Over the next two weeks, the Farmers Weekly team unpacks what the drench resistance threat means for the sector, from revisiting farm systems to heeding the sheep farmers whose experience could offer a solution.
Scientists and vets have added their voices to concerns raised by AgResearch parasitologist Dave Leathwick about triple resistance to drench treatments emerging in New Zealand’s cattle population.
Leathwick raised his concerns in Farmers Weekly later last year in “Drench time bomb blows” after his research revealed definitive resistance across all three drench classes in the majority of tests done on four farms where resistance had become clinically evident.
Drench efficacy on some of the sampled farms had dropped to only 40-70% for levamisole, a previously effective treatment against cooperia – only 15 years after levamisole was touted as the most effective means of control against the worm in cattle.
Since releasing his work, Leathwick has continued to witness resistance issues throughout the summer.
“One farm we visited only the other day had taken a faecal egg count only 10 days post-drenching and it was already up at 650eggs per gram. The drench simply had not worked,” he said.
Dr Ian Scott, Massey University’s senior lecturer in parasitology, shared Leathwick’s surprise that it had taken this long for triple resistance to arrive.
He also shared Leathwick’s doubts that a solution will come through another chemical active emerging in the near future.
“People say the current level of drench use is not sustainable. It would have been if companies had been coming out with a new active every 10 years.”
The most recent releases in New Zealand have been Zoetis’s Startect in 2010, and Novartis’s Zolvix in 2009, both used for sheep.
“We have had no new drenches for over 10 years, but we had seen lots of new companion animal flea treatments on the market,” Scott said.
He believes research into options for pastoral parasite control tipped over around five years ago when companion animals became a higher profit, higher sales area for animal health companies.
There has also been a concentration of animal health companies down to three main ones, limiting the number of participants willing to take a punt on R&D for a new product aimed at the pastoral livestock sector.
But Scott said he had some sympathy for the companies’ reticence about investing.
He pointed to the huge outlay made into Zolvix and Startect as alternative sheep treatment actives, only to receive a merely moderate level of uptake by farmers.
“So, to ask them [companies] to step up and do the same for cattle, that may be unlikely to happen.”
He likened the emerging resistance problem as being a roller coaster.
“When efficacy drops from 99% to 97% it’s not moving very fast, then it drops to 95% – then it really accelerates.
“But we needed to have been aware of it well before then, back at the 99% stage. But it’s hard as a farmer to try to deal with it then, because it’s not a problem and you have other issues to address that are.”
Leathwick has also placed some of the blame on a sales-focused marketing system that has incentivised the sales of certain drench types, driving the wrong purchase decisions.
Like Leathwick, Scott is not optimistic about the issue being resolved chemically.
Nor is he crossing his fingers for vaccines like the ones that control barber’s pole and lungworm worm, due to complications around how the worms more endemic to NZ livestock function in relation to vaccine antibodies.
“I don’t see it as the end of sheep and cattle farming. The challenge is how can we continue to farm without so much reliance upon drenches?”
He sees concerted efforts to breed livestock more tolerant to parasite loads as a more sustainable pathway.
For farmers already struck with resistance in cattle the way back is a long one, but possible.
“Some evidence suggests it is not irreversible, but it could take many years, and take longer than it did to get to where we are today.”
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