Wormer resistance in NZ is no concern for UK yet

Widespread anthelmintic resistance in cattle may be causing problems for farmers in southern hemisphere countries, but UK farmers should not be overly concerned because few cases have been reported here so far.


According to a report in the New Zealand Dairy Exporter, the initial results of a survey in New Zealand suggest that Ivermectin failed to control worms in 92% of beef cattle, while benzimidazole products failed in 74% and a combination of both treatments failed in 72%.


The survey is still being completed and full results are expected soon.


Frank Jackson of the Moredun Research Institute says the results are an indication of how resistance could develop in UK herds, but he believes New Zealand farming practices are likely to have accelerated the development of resistant worms.


“The longer growing season, together with irrigated pastures and high stocking densities mean cattle are wormed more frequently in New Zealand than on UK farms,” he says.


“Increased worming frequency will have led to greater selection pressure on worms, raising the chances of resistant worms surviving.”


The contract rearing operations found in New Zealand could also be partly to blame, he adds.


“Many cattle are reared on contract units where the operator is paid by the head for rearing cattle to a set weight.


This means worming is carried out routinely to ensure growth is uninhibited by parasites, increasing selection pressure on the worm population and accelerating the build-up of resistance.”


Mr Jackson says cattle producers could consider following the lead of their sheep-farming counterparts and rotate wormers to avoid using the same active ingredient for two years running.


“The most important thing is to be aware of how resistance has developed in the sheep industry and plan ahead to assess the likely options.”


Meanwhile, Gerald Coles of Bristol Vet School says implementing a strict quarantine strategy for incoming cattle is one of the best ways of slowing the development of resistance.


“Treating bought-in cattle with both a benzimidazole and levamisole product on arrival should ensure any resistant worms are eliminated before cattle are mixed with resident animals.”


But while agreeing that resistance is not currently a problem in the cattle sector, Mr Coles says failing to recognise the threat it poses could lead to it developing more rapidly.


“There is little doubt resistance will occur, so we must do everything we can to delay it as long as possible.”


Andy Forbes of Merial Animal Health believes ensuring animals are given the correct dosage could help delay resistance, because animals will be neither over- nor under-dosed, both of which can encourage resistant worms to survive treatment.


Independent vet consultant Tony Andrews says the best measure most cattle farmers can employ is to worm cattle only when they need it.


“Thankfully, few cattle are wormed as frequently as sheep have tended to be, so selection pressure on worms has been lower.


“But to protect against resistance developing, farmers should undertake faecal egg counts to assess whether cattle need treating.


Tests can be done reasonably cheaply these days and a test that shows no need to worm cattle will be much cheaper than simply treating a group of animals which you think need worming.”


Looking ahead, Dr Jackson says researchers at Moredun’s Edinburgh facility are hopeful of developing vaccines to counter internal parasites.


“Progress is being made and a number of useful antigens have been found, but producing a vaccine is a complex process.


While vaccines are a future option, progress is not being made as quickly as many would like.”


jonathan.long@rbi.co.uk