Drench resistant worms in sheep – advice, old & new, good & bad

Sheep, farming, husbandry, disease, animal health, worms, internal parasites, advice, old & new, good & bad

By Dr Clive Dalton

What's wrong with what we used to do? And what needs to change?

Good old advice

• Read the label on the drench pack to make sure that you bought the right product and you understand what the product will do. The label should also tell you the withholding period for meat. This is now more important than ever with so much concern over food safety and traceback from markets and consumers.
• Give the container a good shake before loading the drench gun back-pack, and put what you don’t use back into the bulk container.
• Check the drench gun is working properly and delivering the correct dose.
• Squirt ten doses into a measuring jug and check the volume for accuracy.
• Pressure check the gun by filling the barrel fully with drench, seal the nozzle with a finger or thumb and attempt to depress the plunger. It should not move. If it depresses then drench is escaping back past the seals and the gun is defective. Some animal health labs now offer a calibration service.
• Give the correct dose rate to each animal. This is based on live weight so ‘drench to the heaviest’ animal in the mob. So if the heaviest gets its correct dose, then all the others will be guaranteed to have been given enough.
• If there is a wide range of weights in the mob, split them into two and this will save you wasting drench on massively overdosing the lighter ones.
• Use a good drenching technique. Gently place the gun over the back of the tongue and depress gently so all the drench goes down the throat and swallowed and not spat out. You must avoid drench getting into the lungs.
• Clean all drenching gear after use following the manufacturer’s instructions to prevent the washer on the plunger drying out and perishing.
• For drenches delivered by injection, make sure you give them correctly. This should be ‘subcutaneous’ which is under the skin and give them in the neck (the cheap end of the carcass in case abscesses form).
• Do not inject wet or dirty sheep as the injection sites may become infected.
• Take special care when dosing with capsules or boluses to make sure the sheep’s head is in the correct position. It should be level and not pulled upwards. Check with your vet for directions and assistance.
• Key point: Feed stock well and keep the stress levels down. It may be old advice but is still valid.
  

Bad old advice
Here are some things we now know NOT to do:

• Put stock on what you think is clean pasture immediately after drenching.
• Starve them before drenching to reduce gut fill.
• Make sure you drench every animal in the mob and don’t miss any.
• Use a different drench chemical family every year.
• Have a separate lamb block where only lambs graze.
• Drench ewes regularly or use low-dose long-acting products.
• Drench ewes especially before lambing and at docking to stop them polluting pasture for the lambs.
• If the drench used does not appear to be working – drench more frequently.
  

Good new advice

• Urgently find out the drench resistance status of the sheep on your farm using a FECRT. Seek veterinary advice on how to arrange this.
• Use this knowledge to fill in the Farm Resistance Summary (see later blog) and carry it with you when you buy your next drench.
• Realise the limitations of FEC testing and use the Faecal Consistency Score (FCS) to improve the accuracy of decisions.
• Stop drenching by the calendar and don’t drench stock unless you are sure there is a problem, and that the drench used will fix it.
• Discuss your intended drenching programme with a veterinarian or farm consultant, regardless of where you purchase your products.
• Fully use the concept of refugia.
• Don’t expect rotational grazing on 3-6 week intervals to be effective, as larvae can be viable for 4-6 months or longer. But some spelling of the pasture is better than none.
• Avoid drenching mature stock and especially ewes before lambing unless there are major problems which will compromise animal welfare. Many farmers find this especially difficult to accept.
• Use short-acting drenches and not long-acting ones.
• After drenching, put stock back on to a dirty paddock first and then 4-5 days later put them on a clean paddock. Then any resistant eggs will hatch into larvae and mix with susceptible larvae preventing the resistant ones enjoying a reproductive advantage.
• Use drenches (in double or triple combinations) to cover all the worm species that are killed by the products. But get professional advice before you do this. As a general rule use double combinations in preference to triples as you need to keep the triples as a very last resort.
• Consider using a long-acting ‘exit drench’. This is where you use one family of drench, and then after at least 30 days, use another drench family to kill any worms left that were resistant to the first family used. This is because long-acting drenches are reputed to be a major cause of advancing drench resistance (currently only valid for moxidectin).
• Use ‘integrated management’ - which means do everything you can think of.
• Use FECs over time to show which parts of the farm are heavily contaminated with worm eggs, so you can avoid grazing stock on them at critical times, or graze them with different species of animals.
• Start putting maximum emphasis on breeding sheep for improved host resistance.
  

Use ‘quarantine drenching’ for purchased stock
There has been a major change in thinking here. The old recommendation was to drench everything that came on to the farm (with a double or triple active) as soon as they got off the truck, and keep them isolated for a few weeks. This is not ‘best practice’ any more.

Some current recommendations:


1. When the previous property status is unknown.

• Drench with a triple-active.
• Put on highly contaminated pasture for at least 3 weeks.
  

2. When the previous property status is worse.

• Avoid buying stock from them.
• Drench twice at 12-24 hour intervals with triple active after arrival.
• Put on highly contaminated pasture for at least 3 weeks.
  

When the previous property status is better.

• Don’t drench sheep on arrival.
• Tell the vendor not to drench on departure.
• You need their susceptible worms to breed with yours to provide maximum contamination of your pastures!
  

Major snag

• This is good advice but there’s one major snag.
• How do you find out the drench resistant status of the farm the stock are coming from?
• Few vendors at present would have any idea of this, and even if they did, would they be willing to tell you? So the safest option is to assume their property is worse than yours.
  

Purchased rams

• Ask your ram supplier to avoid quarantine drenching your rams before delivery, and don’t automatically do them on arrival.
• Instead, do a FEC/FCS on them and don’t use any with more than 500 epg and producing marbles or hand grenades.
  
• But for accurate results, rams should not have been conventionally drenched for 2 months. And how can you rely on this being done?
• See blog on dags.
  

A Fin cross pet sheep called 'Finnie" that lived for 10 years and had a daggy rear
end all her life. She dried up for a while after drenching before scouring again
She was a 'genetic dagger'!

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.