Drench resistance – who cares?

Every time you drench, you increase your chance of developing resistance. High frequency dosing is not your friend. It exposes your resident worms to drench chemicals more often - with no time in between for ‘good’ worms to come back into the mix and slow the slide towards resistance. Frequent drenching speeds the build-up of resistant worms in ever-increasing numbers. If you need very frequent drenching to keep animals healthy, ask – what is wrong or broken in my system? 

• Never drench young stock more often than every 28 days, unless it’s a short-term measure and animal welfare is at stake. 
• Avoid drenching adult animals unless it’s a short-term measure and animal welfare is at stake.  
• Manage parasites in adults through good nutrition and body condition management. In sheep, worm resistant genetics will help reduce adult ewe drenching. 

Long-acting drench products are similar in effect to drenching too often. They linger in your animals, exposing worms to drench chemicals for extended durations. Persistence fuels resistance.     

• Use long-acting drenches cautiously. 
• Avoid whole-mob treatment of adults.   
• Look for other ways to reduce parasite intake for young stock to reduce dependence on long acting products. 

You’ve heard the phrase what doesn’t kill us makes us stronger, right? Same for worms. A sub-lethal drench dose skews your worm population further in favour of those with resistance.  

• Calibrate those drench guns. Watch this video with Andrew Dowling to learn more. 
• Weigh animals before treatment. Drench to the heaviest. Never under-dose, accidentally or deliberately. See this video on drenching techniques. 

Have you ever drenched animals, then shifted them straight onto ‘clean’ paddocks? Resistant parasites love this. It means the only worm eggs deposited into these paddocks are drench survivors. You’ve just further concentrated the resistance gene pool.   

• Avoid* drench and shift onto ‘clean’ areas: 
• New grass 
• Long-term crops (e.g lucerne, plantain)** 
• Pasture that has been grazed only by another species for 2+ months 
• Pasture that has regrown after an extended dry period 
• Hay aftermath.  

*If the first graze of these areas has to be with treated lambs/calves, follow them with untreated adult stock. 

*Drenching onto short term crops that leave largely bare ground and are replaced with another forage in the same season are much lower risk. Few larvae survive on open and cultivated ground. 

Also see multi-year crops and newly-sown grasses. 

Drench resistant worms like to travel on stock trucks, and often do. It’s easier to import a problem than many people realise.  

• Assume newly bought-in animals are carrying resistant worms. Have a comprehensive Quarantine Drench Protocol.
• Seek up-to-date drench testing information from the vendor – a quarantine may not be necessary if they have a good drench efficacy status. 

Worms adapt more easily to one chemical compound at a time, because resistance to each compound involves a different gene or set of genes. The more compounds, the harder and faster their genetics must evolve to survive.  

• Don’t rely on single active drenches, especially in young stock. But don’t over-rely on combinations either. In the right conditions, worms become resistant to these quite quickly, too.  

If your routine drench is leaving worms behind, an increasing number of worm eggs can be ‘leaking’ between drenches and building a resistant worm population on your pastures. Use drenches that you have tested on your own farm, that are not ‘leaking’ resistant worms. 

In an easy world, we could accurately check drench efficacy without it getting messy! But that’s not possible.  

So we have to go to the source – the hind end of your animals. What comes out of there before and after a drench can reveal a lot.  

Faecal tests are by no means infallible. But they’re the best available tool to learn if drenches are still working. If you’re not testing, you might as well drench with a blindfold on.  

Yes, the results may challenge you. Deep down, though, wouldn’t you rather know if resistance is attacking your business by stealth?  

A Faecal Egg Count (FEC) measures the number of worm eggs in your animals’ poo. If a drench has been effective, the egg laying adult worms will be gone, and there’ll be no eggs in their poo from a few days after treatment. 

Here’s a video on how to collect samples.

You can use post-drench FEC checks as a basic way to determine if your drenching is working. Simply take fresh faecal samples 7 to 14 days after drenching, and see how many eggs are present. 

No eggs? The drench did what it was supposed to do.  

Eggs in post drench samples aren’t always there because of drench resistance.  

Other reasons could be: animals that missed a drench, spat drench back, or were under-dosed. There could have been problems with the dosing equipment.  

Things to check if there are eggs present after drenching: 

• Were animals missed or spat drench out? This can often show up as one or two high FECs when the rest are zero.  
• A larval culture.  A larval culture can show us which worm species the eggs have come from. With drench resistance you’ll usually just 1 or 2 species in the culture. With a drenching gear failure or human error, you’ll often see several species. 
• Were animals underdosed? Check back on exactly what was done on the day. Read the product label and check animal weights – were they dosed to the heaviest? The average? Was weight measured or guessed? 
• Check the dosing equipment. Was the drench gun checked and calibrated? See the Tech tips on sheep drenching video for how to do it! Note, syringes are more accurate for measuring the volume delivered than many measuring cylinders (unless you have proper calibrated ones from a high school/university or a laboratory supplier).  school/university or a laboratory supplier). 
• Were the treated animals normal? Young animals affected by very high numbers of worms or with compromised health may not metabolise drench as well as healthy animals and occasionally some worms may get left behind.  

Say hello to the Faecal Egg Count Reduction Test (FECRT). This bad boy is the boss of drench testing. It’s more complex than a FEC. But that extra time and effort gives more precision, and it only needs to be done every few years.   

Watch this video on testing for worm burden in sheep.

The FECRT is a before and after test based on faecal samples taken from individual animals. A trained technician or vet collects poo from each animal’s rectum, then each is weighed, drenched to the exact dose and recorded.* 

 Seven to 14 days later, those same animals are sampled again. Now we have two lots of egg counts to compare from each drenched animal. If your worms are drench resistant, they can’t easily hide from this test.  

And that’s a good thing! As soon as you discover their presence, you can get to work limiting their impact.  

In a ‘full faecal egg count reduction test’ the main drench families are tested individually, and often some combinations too. 

In a ‘mini faecal egg count reduction test’, one or two of the drench products in common use on the farm are tested. 

*There may be reasons to modify this protocol based on time and cost. Link to agreed best practice principles based on WAAVP review.