Zolvix cases prompt practical messages

While it hasn’t been directly involved in the investigation of the cases, on goat farms, albeit one with sheep too – technical services manager Australasia, Justin Bailey – told Rural News it appreciates the collaborative approach taken by the organisations that have.

“We are working with them to better understand what has occurred and why.”

He points out farms where the resistance has been confirmed were using monepantel off-label; as it is not approved for use in goats. Also, the reason they were doing this was because worms on the farms had already developed resistance to all other broad spectrum actives.

“It is important to note that monepantel is not registered, recommended or promoted for use in goats in NZ and there are no recorded cases of resistance to monepantel having developed in sheep,” Bailey says.

Nor is the firm aware of any reports of reduced or failed control of internal parasites with Zolvix in sheep treated with the product, he adds.

“The most important message for New Zealand sheep producers, and one clearly highlighted by these cases, is to use monepantel now in rotation with any other fully, effective drench options that they may still have.”

Bailey stresses the “now” part of that message.

“It makes no sense to wait until every other drench has failed on farm, because continual use of any one product or active has been identified as a key driver of drench resistance.”

The resistance cases haven’t changed Novartis Animal Health’s advice on the use of monepantel.

“This approach has been well validated with computer modelling as likely to extend the useful life not only of monepantel, but of all other actives used within the rotation.”

Novartis Animal Health does have a monepantel combination product in the late stages of development and aims to bring that to market as soon as possible.

Bailey says it is too early in the investigation of the goat cases to comment on the mechanism the parasites have developed to become resistant and whether it was, in fact, as rapid as it appears at first glance – given what’s now known about the management practices in place.

A paper accepted for publication in Veterinary Parasitology by Massey University’s Ian Scott et al, details the first confirmed case of resistance.

Zolvix was administered on 17 separate occasions over two years to various stock classes on a 3ha lower, North Island property carrying 20 breeding goats and their kids, and 6-10 breeding ewes and their lambs.

A problem was suspected when one adult goat became unwell and was found to have an egg count of 5500 eggs per gram (epg); despite having been treated with monepantel at one-and-a-half times the sheep dose a week previously.

Tests of other goats treated found them to be shedding an average 2700 epg of faeces and a number of them were also off colour

Subsequent tests on lambs and goats found monepantel “ineffective against at least two gastrointestinal nematode species [on that farm], Teladorsagia circumcincta and Trichostrongylus colubriformis,” states the paper. That’s despite the lambs having been treated with 2.9mg/kg liveweight, higher than the recommended 2.5mg/kg rate, and some of the goats with 7.7mg/kg – over three times the recommended rate on sheep.