It’s one of those days again. Veterinarian Ross Loveridge is waiting at the gate, while Waikato farmer Robert Horseley is bringing all his lambs together.

Doubt has been thrown on New Zealand farmers' use of worm drench. Photo: Tim Pokorny/Flickr

In the vet’s hand is something that looks like a gun.

One by one, Loveridge opens the sheep’s little mouths, points the gun, and pulls the trigger.

The lamb chews and licks and swallows  the white paste and bleats. Then it runs away. For now.

But in four to six weeks, it needs to be wormed again.

It is common practice in New Zealand to drench lambs this often. But some scientists and international experts are warning this frequency makes parasites resistant to the treatment.

Martin K Nielsen is an Assistant Professor for parasitology at the department of veterinary science of the University of Kentucky. Before he went to do research in the United States, Nielsen worked in the same field at the University of Copenhagen in Denmark.

Nielsen says Denmark introduced laws to stop preventative drenching in 1999 and only treats animals on a case-by-case basis. He is very critical of New Zealand’s approach to worming.

“I can guarantee that you can go to any sheep farm in New Zealand and find resistant worms,” says Nielsen.

While New Zealand studies don’t completely confirm this, a 2006 result that 74 per cent have resistance is alarming

Nielsen says the resistance comes from over-treatment for decades and if this approach continues, new medication will also face the same problems in just a few years’ time.

In contrast, professor for veterinary parasitology at Massey University in Palmerston North William Pomroy recommends drenching monthly from when the lambs are about three months olds until they come into weening – at about one year old.

After weening, the sheep should have developed an immunity to parasites.

“Anything over 18 months should generally be able to look after its own. But adult sheep still get parasites from time to time,” Pomroy says. But due to the immunity created by the early treatment, drenching adults on a case-by-case basis is enough.

Sheep resistance
Pomroy says the preventative drenching programme is designed to develop immunity and resistance against worms in the sheep.

In 2006, Beef and Lamb New Zealand established Wormwise as a communicator between farmers, veterinarians and the industry to maintain productivity and sustainability in cattle and sheep.

Wormwise’s key message on its website is that “healthy animals harbour worms and always will – eradication is neither an appropriate goal nor achievable”.

Therefore, the chairman of the Wormwise implementation group, Gary Walton, says animals shouldn’t be wormed preventatively.

“Ultimately, we don’t want to drench healthy sheep. We would recommend that you don’t drench sheep that are not suffering in terms of live weight gain or animal performance.”

But he knows only few farmers follow these principles “religiously”.

“The recommendation of Wormwise is to use combination drenches in a way that gives the best effect and has the lowest chance to develop resistance [in worms] over time.”

This is in line with Nielsen’s recommendations. He remembers a study among veterinarians in Denmark in the 1990s when the resistance of worms in horses was surveyed.

“The situation was similar with nowadays in other countries,” Nielsen says.

As a result, Denmark introduced legislation in 1999 allowing only prescription-based drenching, and only after a positive test for worms.

In 2009, Nielsen and five international colleagues conducted a comparative study between Denmark and Germany to assess differences in worming procedures. In the abstract to the yet unpublished study, Nielsen and his colleagues find that a higher rate of diagnostics has resulted in less use of medication in Denmark.

But he says changing the minds of Danish vets was a long process. A lot of communication with veterinarians across the country was necessary, but it helped that Denmark is small and only has one veterinary university. This way, it was easy to spread information among students in an early stage of their education.

But with a population of 6 million, Denmark is actually bigger than New Zealand. And Massey University in Palmerston North is New Zealand’s only university offering veterinarian education.

So with a similar situation in New Zealand, what’s the problem?

NZ situation
Well, Pomroy says Nielsen does not understand the situation in New Zealand. Preventative drenching has proven successful since it was first recommended in 1982.

And Loveridge says the issue is not with educating veterinarians. He says it is a matter of educating farmers, breaking down ancient habits and building the infrastructure for a more sustainable culture of worming. He recommends egg counts before drenching.

But doing egg counts means the farmer needs to collect samples of faeces from his sheep, bring them into a laboratory, and then wait some days until the results come back. And that is simply a much bigger undertaking, than deciding to drench right away.

“Apart from drench checks or resistance testing when we do it on the farm ourselves – but that’s usually only when they have some sort of a problem – to get them to bring in a sample before every routine drench has often been just about impossible to establish.

“I’ve had probably 20 or 30 years of trying.”

Loveridge says the few farmers who actually do the egg count often have testing facilities on their property. But most can’t afford that.

And Wormwise spokesperson Trevor Cook disagrees. He says it is not only the farmers not wanting to go through the long procedure of testing, but he says standardised testing is “not useful”.

“It’s silly because every farm has a different situation. I wouldn’t agree with that at all.”

He says farms in New Zealand need individual plans that take their situation into account. There is not a single approach; there are numerous means that can be combined depending on the need.

Cook says the biggest help is refugia.

Treating part of the population
Refugia is an approach in which farmers only worm a part of their sheep population. This helps reduce the number of drenches both per sheep and per worm. Parasites are less likely to develop a resistance if they are not confronted with the same medication too often. Adding to this, drench-resistant worms breeding with susceptible ones slows down the resistance process.

Another measure is the use of combination drenches. These offer a mixture of any the established three active components – ivermectin, levamisole, and albendazole.

Also, once the sheep have grown up and developed their immunity over the parasites, drenching can be minimised.

And farmers need to make sure they keep new resistant parasites off their land. Cook says new sheep need to be in quarantine before they mix with old stock.

But he says individually, all of these measures are easy to be introduced on farms.

“The biggest difficulty is to get farmers to make a plan on how they apply these principles to support a level of sustainability. There is only a very small percentage who have a valid worm management system in place.”

He says communication is crucial. And in terms of Wormwise, he estimates that about 50 per cent of New Zealand sheep farmers will have heard about the implementation group, but only about 15 per cent apply a lot of its principles.

Communication the key
Communication truly seems to be the key but the question is what to communicate? And to whom? There is a huge number of options to optimise drenching, but every involved party recommends on something different and condemns other ways.

And after all, there are no governmental guidelines. A spokesperson for the Ministry of Agriculture and Forestry, Lisa Gibbison, says: “Healthcare and things like that isn’t what we look after.”

However, Loveridge says if the regulation works in Europe, maybe that would also solve some problems in New Zealand.

If nothing happens, he assumes veterinarians will continue to drench the sheep of farmers like Robert Horseley preventatively – every four to six weeks.

Loveridge himself has given up the battle. He sold his practice and will retire soon.