13 March 1998

Resistance in cattle worms found in UK

ANTHELMINTIC resistance has been found in cattle worms for the first time on a farm in the UK, reports a letter in the Veterinary Record (p255, vol 142, number 10, Mar 7, 1998).

It is also believed to be the first case in the northern hemisphere where a group three anthelmintic (avermectins/milbemycins) has failed to control the intestinal worm cooperia, according to report author, Gerald Coles of Bristol University.

"Usually ivermectin is 95% effective against adult cooperia, although in this herd it did not kill any of the adult worms," Dr Coles told farmers weekly.

He explains in his letter to the Vet Record that he has been surveying worm control strategies used by Somerset dairy farmers. His aim has been to investigate whether there is any sign in this country of anthelmintic resistance developing in cattle nematodes as has been the case in New Zealand. He found ivermectin resistant cooperia on one of the farms.

Cultures from two calves, showing only a 65% reduction in worm egg counts after treatment with ivermectin, were used to infect eight worm-free young Friesian male calves, writes Dr Coles in his letter. On day 28, the calves were split into two groups based on faecal egg counts. One group of four was weighed and treated with ivermectin at the recommended dose, and at postmortem examination on day 35 the faecal egg count had reduced by 44%. However, there was no significant reduction in worm numbers as determined by worm counts in a 10% sample from the small intestine from each calf, writes Dr Coles.

"We believe this to be the first overt failure of any anthelmintic group in cattle and the first case of resistance to group three anthelmintics in the northern hemisphere.

"In view of our finding there is clearly a need for further research into the development of anthelmintic resistance in bovine nematodes in the UK and the optimal management strategies to slow the development and spread of resistant worms," says the letter.

Cattle worms resistant to ivermectin found for the first time in the UK.