resistance against ionophores. For their part, Elanco, a major manufacturer of ionophores, has asserted that this would be unlikely. In the words of Chief Medical Officer Professor Shabbir Simjee, “it’s not a single mode of action, they target multiple different ions which can go in and out of cells, so the whole mechanism is very complex…For bacteria to have any resistance to ionophores, they would need major structural reorganization both at DNA level and cellular level, and that is very, very cost-inefficient for the bacteria.”
One contrary note is sounded, though. According to Dr. Nesse, noteworthy levels of resistance* have been identified in enterococci, gram-positive bacteria used as an indicator species for the gut. However, enterococci are not usually pathogenic. Therefore, in short, it might be said that while the development of ionophore resistance mechanisms among gram-positive bacteria is demonstrably possible, this has not yet happened among poultry pathogens, nor is the threat of it seen as problematic enough to cause a change in their usage.
However, the second of these questions is more complex. Does ionophore use promote co-resistance against other antibiotics which are used in human medicine?
As Dr. Nesse explains, statistical evidence at the time of her 2015 report suggested a tentative “yes”. For years after eliminating avoparcin (an analogue to vancomycin,an antibiotic considered critically important in human nutrition) from veterinary use in Norway, the national AMR surveillance program NormVet still picked up puzzling levels of vancomycin-resistant enterococci in bacterial samples from Norwegian broilers**. It was also observed that that resistance tended to be correlated with increased MIC to narasin, an ionophore used in poultry (MIC is minimum inhibitory concentration; when it is increased, it is a sign that antimicrobials are less effective).
There also appeared to be evidence of a connection between narasin resistance and vancomycin resistance beyond this statistical coincidence. In 2012, while working on vancomycin resistance, a researcher named Dr. Oskar Nilsson of the National Veterinary Institute of Sweden accidentally uncovered evidence2 that narasin resistance was transferrable; as he noted to Feedinfo, “before that, we had assumed that increased MIC to narasin was not transferrable, that it was a chromosomal mutation or something.” Then in 2016 Dr. Nilsson and his coauthors3 identified a plasmid whose presence is also correlated with an increased MIC to narasin—not the same thing as clinical resistance, but a worrying sign of lowering susceptibility nonetheless—which also contains the gene coding for resistance to vancomycin. Plasmids are a mechanism allowing bacteria to swap genetic material, and play a key role in the spread of resistance. It was therefore hypothesized that narasin (ionophore) use could favor the survival of narasin-resistant bacteria, and that bacteria could be spreading vancomycin resistance at the same time as they swap that narasin resistance plasmid.
There is, however, a hole in this story, as later evidence would show. Dr. Nilsson’s surveillance of Swedish vancomycin resistance—which was, after all, the starting point for this discovery—does not show that the population of vancomycin resistant bacteria is increasing in correlation with use of the ionophore narasin. “In 2015 we did a follow-up of how much vancomycin-resistant enterococci in broiler production and found it had decreased substantially since 2010—and if anything, narasin use in Sweden increased over that time period because the withdrawal period was shortened.” As he concluded in a paper4 this year: “the hypothesis that use of narasin plays a role in persistence of vancomycin resistance among enterococci in Swedish broilers is weakened.”
“I don’t think [the link between narasin and vancomycin resistance] is such a big problem that you would stop using narasin because of that,” he told Feedinfo. “Should broiler production in the world be dependent on chemicals like narasin, that is another question. But I can’t really see that AMR is an argument strong enough to stop the use of narasin.”
Elanco underlines a similar point; ionophores have been used extensively in production systems worldwide for over 40 years now, and if narasin really was driving resistance to vancomycin, these trends should have been quite visible. “If this was seriously an issue, we would have picked it up quite a while back, and we would have seen more resistance to vancomycin,” says Professor Simjee. “We’re just not seeing that being reported.”
Meanwhile, in Norway—which is culturally averse to the use of antibiotics to a much greater extent than most other places, according to Dr. Nesse’s observations—consumer pressure forced broiler producers to end the use of narasin in broilers in 2016. This will provide a natural experiment to illustrate what the impacts on AMR will be. “Now Norway has stopped using ionophores in broilers, we’re waiting to see what happens with vancomycin resistance,” she says.
Indeed, while the most current evidence now seems to indicate no link between ionophore use and AMR, further surveillance is always prudent. Even the Federation of Veterinarians of Europe, whose position paper in 2016 advocated for changing the rules in Europe in order to require a veterinary prescription for coccidiostats, bases this argument in the desire to see these drugs brought under supervision in order to allow their inclusion in the ESVAC monitoring system. Importantly, they do not believe that any rule change should render coccidiostats less available to the market. “The (prophylactic) use of coccidiostats or anticoccidial drugs remains for the time being necessary in modern animal husbandry in the EU,” it says.
*Elanco argues that there are no clinical breakpoints for ionophores, so we cannot actually say what is resistant and what is not; however, the Danes did years back set a tentative breakpoint of 16ug/ml of ionophores against enterococci, a level which, according to Elanco, has not yet been observed. The company also argues that although increased MICs have been noted, these are at levels where Narasin is still 100% effective at killing the enterococci.
**In a similar vein, Dr. Nesse points out that evidence, including statistical co-occurrence in the Norwegian surveillance data and older laboratory results, to suggest that resistance to bacitracin, another antibiotic used in human medicine, may have some connection with the use of some ionophores. As the genes coding for resistance to bacitracin has not yet been identified, the possibility of demonstrating a connection between these ionophores and the bacitracin resistance does not yet exist. However, bacitracin is not considered a critically important antibiotic for human medicine, being found primarily in topical skin creams.
1 Panel on Animal Feed of the Norwegian Scientific Committee for Food Safety [VKM] (2015). The risk of development of antimicrobial resistance with the use of coccidiostats in poultry diets. Oslo.
2 Nilsson, O., Greko, C., Bengtsson, B. and Englund, S. (2012). Genetic diversity among VRE isolates from Swedish broilers with the coincidental finding of transferrable decreased susceptibility to narasin. Journal of Applied Microbiology, 112(4), pp.716-722.
3 Nilsson, O., Myrenås, M. and Ågren, J. (2016). Transferable genes putatively conferring elevated minimum inhibitory concentrations of narasin in Enterococcus faecium from Swedish broilers. Veterinary Microbiology, 184, pp.80-83.
4 Nilsson, O., Alm, E., Greko, C. and Bengtsson, B. (2019). The rise and fall of a vancomycin-resistant clone of Enterococcus faecium among broilers in Sweden. Journal of Global Antimicrobial Resistance, accepted manuscript (in press).