After a lifetime in the animal health industry working with internal parasite treatments, Colin McKay’s sense of frustration is tangible. Andrew Swallow talked to him about resistance risks and prevention tools.

Despite repeated warnings about resistance risks, most farms still only move to the next level of parasite treatment when the previous one fails.

That is costing dearly in lost production year-on-year and it will increase costs of parasite control in the future. Without change, the very viability of their operations could be at risk, he believes.

“How do we deliver better messages so we can get earlier and better uptake of these tools so that we’re not using them as rescue drenches?” he asks.

From launch in 2002, triple combinations took 10 years to achieve worthwhile sales, despite independent national survey data published in 2006 and widely publicised in 2007 showing resistance to benzimidazoles (“white” drench) and levamisole (“clear” drench) was present on 40% and 25% of farms respectively, and 8% had populations resistant to both.

Now, internal parasites resistant to macrocyclic lactone (ML or “mectin”) drenches are also increasingly widespread and often with resistance to white and clear drenches, ie resistant to triple combination products.

That realisation is finally prompting increased use of newer drench families, the so-called “orange” drenches monepantel (as in Zolvix Plus) and derquantel (as is Startect), but again, it is 10 years after their launch.

Such late uptake, and only after other actives have given way, piles pressure on the new modes of action and increases the likelihood of selecting for populations of parasites resistant to them.

McKay believes part of the problem is “mission fatigue” in getting resistance management messages across to an audience that has tuned-out after 40 years of “white noise” about resistance risks, not to mention past and ongoing mixed messages.

For decades animal health professionals warned drench resistance could end sheep farming as we know it yet, until recently, for most it appeared little had changed.

Now, a “significant number of farms” are finding populations of internal parasites resistant to the three older families of drench on their properties, leaving the newer drenches monepantel (as in Zolvix Plus) and derquantel (as in Startect) as the only fully effective actives.

As such, they’re on very thin ice and unless practices change significantly, it’s probable parasite populations will evolve to survive those too and then productivity will really start to suffer, he warns.

“Is the sky starting to fall?” he asks, rhetorically referring to the tale of Chicken Licken. “Yes, and no.”

He believes a broader understanding of drench use is needed. While most farmers see them as tools to make more money through better stock health and productivity, and McKay says that’s fair enough, a greater appreciation of how they do that is also needed.

“People forget that we use drenches to manage pasture larval contamination as well. The reproductive stage of roundworms is in the animal but the infective stage is on pasture and a really decent drench programme manages both.”

The aim is to minimise intake of infective larvae by at-risk classes of stock, such as lambs, while allowing a low-level parasite population to persist in undrenched and parasite tolerant classes such as ewes.

The latter measure ensures most of the parasite population on a farm isn’t repeatedly exposed to drench treatment, reducing the risk of drench resistant strains coming to dominate. A practical way to do that is to tidy-up pasture previously grazed by drenched lambs with undrenched ewes.

As for how often to drench lambs, long-standing AgResearch work shows strict adherence to 28-day interval drenching is the best approach, and the drench used needs to be fully effective. McKay recalls an Agresearch trial where the parasite population was known to be resistant to albendazole. Lambs drenched with a fully effective programme were worth 14% more, with lower dag scores and faecal egg counts at the end of the programme compared to the albendazole-treated mob, yet visually there was no difference.

“Farmers have to really appreciate they could be losing money through using less than fully effective drenches without them even knowing, so testing is paramount.”

What’s “scary” is that faecal egg count reduction tests (FECRT) by FECPAK on lambs destined for a leading UK supermarket found 37% of farms using an ineffective drench, a figure that “largely marries” with incidence of resistance coming through in data from testing labs such as Gribbles, McKay says.

“That’s crazy … you’re talking about serious amounts of production being lost.”

It’s a situation that’s left him questioning extension systems.

WormWise, launched in 2007 following the 2006 survey, introduced a risk management approach to parasite control, which was a step forward, but as an extension service it has been under-resourced and slow to react to changes happening on farm, McKay says.

For example, it was still recommending use of triple combinations for quarantine drenching in 2017, when strains of parasites resistant to whites, clears and MLs were already becoming widespread.

“So why would you use a triple as your quarantine drench? It was just nuts.”

However, it has since been amended and now recommends quarantine drenching with four actives, including a novel drench, which, with all the other risk management procedures outlined in the latest version of WormWise, is sound advice, he says.

The veterinary profession’s not blameless either, with some clinic newsletters delivering mixed messages and less than sound advice, McKay says, citing examples of only supplying one manufacturer’s products, and giving outdated quarantine advice.

“We seem to have made a bit of a mess of it all I think.”

That said, such is the complexity of understanding and managing resistance risk on-farm a vet is essential, he believes, particularly to tease out subtle differences in product use.

For example, when to use Startect, and when to use Zolvix Plus. Both have a place, but derquantel (as in Startect) isn’t as broadspecturm as monepantel, which was why when derquantel was launched it was in combination with abamectin, whereas Zolvix was initially stand-alone.

Vets can also look at grazing strategies and stock class ratios and pin-point high risk activities.

“All the tools have to be used. We can’t just talk about drenches.”

Drench history

The early 1960s launch of Thibendazole, the “grandfather” of modern broad spectrum drenches, “is regarded as one of the major steps forward for sheep farming in New Zealand, right up there with electric fencing, top-dressing and everything like that,” Elanco’s Colin McKay says.

It all but eliminated the annual problem of hogget ill-thrift and despite a considerable price-tag relative to today’s prices, was widely adopted. In 1979 the first case of resistance was recorded, but by then levamisole, the first clear drench, had been on the market for a decade and, as it turned out, Ivomec, the first of the macrocyclic lactones (ML drenches) was just round the corner, becoming available in 1981.

Before Ivomec’s launch, resistance management advice had been very simple: use one family of drench a year, and alternate year to year. Drums were colour coded – either yellow or blue – to make it even easier. Ivomec, which came in brown drums, was to be used as a quarantine drench.

The first combination products, delivering white and clear actives in a single dose, were launched in 1990 but struggled to gain market share until a combination of resistance and low prices for clear drench levamisole saw straight white products withdrawn.

“No one in the drench supply chain was making any money so they were taken off the market,” McKay recalls.

From the mid 90s, when two more ML drenches came on the market, abamectin and moxidectin, the standard strategy became to use an ML alternating with a BZ (white)/Levamisole (clear) combination product until, in 2006, the national survey (see main story) confirmed growing concerns resistance to white and clear drenches was increasingly widespread.

When Zolvix (monepantel) and Startect (derquantel) were launched in 2009 and 2010 respectively, it had been nearly 30 years since a new family of drenches had been launched.

McKay says that delay reflects changes in structure of the animal health industry. Whereas once there were dozens of companies investing heavily in new product R&D, now there are just a handful of companies left and they’re multi-national so the New Zealand market is just part of their portfolio, rather than the focus it once was for some.

Also, whereas once the companies’ R&D work spanned many sectors, and there were several cases of actives initially developed for crop protection finding a place in animal health, now most companies are dedicated to one sector or the other.

The incentive to search for new actives isn’t what it used to be either because costs of bringing a product to market have soared while profit margins have eroded due to drench prices today being a fraction of what they were.

Consequently, it’s less likely new actives will come to market anytime in the near future, if ever, he says, particularly for sheep as globally they are a minority species in animal health compared to cattle, cats and dogs.

“So using best practice to protect the drenches that still work on your farm is absolutely vital.”