Managing Internal Parasites in Sheep - Part Two – A Strategy for Your Flock

Having looked at the problem of internal parasites that affect sheep last month, this month’s article will describe some of the strategies that you can use to control them in your flock.

The most obvious control method is to use wormers (also called drenches, or anthelmintics) that kill the parasites in the sheep’s gut. Until recently there were 3 chemical groups of wormers: White (BZ), Yellow (LM) and Clear (ML) but, as you may have read, the number of farms carrying worm populations that are resistant to one or more of these 3 groups of wormers is increasing, and there are some farms where parasites are resistant to all 3, making sheep farming a challenge, to say the least!

In 2010, a fourth group was launched: Orange (AD) sold under the brand name ‘Zolvix’. In Spring 2012 a further product was released, called ‘Startect’ based on a combination of two chemical compounds, one new (SIs) and one from the ML group not previously used in the UK. So, you might think that’s solved the problem of wormer resistance - but in reality it is only a matter of time before some of the parasites develop resistance to them; even if they are effective for another 20 years or so, we have a responsibility to preserve their efficacy for as long as possible. If we fail in this, it would be a careless legacy to leave to the next generation of sheep farmers.

How does resistance occur?

Let’s assume we start with a brand-new chemical, and a group of parasites that have never been exposed to it. As with any drug, there is an optimum dose; any more would be wasteful, and any less would only affect the weaker, most susceptible parasites. This leaves the stronger ones unaffected and able to reproduce and populate the environment. Worse still, having received a sub-lethal dose, they start to become immune to future treatments. So, correct dosing is an essential first step.

Make sure you administer the correct amount of wormer.

internal-parasites-2-weighing-sheepTo do this you need to know the actual weight of your sheep (weigh the heaviest, and use this to calculate the dose as advised on the pack).







internal-parasites-2-callibrating-drenching-gunNext, check that your drenching gun is correctly calibrated to deliver the right dose. A simple way to do this is to use the outer of a large syringe: hold it upright and block the pointed end with your finger, then set your drench gun to 40ml and squirt a single dose from your drench gun – it should match the measure on the syringe. It’s worth checking this each time you drench your sheep. These simple steps will ensure that when you administer a wormer, it is the correct dose to kill all the susceptible worms.


Make sure you drench correctly

internal-parasites-2-drenching-eweYou need to restrain your sheep so they don’t wriggle away from you half-dosed! Plan your working area carefully, and try to arrange a ‘race’ through which sheep can only pass one at a time, so you don’t get jostled. Grasp the sheep firmly under its chin and put the drench gun nozzle into the mouth from the side – remember sheep have teeth on the bottom of the front of the jaw, then a gap where the nozzle goes, and very sharp grinding teeth along the cheek part of their jaw – don’t ever put your thumb here or you will get badly lacerated! Put the nozzle over the tongue and squirt the drench into the back of the throat.


internal-parasites-2-drenching-lambIf you’re treating lambs, use a small nozzle designed for lambs as a full-size one could damage their throat.

Now you know how to drench, the next critical question is, when to drench? There are two set occasions when it’s recommended to treat sheep for worms: the first is a quarantine drench for all new sheep coming onto your holding, or returning from land that has recently carried other sheep (to prevent importing resistant worms with them), and the second is for newly-lambed ewes before they go out to grass with their lambs (see last month’s article). At all other times you should have evidence of the need to drench.

Scouring in sheep doesn’t always mean they are infected with worms; the only way you can tell for sure is to carry out a Faecal Egg Count (FEC). This is the most reliable way to get well acquainted with the parasite population on your holding, and will give you a snapshot of how it’s currently affecting your sheep. By carrying out a number of FECs over a period of months, and relating the results to a set of known ‘trigger events’ (such as time of year, age of lambs, breeding status of ewes, weather, grazing patterns, etc.) you will begin to build a clear picture of how these events affect the worm population on your holding. Armed with this information, you will then be able to make informed decisions about when to worm, and what parasite(s) to target.

How To Do A Faecal Egg Count

First of all, check with your vet to see if they offer an FEC service, and what the cost is – ours charges just over £10 per sample, including the little plastic pots to collect the dung in. This may sound a lot, but you can quickly recoup the cost by not using wormers when it’s not necessary, and using the right wormer when it is. This means your flock will be healthier and more productive, and if you’re raising lambs for meat, by managing the worm burden of your lambs, they will reach slaughter weight faster and put less pressure on your grazing.

  • internal-parasites-2-faecal-egg-countCollect fresh dung samples from a representative group of sheep (ideally no less than 10 from a large flock, or all the sheep in a smaller flock) by gathering the flock in a small, clean area of the field.
  • Hold them there for 10-15 minutes, then return them to the pasture and randomly gather fresh dung samples (about a teaspoonful) individually into small plastic pots.
  • If you have ewes and their lambs together, collect the ewes’ samples separately from the lambs’ samples, as they will have very different results – it’s easy to tell the difference as the lambs’ poo will be much smaller than the ewes’!
  • Keep these cool and deliver them to your vet as soon as possible on the same day – often if you can deliver early in the morning, you will have the results by the end of the day.
  • Try to give your vet as much information as possible, including the date they were last wormed and which product was used, the age of the animals and how long they’ve been on the current pasture
  • The vet laboratory will ‘pool’ the samples (ewes and lambs separately) and examine them under a microscope, noting the different worm egg species found, and their numbers    

If the FEC highlights a problem, you can treat it appropriately using the right chemical, which may be a narrow spectrum drug that targets the specific parasite. For example, if Nematodirus battus is identified, a white (BZ) drench would be recommended as resistance by this species is uncommon. To use one of the newer (more expensive!) wormers in this case would be unnecessary and could help accelerate the process of resistance by other parasite species on your holding. If, however, the blood-sucking parasite Haemonchus contortus was identified, either a clear (ML) drench or one of the new classes of anthelmintic would be more appropriate as this species has developed a high level of resistant to many of the older wormers.

One final word concerning the use of anthelmintics – they are powerful chemicals and so can be harmful to people, pets and wildlife, especially aquatic life. Always follow the instructions on the pack, and be careful how you dispose of empty containers and any waste product.


sheep cover2 smMore information on managing your flock can be found on the “Managing Your Flock for Peak Health” DVD.







Associated articles:

Managing Internal Parasites in Sheep Part One – Understanding the Problem

Managing Internal Parasites in Sheep - Part Two – A Strategy for Your Flock

Managing Internal Parasites in Sheep - Part Three – Sustainable Control


This article first appeared in Smallholder Magazine, September 2012 - written by Carole Youngs of The Smallholder Series.