Managing Internal Parasites in Sheep - Part Three – Sustainable Control

ewe-and-lambsMany smallholders are keen to rear their flocks in as natural a way as possible, choosing to manage their sheep in a way that promotes good health, rather than relying on the routine use of drugs and chemicals. This third article looks at specific flock management regimes that will both reduce the routine use of chemical wormers, and improve flock health.

Last month’s article - part two - looked at the problem of wormer resistance and described how to administer a drench effectively, and do a Faecal Egg Count (FEC) to ascertain the need to drench, or not. One of the most effective uses of an FEC is to carry out a ‘Worm Egg Reduction Count’, or drench test, to find out if you already have a problem with wormer resistance. If it turns out that you do, you can then both save money and get better performance from your sheep by targeting parasites with a product that actually works on your holding!

Wormer groups – an extended range

There is a ‘hierarchy’ of the wormer groups, starting with the oldest group, the BZ or ‘white’ group. Next is the LV or ‘yellow’ group, followed by the ML or ‘clear’ drenches. The two newer groups are AD or ‘orange’ (Zolvix) and finally SI or ‘Purple’ which is only available as a ‘multi-active’ compound (Startect) – these last two groups currently have no known worm resistance to them and so are ideal to use when you bring new sheep onto your holding as a ‘quarantine’ drench. However, on many farms, especially where sheep have been grazed for some years, the gut parasites have developed a degree of resistance to some of the older groups of anthelmintic.

A flukicide is a narrow-spectrum product, designed specifically to act against liver fluke, which has a unique lifecycle that relies on an intermediate host, a tiny water snail, to complete its life cycle. It’s most prevalent in wet marshy areas, and can be a serious problem following a period of high rainfall, such as most of the country has experienced this summer!

How to test for wormer resistance – the ‘drench test’

  1. Collect fresh faecal samples from a group of sheep (or lambs, but not both together) that hasn’t recently been treated with any worming products
  2. Send samples to vet for faecal egg count (FEC)
  3. On the same day, drench this group of sheep with a wormer from one of the older groups listed above (ideally, restrict feed for up to 24 hours before drenching with BZ and ML, but not for pregnant ewes)
  4. Identify these sheep with a marker spray
  5. After a set number of days (see table below) repeat the FEC
  6. The difference between the 1st and 2nd egg count will indicate whether the wormer used is reducing the worm burden in the sheep.


Drench Class

Days before repeat FEC

Group 1 – BZ

10 -14

Group 2 – LV


Group 3 – ML

14 - 17



It makes sense to test a product from Group One (BZ) first, as this is the oldest group of wormers and therefore it’s the most likely one that parasites will have developed resistance to. If you do find you have resistance to this group, repeat the exercise with a product from Group Two (LV) – then ditto for Group 3 (ML), but hopefully you won’t need to.

Grazing management

Organic production systems rely on careful grazing management to minimise the sheep’s exposure to gut worms, but bearing in mind that worm larvae and eggs can survive on pastures for a year or more, this is not an easy task for the smallholder on limited acreage.

sheep-grazing-next-to-electric-fenceRotational and strip grazing are widely used methods of ensuring that the flock always has fresh, clean grass ahead of it and doesn’t graze the same pasture for any length of time, thus reducing their exposure to ‘dirty’ pasture. ‘Mob’ grazing is a more extreme version of strip grazing where sheep are heavily stocked but confined by electric fencing to a narrow strip of pasture. The electric fence is moved ahead of them frequently, so that there is no heavy build-up of parasites and the sheep don’t return to the same grazing for several months.

New ‘leys’ and the aftermath that grows following the harvest of a crop, or hay, can be considered ‘safe’ and are ideal for weaning lambs onto. Rotating grazing with another species, such as cattle, also offers safe grazing as sheep-specific gut worms will be destroyed when ingested by the cattle. Care must be taken however, if sheep and goats share pastures, as a goat’s digestive system differs from a sheep’s and can accelerate the problem of parasite resistance to worming drugs.

‘In refugia’ – no more ‘dose and move’

For generations, it was common practice to drench sheep before moving them to fresh pasture (‘dose and move’), in the belief that by ridding them of internal parasites they wouldn’t contaminate the new grazing. However, this strongly selects in favour of drench-resistant parasites that are carried forward in the sheep’s gut to populate the new grazing. Current advice is that a flock should carry a small worm burden (called ‘in refugia’) to any new pasture – this means that any ‘superworms’ will cross with susceptible worms and not colonise the pasture. There are two main ways of achieving this: firstly, drench only those animals showing physical signs of parasitic infection (eg. poor condition, scouring), leaving fit animals in good body condition untreated; and secondly, by leaving the flock on the same pasture for 24 hours after drenching so that they re-ingest some susceptible worms, then move to new pasture.

The role of nutrition

Nutritional stress, or imbalance, can cause poor body condition in adult sheep and restricted growth rates in lambs, and if gut parasites are also present, the sheep is less able to mount a defence against them. Forage and soil analyses can identify any shortfall in your grazing, as well as the presence of ‘antagonistic’ elements that can block the availability of essential minerals & trace elements.

chicory-&-plantainIn recent years, ‘bioactive forages’ have been shown to help sheep repel gut parasites, and become more resilient to their harmful effects. Suitable plants, which can be incorporated into existing permanent pastures, include chicory, sainfoin and plantain. These forages, which are relished by sheep, also have deep tap-roots that increase the availability of minerals and trace elements to grazing animals.


The Future – Selection & Breeding

ewe-with-twinsImmunity against parasitic worms is the ‘holy grail’ for sheep breeders; but how achievable is this goal? In reality, it takes a great deal of energy for a sheep to mount an immune defence against gut worms that would result in a worthwhile reduction in worm egg output. This energy use would inevitably be at the expense of some other valuable trait, such as prolificacy – the ability to rear twins.

On the other hand, most shepherds will be aware that certain lambs are always ‘clean’ and grow well, while others in the same group suffer from scouring and are unthrifty. Although exposed to the same challenge, some lambs inherit their mother’s resilience to the effects of gut worms, in other words, they tolerate a moderate worm burden without ill-effect. In our own flock of Hampshire Down sheep, we’ve always kept the best ewe lambs that also exhibit this trait to join our breeding flock, and within a few years we’ve noticed how their ability to thrive without routine worming treatments is passed down to their lambs, resulting in a healthier flock, less drenching, and a lot less bother for the shepherd!

One final word of caution in interpreting FECs is that they don’t show the presence of immature worms that are not excreting eggs, so you must regularly monitor your flock.

In conclusion, if you are in any doubt about the parasite status of your flock, consult your vet, you’ll find the money you spend will pay itself back with fewer worming treatments, a healthier flock, and faster-growing lambs!


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, October 2012 - written by Carole Youngs of The Smallholder Series.