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Weaning is a stressful time for lambs.  Not only do they have the anxiety of separation from their mothers but they also suddenly have to adapt from a milk-based diet to a pasture-only diet.  There can be significant disease problems around this time and one of the most common problem is “worms”.  Worm problems can be prevented by a combination of good management, monitoring and strategic drenching. 

Worms in lambs at weaning are a risk for several reasons. 

  • Pasture can be heavily contaminated with worm eggs by the ewes because their immunity is weakened around the time of lambing (causing a ‘peri-parturient rise’ in ewes’ faecal egg counts)
  • Lambs haven’t had time to develop age-related immunity to worms and they are relatively susceptible to them. 
  • The stresses of weaning can weaken lambs’ immune systems and make them more vulnerable to worms.

Sheep on lifestyle farms are particularly at risk if they are set stocked on the same small area of land for long periods of time.  Worm eggs build up as the pasture is never rested long enough for the worm eggs to die off. 

On the other hand, many lifestyle farms have several species of livestock sharing the same pasture.  By and large, cattle and horses don’t share the same types of parasitic worms as sheep so they ‘mop up’ sheep worm eggs on pasture and this can reduce the worm challenge for the sheep. 

It is important not to let worm burdens build-up to the extent that they cause problems like weight loss or scouring and even death (clinical signs).  Worms also have less obvious effects (called ‘sub-clinical’ effects) such as reduced growth rates and reduced weight gains.  The only way to find out what level of worm burdens your sheep have is to arrange for faecal egg counts to be carried out.     


  • Treating sheep that have significant burdens means giving them an appropriate dose of an effective drench (anthelmintic). 
  • There is no benefit in giving more than the recommended dose (this is a waste of money) and giving less can encourage the development of drench resistance. 
  • It is important that the correct dose is given to each sheep according to its body weight.  Weigh individual sheep if necessary.

Suckling lambs

Whether or not to drench suckling lambs should depend not just on how many worms are present but on the worm types.  Faecal egg counts can indicate this.  In the South Island, one particular type of worm (Nematodirus) can be a particular problem in lambs just before weaning.


  • If weaning onto a clean area, either don’t wean directly onto it or leave the most robust lambs undrenched.  This seems counter-intuitive but it helps prevent drench resistance by ensuring a level of challenge that will help boost natural immunity without causing heavy infections.
  • If lambs are weaned onto a lambing area, plan to share the grazing with ewes.  They will help mop up worm eggs. 

Weaned lambs

There are two ways of helping prevent heavy worm burdens in lambs. 

  • Monitor and test: Faecal egg count test at 4-week intervals with follow-up drench if indicated
  • Preventive drenching: Four or five drenches at 4-week intervals from weaning.  This is an easier plan to follow but it may not always be the most effective use of drench

Drench resistance

Drench resistance is a very real problem on many farms both large and small.  Drench resistance means that some of the worm species that can cause problems are resistant to one or two of the three types (families) of worm drench commonly used, so using these drenches is not successful.  There are now combination drenches containing drenches from two or even all three of the drench families and these are usually completely successful if used correctly.  Some drenches have the added benefit of mineral supplements such as selenium.

If you don’t know if you have drench-resistant worms on your farm, find out by arranging a ‘Drench Check’ on your lambs in summer.  This means getting ten faecal samples tested 10 days after drenching.  If you are lucky enough to have no drench resistance on your farm, you should try to keep it that way!  Don’t allow any introduced livestock onto your paddocks until they have been effectively drenched and emptied out on a hard surface.  Your vet or animal health advisor will help you do this. 

Ewes and hoggets

Recent changes in recommendations relating to worm control include leaving the most resistant sheep undrenched and worming only those that need it.  This involves the concept of ‘in refugia’.  Basically, it means accepting a certain level of worms and encouraging natural immunity to them, for example by allowing a low level of infection in the most robust sheep.  This means that there is no need to drench ewes that are in good body condition.  Drench only ewes that are thin or underfed.  Ewe hoggets are still developing age-related immunity and faecal egg counts will indicate if drenching them is necessary.

The bottom line

Talk to your vet or animal health professional for advice on worm control that is tailored to suit your particular lifestyle farm.  This will include advice about pasture and stock management, monitoring sheep for worms using faecal egg counts, and treatment options.


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