An antibiotic used to treat horses with life-threatening conditions is to be withdrawn from the equestrian market due to a new EU ruling.

The drug, Metronidazole, is produced by the firm Pfizer. It is used to treat humans and other animal species as well as horses. Equine vets use it to combat severe chest infections (pleuropneumonia, which can occur in competition horses after travelling) and serious abdominal infections (peritonitis).

From November, Pfizer will no longer be allowed to produce and market the drug for horses in this country. This is because no safe level (maximum residue limit or MRL) of the drug has been set in meat since it is considered carcinogenic.

Metronidazole sits with a list of 10 other drugs with no MRL in category Annex IV of Regulation 2377/90/EC. Annex IV drugs cannot be given to food-producing animals — but from 1 November none of the drugs can be marketed to treat non-food producing horses either.

“It’s illogical that the drug can’t even be used in a horse that is never going to be for human consumption,” said Dr Alistair Barr, former president of the British Equine Veterinary Association. “It is a one-off — there isn’t any drug that’s directly comparable.”

Pfizer markets Metronidazole for horses as Metronex in the UK. The firm is concerned by the loss of this drug but will not undertake research to establish an MRL.

“There is no point in running toxicology studies on Metronidazole because a carcinogenic substance will never get an MRL,” said Edward Ferguson, regulatory manager for animal health at Pfizer. “It has left us very frustrated.”

“It would require a lot of work to prove Metronidazole was safe — despite the fact that it is licensed for use in humans,” added Heather Oliver, head of policy development at the Veterinary Medicines Directorate, which executes the legislation in Britain.

“The UK argued strongly that this is illogical, but had no support and lost. Vets use it with a number of antibiotics, so they couldn’t pinpoint exactly what it does or give us any figures [to back up the argument].

“But it is still authorised for use in humans, so a vet can use the human medicine for horses if they are declared to be non-food producing animals.”

Alistair Barr added: “The human form has not been tested on horses, and may be a different concentration, requiring larger quantities.”

  • This news story was first published in Horse & Hound (7 April, ’05)