Colistin use in animals: “no evidence of risk to public health” - Veterinary Practice
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Colistin use in animals: “no evidence of risk to public health”

The 2020 edition of the EVAC focused on the much-debated colistin molecule

Recordings of the European Veterinary Antibiotics Conference, organised online on 30 July by Inovet, are now available. The 2020 edition of the EVAC focused on the much-debated colistin molecule and the potential consequences of overregulation. The webinar featured presentations from pharmacological, clinical and epidemiological experts on the topic.

Colistin, the widely used Gram-negative antibiotic in pig and poultry medicine, is on its way to be classified as a category B antibiotic (“restrict”) by the European Medicines Agency, due to a reported increase in resistance in human medicine and the discovery of a mobile resistance gene (mcr-1). There is a real fear among veterinary experts that in the near future this antibiotic – and maybe others – could be banned from use in food-producing animals altogether. A ban that would take away all effective treatment options against major pathogens.

Colistin: no alternatives for poultry health

And this might prove devastating for animal health and welfare – as well as public health. “There is no real alternative to colistin for the group treatment of poultry affected by Gram-negative pathogens such as E. coli,” Xavier Chatenet, field veterinarian and poultry expert, confirmed. “All other antibiotics available to us either have no or only a weak Gram-negative action or would need to be used off label.” Citing expert opinion, Dr Chatenet confirmed that colistin is an essential veterinary medicinal product without a history of resistance or treatment failure in the field if used at manufacturer’s recommendations. “If colistin were to be banned, this would lead to greater resistance problems with the alternatives.”

Antibiotic resistance: the misconception

“There is a widespread misconception regarding resistance,” stressed Dr Pascal Richez DVM PhD, European specialist in veterinary pharmacology and toxicology. “The term ‘resistant’ does not mean you cannot kill the bacteria with a certain antibiotic, it just means you may need to increase the concentration,” he said, adding that the minimum inhibitory concentration (MIC) was more useful to clinicians than just the “Sensitive-Intermediate-Resistant” classification. ”For a clinician, only using this SIR classification, which is based on an epidemiological cut-off value, may result in treatment failure or even contribute to creating resistance.” Therefore, ”it should not be considered a reliable basis for the choice of an antibiotic therapy.”

Clinical breakpoint: the gold standard

Instead, “the clinician needs an informed clinical breakpoint to determine if a pathogen is likely to respond to treatment.” This breakpoint depends on a number of parameters, including the susceptibility of the strain or MIC, the pathogen response, host-pathogen interaction, dosage and pharmacokinetics. Dr Richez described a new study investigating such a clinical breakpoint for colistin in chickens. “All E. coli isolates in the broilers’ intestines retained a high susceptibility to colistin given at the recommended therapeutic dosage, without any induction of resistance.” However, “in case of resistant mcr-strains, which can have a much higher MIC, treatment would require higher doses of colistin.”

Colistin use “no need for concern” in human healthcare

“Despite all the attention going to multi-drug resistant organisms, the vast majority of cost in terms of human healthcare and deaths is actually due to multi-drug susceptible organisms,” reminded Dr Boudewijn Catry, epidemiologist and head of the Belgian service for healthcare-associated infections and antimicrobial resistance. According to him, the current major drivers of resistance in human medicine are beta-lactam antibiotics and fluoroquinolones. “Today, the need to be concerned for colistin resistance in public health care is very low”, he confirmed. This is particularly true since, in human medicine, “many alternatives to colistin are coming on the market.”

Highlights of the EVAC webinar, organised by Inovet, can be viewed online.

Dr Chatenet concluded: “We can help reduce resistance to antibiotics by prescribing appropriate antibiotics at the appropriate dose, adjusted to the MIC of the pathogen concerned. However, this is only possible if we know the nature of the pathogen – and whether or not any resistance genes are present.” He added that, to help veterinarians in their therapeutic approach, a PCR kit for the detection of the on-farm presence of mcr-1, mcr-2 and mcr-3 resistant strains has been developed by Inovet in collaboration with

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