'I’ve seen farms where all drenches are failing': How to respond to resistance

11 December 2023
It is  unlikely there will be a new drench hit the market for the next 10 years, which is why it is critical that farmers have good parasite practices in place. Pic: Agrishots
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Rain across the East Coast of Australia has been welcomed by sheep farmers, but with the moisture comes a very unwelcome guest – parasites.

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Rain, followed by days of warm weather are ideal for proliferation of parasitic worms like Barber’s Pole.
Resistant worms have become increasingly problematic and are now being detected in areas they've never been before due to insufficient induction processes following transport.
According to Dr Timothy Elliott, Livestock Technical Product Manager – Sheep at Virbac Australia, one of the most commonly used drenches, Abamectin, no longer provides the efficacy it once did.
“Abamectin is priced very economically and as such has been in widespread use since the 1990s,” Dr Elliott said. “But it’s failing big time. We likely won’t see a new drench hit the market for the next 10 years, which is why it is critical that farmers have good parasite practices in place to help protect the drenches we have.”
Dr Elliott warned that many farmers are still drenching under the assumption that the older drenches are still effective. Unfortunately, that is no longer the case, with 50-70% efficacy now regularly reported, well below the 95% or above that is needed for a drench to be classified as effective.
“Resistance is getting to be a significant problem,” Dr Elliott said. “I’ve seen some farms where all drenches are failing.”
It adds up to be an unnecessary and ineffective expense, including the cost of the drench itself at 30c - $1.20 per animal, the time spent mustering a mob, and the production drop due to stressed animals. 
“A lot of the cheaper drenches are no longer working due to being on the market for 20+ years which makes them seem cheap but will actually cost the farmer money as the most expensive drench, is the one that doesn’t work,” Dr Elliott said.
For a worm like Barber’s Pole that is often drench-resistant, that also means lower rates of growth and lower wool production. 
In worst cases, as many as 40% of the flock may not survive.
“Death is the tip of the iceberg. Because Barber’s Pole is a bloodsucking worm, the animal is anaemic. In an outbreak you’ll see sheep really lag behind or fall down during a muster. On the animal itself, it is associated with bottle jaw and pale gums.”
Prevention key

The most important thing a farmer can do is to ensure parasites don’t get onto the property in the first place.
Failing to do so can have drastic consequences, Dr Elliott said, citing a recent case he had seen.
“A farm with an effective drenching practice had taken delivery of six rams. Those rams were given an ineffective quarantine drench, and they all had highly resistant Barber’s Pole. The rams were joined with the ewes, who picked up the parasite, and their lambs picked them up too. Within 12 months, those multi-resistant Barber’s Pole were all over the farm,” he explained.
TridectinDr Elliott advised that any new shipment of animals (as long as they are in good condition) should go straight down the race and be administered two different drenches, including one with the one of the newer active ingredients (Derquantel in Startect or Monepantel in Zolvix Plus) and a Tridectin.
Then they should be left in the yards with feed and water for up three days to allow any surviving eggs to be passed out in the yards.
Following the yarding, the mob should be released into a pasture previously occupied by the home flock for 14 days before conducting a faecal egg test to ensure the parasites have been cleaned out.
Monitoring is essential
The best approach a farmer can take is to regularly test for faecal egg count (FEC), particularly after rain, when the moisture causes the eggs to hatch into larvae which is the infectious stage of the worm. 
It costs around $50, and good operators in high rainfall areas conduct them monthly, allowing them to build a trend model.
Unfortunately, less than 20% of farmers do FECs as a montoring tool, Dr Elliott said. Even less – a few per cent – have performed and Faecal Egg Count Reduction Test (FECRT) test to see if the drenches they are using are effective.
A FECRT, also known as drench resistance test, can help a farmer determine what drenches work on their farm. This is done by selecting a mob of young sheep that are known to have worms and treating 12 animals with a certain drench, then another 12 with a different drench and so on, resulting in multiple drenches tested against a control group of 12 untreated animals. After 14 days, a collection of individual faecal samples from the animals for a new Faecal Egg Count will allow for efficacy to be calculated. 
Another way that efficacy of a drench can be checked on a mob basis is by taking a mob sample on the day the animals are treated (day 0) and then 14 days later.
Counting the remaining eggs will allow the drench efficacy to be calculated.
For example, if a mob of sheep has 1000 eggs in a gram of faecal sample at day 0, then 500 eggs in a gram of faeces at day 14, the drench is 50% effective.
Companies like Virbac can assist with performing a FECRT on farm so contact your local area representative for details.
If a paddock has had animals with a very high worm burdens, then the paddock is more than likely contaminated with a large number of worm eggs and larvae which can infect the sheep that graze the paddock.  Dr Elliott advised spelling the paddock for a couple of weeks or months to allow the worms to die on the paddock. “Without a host they cannot survive, especially in hot weather,” he said.
Alternatively, the paddock can be grazed with adult cattle, as most sheep parasites cannot infect adult cattle. That can reduce the overall larval contamination on the pasture.  

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