Health & Welfare

Guidelines for Trying to Reduce Resistance to Worms

I know farmers get fed up about vets going on about this but it is very important, as resistance to the wormers which we have relied on for over three decades is becoming a serious problem.

There are various strategies you can employ in controlling the most economically important group of internal parasites, the nematodes (worms); these are parasites that inhabit the gastrointestinal tract and lungs, causing paratitic gastroenteritis (PGE) and parasitic bronchitis. In the UK there are a number of species of nematode that are responsible for PGE, the symptoms of which you will all be aware of, including weight loss, poor performance, and diarrhoea. The adults of these species feed on and inflame the lining of the host's gut; developing larval stages also cause damage to the gut mucosa. Some of the species and diseases you will have heard about include Ostertagiosis, commonly seen in the summer and early autumn in lambs and calves; Trichstrongylosis, or black scour, a problem in store and replacement lambs and calves in the autumn; Nematodirus, typically seen in young lambs 6 - 12 weeks of age in the late spring, and occasionally in calves; and lung worm infections seen usually in the autumn, characterised as breathing difficulties and coughing in grazing animals. These are usually less severe in sheep than those seen in cattle, but I have heard of severe clinical disease in sheep due to lungworm. In sheep there is a also a nematode that feeds on blood - Haemonchus. The symptoms caused by this worm include weakness, anaemia (look at the mucus membranes; they will appear pale or even white) and sudden death in lambs. These symptoms can even be seen in adults if pasture contamination is high.

Wormers, on the basis of their chemical structure and mode of action, can be divided into three groups, and as such are labelled one of the following: 1-13Z (white drenches), 2-LM (yellow drenches), 3-AV (clear). Once you have resistance to any one anthelmintic in a group, all members of that group will have reduced efficacy; once you have resistance to all three then you have no drugs available to treat these diseases.

How to reduce the spread of anthelmintic resistant nematodes on your farm

• Make sure you always give a full dose. Obtain accurate weights of animals in the group and ensure that dosing equipment is correctly calibrated to administer the full recommended therapeutic dose of anthelmintic. When using drenches, ensure that the barrel of the gun is filling properly and isn't full of air bubbles and that you place the nozzle of the gun at the back of the tongue to ensure that the entire dose is swallowed into the rumen. Ensure with injectables that you are going intramuscular rather than intra-fleece in sheep. Do not use an oral dosing gun for anything other than its intended purpose; i.e. don't use it to open paint tins, etc., as damage to the end of the dosing gun can have devastating consequences with damage caused to the soft tissue at the back of an animal's throat, leading to septicaemia and even death.
• Introduce regular checks to ensure anthelmintic efficacy is being maintained. Your vet can advise on which tests you need in order to see if you have resistance.
• Draw up a strategic dosing plan with your vet to avoid unnecessary dosing and reduce the frequency of usage. Can you use grazing strategies? Ask about faecal egg count monitoring. Monitoring normally involves collecting ten fresh faecal samples from a group of animals and then sending them to your vet or to a laboratory under direction of your vet.
• Dose all bought in sheep with ivermectin or moxidectin (3-AV or clear) plus levamisole (2-LM or yellow) - i.e. two separate drenches at the same time - and yard for at least 48 hours before introducing to the flock or contaminated pasture.
• If you can reliably establish the resistance status of the farm from where animals are being sourced, or determine that there is no resistance and the sheep will be transported directly from the farm of origin, leave them untreated for as long as possible. (Obviously they should still be isolated before being introduced to the rest of the flock to prevent the introduction of other infectious diseases, ideally for three weeks). The reason for not treating those sheep that come from farms where resistance isn't a problem is that they will introduce worms with genes for susceptibility. Ask your vet for more details. Don't forget bought in rams!
• If resistance is already established on the farm, discontinue the class of anthelmintic in question and inform your vet immediately.

Especially important in cattle is to remember that pasture management control measures are not as effective for lungworm, so consider vaccination.

Printed by kind permission of Stock 1st Veterinary and Livestock Services, www.stock1st.co.uk

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