Sheep worms – breeding worm resistant sheep

Page last updated: Friday, 30 June 2017 - 11:41am

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Lost productivity due to drench resistance in sheep worms has been recognised as a widespread problem in Western Australia (WA) since the 1980s. The first member of the macrocyclic lactone (ML) group, ivermectin, was released in Australia in 1988 and by 1993 the first case of resistance to this group was discovered. It is now thought that more than 85% of WA sheep farms are likely to have ML resistant sheep worms. In other parts of Australia, resistance to barber's pole worms is especially severe and involves all drench groups except the newly-released drench, monepantel (Zolvix).


Following experience with previous drenches, it is essential that sustainable drenching practices are followed. Continuing over-reliance on chemical products will eventually result in less effective worm control, leading to losses in liveweight gain and wool production, and even mortalities in some circumstances. In economic terms the cost can be several dollars per head. To extend the remaining life of currently effective drenches, it is important to include non-chemical control options as part of the overall sheep worm control program. As with all other pest control programs, the industry needs to adopt ‘Integrated Parasite Management’ (IPM) procedures. Genetic improvement of the host’s resistance is an important component of IPM.

Worm resistant sheep

The only permanent long-term solution to help manage drench resistant worms is to breed sheep that are resistant to worms. In terms of whole farm management, resistant sheep have several advantages:

  • robust and easy care sheep
  • lower production losses due to worms:
    • less drenching
    • less labour
    • lower worm contamination of paddocks
    • smaller carry-over of worms from one sea­son to the next
  • less reliance on artificial control options:
    • reduced impact of drench resistance
    • increased life span of current effective drenches
  • off-farm benefits:
    • clean and green marketing potential
    • sustainable production systems.

Increased worm resistance in breeding programs involves measuring the faecal worm egg counts (WEC) of individual sheep and taking that into consideration with the other production traits in making selection decisions. Some breeders are concerned that they will reduce the potential genetic gain for other production traits if they include another trait such as worm resistance in their breeding programs. Modelling studies show that including worm resistance in the breeding program results in larger gains in growth and production than selecting on production traits alone. Research results from the Mount Barker Research Station (WA) and New Zealand have confirmed huge beneficial carry-over effects for lowered faecal worm egg counts. This is mainly due to the protection that worm resistant individuals convey to their contemporaries in the mob. Thus, worm resistance should be seen as an extremely important trait in any sheep breeding program where worms could be a problem.

Resistance to worms in sheep is an inherited trait that can be increased by selection. The heritability of worm resistance, as measured indirectly by individual sheep faecal WEC, is at least 25%. This means that 25% of the phenotypic superiority of an animal will be passed on to their progeny. For example, if the average faecal WEC of a flock is 400 eggs per gram (epg) and an individual ram has a true faecal WEC of 200, then the ram's superiority is 200 epg (400 minus 200) as they are 200 epg below the flock average. With this example, 25 per cent of the superiority will be passed on to its progeny. Thus, if such animals are mated with each other, then it can be expected that their progeny will have a superiority of 50 epg (25% of 200) below the average, that is 350 epg.

There is typically a large variation in the faecal WEC between sheep, making it relatively easy to identify the more resistant individuals within a group provided the average WEC is high enough with as few animals as possible with zero WEC.

As with all genetic improvement programs, selecting within a flock can take several years to achieve noticeable improvements, but an increasing number of farmers who have been selecting sheep for worm resistance for some time have achieved very good results. Results from Sheep Genetics show that an increasing number of farmers have sheep that are relatively resistant to worms.

Research results

Since 1988 the Department of Agriculture and Food, Western Australia (DAFWA), with industry support, has been selecting for worm resistance in the Rylington Merino (RM) flock. Figure 1 shows the genetic trend of the decline of the faecal WEC in the Rylington Merino selection line relative to that of the unselected control line.

Figure 1 Genetic trend of Rylington Merino sheep selected for worm resistance compared to the unselected control group

Figure 1 represents a genetic gain of 2.7% per year or about 10% per generation for selecting solely for low hogget WEC in the selected line until the year 2000. This is very good considering that the annual genetic gain for most production traits in sheep is less than 1%. During the main winter worm challenge period, the actual average faecal WEC output from hoggets in the two lines will vary from year to year due to environmental effects and inaccuracies in estimating the breeding values of animals.

Figure 2 shows the genetic merit of the selected line in the worm resistant Rylington Merino flock compared to industry flocks in Sheep Genetics' database. In this case the Rylington flock is compared to the industry average. On that graph the Australian Sheep Breeding Values (ASBV) for WEC of 0% is taken to represent an actual flock average WEC of 500 epg, and the minus 100 ASBV WEC is taken to be an animal that is totally resistant to worm with a WEC of zero epg. It is clear that the Rylington Merino flock is a highly worm resistant flock. The increase in the 2009 yearling worm egg count (YWEC) was due to the use of outside sires with less favourable ASBV for WEC. This then flows through to their progeny sires two years later.

Over an 8 year period, the industry flock hovers around 0% YWEC. The Rylington flock stays in a arrange from minus 90% to minus 75%. This shows that the Rylington Merino flock is a highly worm resistant flock.
Figure 2 Genetic trend of Rylington Merino for yearling worm egg count ASBV compared to tested industry flocks

How to select worm resistant sheep

The overall worm resistance of a sheep flock can be increased in two ways:

  • measuring worm resistance in your own flock and selecting those animals with greater worm resistance to include in breeding programs
  • sourcing stock (usually rams) from breeders who have already been selecting for worm resistance. Sheep Genetics provide breeding values for a wide range of production traits on animals including WEC. This makes it possible to identify suitable rams from different breeders who measure WEC. Selecting for worm resistance can be incorporated into normal sheep breeding practices by comparing WEC of individual sheep. The number of worm eggs in the sheep’s faeces is a guide to the number of adult worms that have developed inside the sheep.

It will generally be most cost effective to concentrate on the young rams, with faecal sample collection at hogget age/weaning time. Combining information from measurements taken at weaning and hogget age will increase accuracy, but in most cases, one measurement will be most cost effective if pedigrees are available.

The choice between weaner and hogget samplings depends on the level of the local worm challenge. For example, in low worm challenge environments the natural immunity of the hoggets combined with the lower challenge can often result in inadequate faecal WEC levels to allow good differentiation between animals. In many situations, faecal WEC testing at hogget age could reduce costs because a proportion of the flock is likely to have already been culled based on other production traits. However, as worm resistance is more important in young lambs, it will be more beneficial over the long term to invest in measuring WEC at weaning. This information can be submitted to Sheep Genetics who will provide a breeding value for worm resistance at an early age to assist in making sound selection decisions.

Testing procedure

Checking the average WEC of the mob before testing

Sheep need to be exposed to worms in order to show their level of resistance. The best time to do this is during the green feed period in the winter rainfall regions of Australia. Worm egg counts of the test mob should be monitored as a group before commencing testing of all the individuals for selection.

Monitoring can be done simply by collecting three to five fresh dung pellets from individual piles deposited by 12-15 sheep in the paddock.

Procedure in lambs

In lambs the ingested infective (L3) worm larvae start to establish themselves in the intestines when the lambs become functional ruminants at about five to seven weeks of age in a normal season. It then takes about four weeks for these larvae to mature into egg producing adults. Therefore the lambs need to be at least 10 weeks old to have the WECs reflecting their worm burden. Faecal sampling should preferably be carried out on the day of weaning, or at least two weeks after weaning. Lambs should only be sampled if the average WEC of the group is at least 300 epg faeces. Thus, start monitoring faecal WECs of the lambs from 10 weeks of age, then re-check every week until the faecal WEC has reached an acceptable level. Don’t sample lambs during the first two weeks after weaning as the stress associated with weaning will temporarily suppress the immune response thus removing genetic differences between resistant and susceptible individuals. This practice was followed in the Rylington Merino flock that has been selected for low WEC at Mount Barker Research Station.

In flocks where mating is longer than five weeks, age differences between lambs can reduce the accuracy of WECs. This can be reduced by giving the lambs a short acting ‘even-up’ drench when the youngest lambs are at least 10 weeks of age. The lambs should then be exposed to a contaminated pasture for a couple of weeks to allow them to pick up some worm larvae. The worm larvae will mature to become adult worms in four weeks and lay eggs. The lambs can then be sampled five to six weeks post-drenching. In a late lambing scenario with an early pasture senescence, this may result in insufficient larvae pickup after drenching. However, if WEC is high enough for faecal sampling then this practice will also require an extra drench.

Procedure in hoggets

In hogget age sheep, start monitoring faecal WECs four weeks after the beginning of the winter rainfall season, then re-check every two to three weeks until the faecal WEC have reached an acceptable level as indicated below, before all the sheep in the flock are sampled.

The two main purposes of monitoring are:

  • to ensure an adequate worm challenge is present that will allow identification of resistant and susceptible individuals. For the non-barber’s pole worm areas, this is achieved when the average faecal WEC of the mob reaches about 300 epg for weaners, and 500 epg in the case of hoggets, and where only a few animals will have a zero count and only a few will have a WEC of over 1000 epg
  • to ensure that the worm challenge does not increase to levels where the production and welfare of the animals are compromised.

Sampling individual sheep

When the target faecal WEC level has been reached, individual samples from each of the candidate animals are collected, over as short a period as possible. Contact the laboratory a few weeks before collection is planned, to notify them when the samples can be expected. That will allow the laboratory to allocate enough time to process the samples as soon as possible after arrival. Samples must also be kept cool (refrigerated but not frozen) and should be sent to the laboratory within 48 hours of collection. Usually a drench will be needed on the day of sampling, as the counts required are at a level where some worm effects will occur in the lambs.

Co-selection against scouring and dagginess

Most cases of winter scours (dags) in grazing sheep are due to the effects of either immature and/or adult worms in the sheep. When scouring is due to immature worm larvae, this will not be expressed as an increased faecal WEC. Also, there is a tendency for some highly worm resistant sheep to be more predisposed to the hypersensitivity (low WEC) scour. Therefore, it is advisable to treat scouring as a separate trait.

Scouring is a heritable trait, which implies that it can be selected against to assist in reducing dagginess. Some breeders have expressed interest in selecting sheep for reduced scouring (as well as worm resistance) on the basis of visual recording of individual dag scores or faecal consistency scores in their sheep using a five point scoring scale. This has been standardised in a Visual Sheep Score classification available on the Australian Wool Innovation (AWI) and Meat and Livestock Australia (MLA) websites.

Using the information

To maximise the gain from selecting for worm resistance whilst maintaining other breeding objectives on the farm (such as lower micron, increased fleece weight), a geneticist should analyse the faecal WEC results. There are a number of service providers in the industry who can do this. Contact the authors (details below) for assistance. A geneticist will convert the raw faecal WEC data into a faecal WEC estimated breeding value (EBV) or Australian Sheep Breeding Values (ASBV) published by Sheep Genetics. The breeding values for each animal are calculated from its own measurements, incorporating information from an animal’s relatives, and correcting for other non-genetic factors such as the age of the ewe and whether the individual was a twin or single lamb. This process increases the accuracy of each individual’s estimate and will therefore improve the rate of genetic gain. The individual ASBV of fleece weight, body weight, fibre diameter, WEC, etc. can be incorporated into an overall selection index. This will allow maximum overall genetic gain in all of the economically important traits.


Ram breeders

  • Ensure a controlled and adequate worm challenge.
  • Select for reduced faecal WEC at weaning and/or hogget age.
  • Selection for reduced scouring in the winter rainfall environment is best done from yearling to hogget age during their second winter challenge period.
  • Increase the selection response by using trait ASBVs incorporated into an overall selection index.
  • Allocate at least 30% of the total selection pressure to faecal WEC selection and a further 20% to scouring if necessary where worms and dags are a problem.
  • Ask for information on worm resistance where a sire was progeny tested in a sire evaluation scheme.

Ram buyers

  • Ask for ASVBs and indexes to suit your breeding objective. If ASBVs are not available, then ask for information on the average faecal WEC of the test group (to check that there was adequate selection pressure from worms when the worm resistance selection was done).
  • Ask for information on a ram’s deviation from the group mean rather than actual faecal WEC (this allows ‘within group’ comparison such as considering if the ram is better than average).
  • Ask for information on worm resistance where a sire was progeny tested in a sire evaluation scheme.


  • Any genetic improvement program, such as selecting on fibre diameter to reduce the average micron of a flock and selecting sheep for worm resistance will take time before major improvements are apparent.
  • By including worm resistance in breeding programs now, significant progress can be made before existing drenches become totally ineffective and it is no longer profitable to run sheep because worms have become uncontrollable.


  • Australian Wool Innovation
  • Rylington Merino contributors
  • Sheep Genetics
  • Curtin University
  • Department of Agriculture and Food, Western Australia (DAFWA) staff

Contact information