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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.
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.
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 & 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.’
Take home messages of the 2020 EVAC webinar
- At therapeutic dosages, there is no acquisition of resistance for water-soluble colistin
- At therapeutic dosages, colistin does not select for mcr-gene bearing resistant strains
- Colistin is essential for veterinary medicine as group treatment through drinking water
- Colistin resistance in human medicine is very low and better alternatives are coming on the market
- Banning colistin from use in animals will increase antimicrobial resistance in the short run, with a detrimental impact on One Health in the long run
Highlights of the EVAC webinar, organised by Inovet, can be viewed here.
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 ID.vet.
November 4, 2020 - Inovet
