Antimicrobial use, resistance and misconceptions
Following the recent release of the UK's usage
of antimicrobials in veterinary medicine, there was the usual storm from the
organic lobby. Are they right, are we sitting on a time bomb waiting to go
Following the recent release of the UK's usage of antimicrobials in
veterinary medicine, there was the usual storm from the organic lobby. Are they
right, are we sitting on a time bomb waiting to go off?
(Veterinary Medicine's Directorate) released the therapeutic antimicrobial usage
figures for 2007 and showed that the figures had fallen for the third
consecutive year - by nearly 15% (67 tonnes) since 2004, to 387 tonnes. This
appears to be very positive news, especially if one adds another 32 tonnes of
growth promoter use, which has also stopped.
They also made a brave
attempt to split the use of antimicrobials by different animal species, which is
difficult, as several major products are approved for use in pigs as well as
poultry and cattle. Pig use seems to be the largest with an estimated 206 tonnes
or about 53% of UK usage and poultry 26%.
The bulk of these would be
oral products such as medicated premixes and water solubles. These tend to be
the older products such as tetracyclines, trimethoprim/sulphonamides etc, which
are not so important in human medicine. Injectable products account for only
9.6% of total usage.
During the same time period
though, the usage of two important antimicrobial groups in man, namely the
cephalosporins and fluoroquinolones, did increase by 92% and 38% respectively to
6.2 and 2.0 tonnes respectively. This is still only a small fraction of what is
used in human medicine, however. Is this going to be hugely damaging to pig or
Cephalosporins are comparatively safe and highly
effective antibiotics, hence their increase in popularity, especially in
companion animal (dog and cat) medicine, which accounts for 70-80% of their use.
The first and some second generation cephalosporins may also be given as
tablets, hence their popularity in pet medicine, especially for treating chronic
The more advanced, third
and fourth generation cephalosporins are primarily used by injection in pig and
cattle medicine, and account for 10-15% of the overall cephalosporin use.
Another feature, which adds to their relative safety is they are primarily
excreted via the kidney and urine, so there is little contact and selection
pressure on the gut organisms such as Salmonella species and
Escherichia coli, which might be inadvertently passed on to man via meat
contamination at the slaughter house. This lack of resistance was confirmed by
two recent surveys in the UK on these two bacteria from pigs and no resistance
to 3rd generation cephalosporins was detected.
fluoroquinolones, more recent data from UK pig and poultry farms identified the
presence of fluoroquinolone resistant E. coli on 58% of pig farms, 91% of
turkey farms and 50% of broiler units (Taylor and others, 2008).
addition, 75% of pig farms had resistant Campylobacter species yet only
38% and 18% of turkey and broiler units respectively.
On the face of it this looks very serious, but they
used a very sensitive method of culturing the bacteria, which selected for
resistant isolates. They included a fluoroquinolone in the culture media to only
allow resistant isolates to grow.
In proportionate terms between
susceptible and resistant strains, this was very variable from 0.0079-53% of pig
E. coli isolates, which were resistant and <10-100% of
Campylobacter isolates. This shows that fluoroquinolones do select for
resistance in gut contents when used by injection and orally.
usually metabolised and excreted via the liver and bile into the gut, so that
these bacteria are exposed. E. coli act as good 'indicator' bacteria for
use and exposure to an antimicrobial but the proportion in normal flora is
usually very low, unless recently treated but this data is not shown.
By normal non-selective cultural methods, recent
UK pig isolates of Salmonella have all been free of resistance to
fluoroquinolones. It must be remembered that fluoroquinolones have been used for
over 15 years, so the number of farms with resistant isolates is high, but the
overall incidence is relatively low.
Campylobacter coli, the
major species found in the pig can develop resistance quite quickly, as it is a
one-step resistance mutation, hence the higher percentage of resistance, in
comparison with E. coli, which has a two-step resistance development.
Fortunately, as pointed out in the paper, 'pig strains have significantly lower
overlap with human-derived strains of Campylobacter than poultry
strains'. They usually die out during meat chilling and storage.
the risk of resistance transfer to man appears relatively small from the use of
antimicrobials in pigs, we must not become complacent and must continue to use
them responsibly to maintain their availability and efficacy in veterinary and
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