The risk of farmers inadvertently contaminating cattle with the common horse drug phenylbutazone through use of shared buckets is very real, research has shown.
Farmers are being urged to take extreme care when using phenylbutazone to treat horses because of the risk of cross-contaminating cattle, leaving the animals with illegal residues of the non-steroidal anti-inflammatory drug that would make it illegal for them to enter the food chain.
Investigative work by scientists at Northern Ireland’s Agri-Food and Biosciences Institute established that cattle sharing a “dirty” bucket can show residues of the drug in their blood some 3500 times greater than the lowest amount detectable using the agency’s method of analysis.
A series of studies undertaken at the institute focused on phenylbutazone, commonly known as bute, in its powder form.
Bute is an inexpensive, yet effective treatment for inflammation and pain in horses. It can be injected intravenously or given orally as a powder or paste to horses.
It is not authorised for use in any animal, including horses, destined for the human food chain.
Despite this, official statistics provided by the European Food Safety Authority show that around 0.1% of cattle tested in the European Union in 2014 had detectable bute residues.
Horses treated with bute must have their European equine passports signed to declare that the animal is not intended for human consumption. This is an irreversible decision.
In common with the EU results, testing for bute in cattle in Northern Ireland has, over the years, identified a small number of animals with detectable residues of the drug.
The institute’s Dr Steven Crooks explained that, following on from several positive findings in cattle in Northern Ireland, there was anecdotal evidence to suggest that, at least in some cases, the cattle may not have been illegally treated with the drug.
In these cases, non-compliance may have arisen through contamination as a result of the legal treatment of horses on the farms.
Based on this evidence, several studies were carried out by institute scientists to determine the likelihood that cross-contamination could be at the heart of at least some of the problem.
Researchers investigated the possibility of illegal residues in cattle arising through three potential routes: a shared pen, contaminated pasture, or a shared bucket – that is, if a horse was fed from a bucket containing bute and then the same bucket was used to feed cattle.
In all cases, results clearly showed that contamination could in fact play a significant role.
For example, cattle sharing the “dirty” bucket showed residues of the drug in their blood some 3500 times greater than the lowest amount detectable using the institute’s analysis method.
Similarly, cattle sharing a pen with a treated animal, in this case an illegally treated bullock, showed detectable concentrations within 24 hours of being penned together.
In the final study, a number of animals were treated with bute over the winter period. The manure and bedding from these animals was spread on to pasture in early spring and untreated cattle allowed to graze the pasture some 10 weeks later. Subsequent analysis of blood from these grazing animals showed that all contained significant concentrations of bute.
Given that bute is often the drug of choice for horses and that many farmers do keep some horses, it was important that those using the drug take extreme care to avoid contamination of their cattle.
Crooks explained that while the therapeutic dose of the drug was high with, for example, a 450kg horse receiving as much as 4 grams of bute on the first day of treatment, a 130,000 times smaller amount (30 μg) of the drug can give rise to detectable residues in the blood of a 500kg bullock.
As such, the institute urged farmers to take extreme care to avoid any form of contamination of cattle as this will result in detectable residues.