In recent years, a lot of progress has been made in understanding how to control worm burdens in livestock. However, it's a hugely complex issue, not least because of the widespread problem of drench resistance!

This makes it difficult for lifestylers to know how to deal with worms in their stock. There is no easy formula. However, in this article, we summarise some of the principles of worm control to help you assess the situation on your farmlet and to guide the decisions you make.

Can you say any of the following about your farm? They are some of the factors that predispose to worm problems:

  • High stocking rate
  • Forcing stock to graze close to the ground
  • One species only grazing (eg cattle only or sheep only)
  • Young stock
  • Stock in poor body condition eg poor nutrition
  • Stressed stock (eg lack of shelter, concurrent disease eg footrot)
  • Warm humid weather (favours survival of worm eggs and larvae on pasture)

Here are some of the factors that make it less likely that worms will be a problem:

  • Low stocking rate
  • Pasture that is not very short
  • Mixed species grazing, eg lambs grazing with cattle or healthy adult ewes or horses
  • Mature robust well-fed stock
  • Cold frosty weather or very hot drying weather (kills eggs and larvae in pasture)

Here is a reminder of the signs that worms are causing real problems in your animals:

  • Diarrhoea or dags
  • Slow growth rate or weight loss
  • Pot-bellied, anaemic
  • Loss of appetite
  • In severe cases, protein loss leads to bottle-jaw (soft swelling under the jaw)

However by the time worm numbers in the stomach and intestine have become large enough to cause any of these signs, their body condition and growth rate will have been affected.

Get veterinary advice

  • Devising an effective parasite control strategy for your farm is a tricky business so it pays to get the help of an expert veterinarian.
  • Your vet may arrange faecal egg counts to help assess the size of the problem but faecal egg counts aren't always helpful because worms don't always lay lots of eggs, even when there are huge numbers of worms.
  • Your vet can help rule out other conditions and diseases that can cause the same signs as big worm burdens (lush spring grass, some bacterial infections)
  • Most farms have some degree of drench resistance and your vet will help you assess your farm and how to deal with any resistance present.


Note that nowadays the terms "drench" and "drenching" are used in relation to giving anthelmintic (worm treatment) - by injection or by pour-on as well as by mouth.

  • There are now five main types or families of drench, and most commercial drenches for sheep now contain at least two of them.
  • It is really important to read the label and follow instructions carefully with no under-dosing.
  • Healthy adult sheep and cattle don't normally require routine drenching because they develop immunity to worms as they grow older.
  • Generally, the idea is to protect young vulnerable animals with strategic drenching while allowing them to be exposed to a low level of parasites (to help to build their immunity to parasites) without risking big worm burdens.
  • Drenches can be given by various routes (by mouth, by injection or pour-on). However, pour-on drenches should not be used as they encourage drench resistance and are generally less effective than drenches given by mouth or by injection.
  • When drenching a mob, make sure each animal gets an appropriate amount of drench for its size. Under-drenching encourages drench resistance. Weigh each animal or get a 'guesstimate' from an expert. Weighbands can be used to measure the girth of cattle and this can be converted into a weight assessment.
  • There is a withholding period after giving any anthelmintic, as there is for most registered animal remedies, and this will have to be taken into account if milk or meat is to be sold.
  • Note that some oral anthelmintics have a longer duration of effect than others so fewer drenches are needed, with a longer period between drenching.
  • If only a few animals are to be drenched it may be difficult to source small quantities of drench. Some vet clinics will dispense smaller amounts or it may be possible to get a group of farmers together to buy drench to share.

Sheep, goats and cattle

  • Lambs should generally be drenched at 4 to 6-week intervals from weaning around December to May.
  • Generally, there are no specific label instructions for goats so unless your vet advises otherwise, goats should be treated as sheep.
  • However, goats don't develop as much age-related immunity as sheep and cattle so they may require regular drenching every year of their lives.
  • Young cattle may require several drenches during their first summer and autumn.

Drench resistance

  • Worms that are drench resistant have developed resistance to a particular family of anthelmintics, so drenching with those anthelmintics is not effective.
  • Your vet can arrange tests for drench effectiveness (and hence drench resistance).
  • When resistance is present or suspected, drench with a combination of anthelmintics as recommended by your vet.


  • In some situations your vet may recommend drenching only the animals that are in poorer condition so that the more robust but un-drenched animals will shed non-drench resistant parasites onto pasture to dilute the proportion of resistant parasites.
  • This is the concept of 'refugia' (Clive Dalton has an informative article about this elsewhere on this website).

Quarantine incoming stock

  • Quarantine drenching is very important. It involves drenching incoming stock with a triple drench combination (your vet will advise on what is appropriate).
  • The treated animals should be held off pasture or in a quarantine paddock for at least 24 hours.
  • Quarantine drenching is especially important if your farm is free of drench resistance to help prevent resistant worms from being brought in with the incoming stock.