Drench resistant worms in sheep – drenching to control worms

Sheep, farming, husbandry, animal health, worms, internal parasites, drenches, anthelmintics.

By Dr Clive Dalton

Well, which one are you going to buy? Will the store assistant know enough to help you?
Beware of being told that 'this one's a good seller at the moment'! Or this one has the free jerkin'.

Drenches on the market
It is mind boggling to see the enormous range of products (anthelmintics) on the market to treat internal parasites in sheep.

Anthelmintics are any products that kill internal parasites, and past annual surveys in farming press show the following:

• There were around 40 different brands on the market.
• These were made by 5 companies.
• 15 were sold by vets only, and 16 were available from all outlets.
• Concentration of the chemicals ranged from 1g/Litre to 40g/Litre.
• Ingredient dose rate ranged from 0.2mg/kg LW to 10mg/kg LW.
• Formulated dose rate ranged from 1ml/5kg LW to 1ml/20kg LW.
• Withholding periods for meat ranged from 3 to 91 days.
• Safety margins ranged from 3x the dose to 100x.
• 15 of the products were ovicidal (killed worm eggs) and 16 did not.
• All of them killed both mature and immature parasites.
• 17 products included minerals, 1 had vitamins and 2 included vaccines
  

Ways to deliver

• Products can be given as an oral drench, as an oral bolus or capsule that stays in the rumen, or they can be injected.
• Check the label about the injection method (it’s usually subcutaneous) and for the best site (in the neck).
• There are no straight pouron ‘anthelmintics’ for sheep.
• Other products are called ‘endectocides’ as they kill both internal and external parasites and can be administered by mouth or injection in sheep. Pouron endectocides are not used in sheep.
• Some products are ‘ovicidal’ which means they also kill worm eggs in the sheep, as well as the worms.
• When checking the survey table of drench brands on the market, remember the ‘active ingredient’ column is much more important than the brand.
• And when you buy, don’t let the promotional ‘giveaways’ bias your decision.
  

Key questions:

• Is it any wonder that we get confused when buying drench? Have sympathy for the shop assistant too having to give advice and make recommendations when customers have no knowledge of the subject.
  
• Is it any wonder the customer buys a product with the lowest price and the best promotional give-away?

The drench chemical families

• Only a few years ago we didn’t need to know much about the chemicals used in drenches, but things have now changed.
• The key issue is that all the different products on the market are based on only three chemical ‘action families’ or ‘actives’. Active ingredients have long names that are hard to remember.
• We’ve got to get over this as it’s now very important to be able to ‘talk chemicals’ to your veterinarian or consultant.
• Calling them ‘white’ and ‘clear’ drench families was once helpful, but now not all white drenches are white and not all clear drenches are clear!
• Labeling does not always tell you clearly which family the product belongs to. You have to know your chemicals to find this out with far too many products.
• The printing on the labels is very small to fit in all that legally has to be presented. So it’s hard to read and no company has provided a small cheap lens to help the ‘average’ farmer who at over 50 years old needs some visual assistance!
• Current knowledge is that each chemical family kills worms the same way – thank goodness for that! So if a worm is not killed by one type of white drench then it won’t be killed by any other chemical in the white drench family.
• The unfortunate result of all this is that farmers now need to be more aware of the active ingredients in the specific brands of drench within each chemical family.
• This is now especially important when buying a branded product. You’ll have to recognise which chemical family the drench belongs to and know what species of worms it is expected to kill.
• Use the Table below as a check list to show which active ingredient is in which chemical drench family. You’ll need to check this when buying branded products. The table doesn’t list the chemicals used in double- or triple-combination products – so check the label. Don’t buy the product if you are not sure what’s in it and you cannot find out.
  

Key point:
Once your sheep have drench resistance to some or all three families of actives, flock management and your life will be a lot more complicated.


How anthelmintics work

• After drenching, the active ingredient is absorbed from the gut and circulates through the blood stream throughout the body, and is then resecreted back into the gut to do its work.
• Some active ingredients can also kill worms through direct contact with the parasite in the gut.
• When blood levels of the chemical drop below an ‘active’ level, then the drench loses its power.
• The length of activity of a drench depends on a range of factors related to the formulation and the worm species in the host.
• ‘Persistence’ is the term used to describe how long after administration a chemical in the drench will still work. For example the chemical in control-release capsules lasts a long time so is very persistent.
  

The gut trigger

• This is a clever bit of engineering in ruminants called the ‘oesophageal groove’.
• It is nature’s way of making sure milk when swallowed by the young lamb goes straight into the 4th stomach or abomasum for gastric digestion where it can clot.
• You don’t want milk going into the other three fore-stomachs (rumen, reticulum and omasum) where bacteria and other micro-organisms are developing to digest fibre.
• In the adult animal certain minerals and chemicals can close the groove so the drench will bypass the rumen where you want it to go.
• Large volume drenches are more likely to trigger the groove than low-volume drenches – so low volume products are a feature you should also look for and check with your veterinarian.
  

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.