The impact of roundworms on sheep production in the UK is an industry-wide concern, with the traditional use of drenches to combat the issue under threat from the increasing prevalence of anthelmintic resistance. Roundworm-infected lambs are subject to reduced feed intakes and poorer conversion of feed, resulting in reductions of live weight gain of up to 60-100%. Infectious roundworm larvae are found on blades of grass in pasture, where they are ingested by sheep and lambs. Sheep may play host to many thousands of worms during a moderate infection, and every female worm in the gut will lay eggs in enormous numbers upon mating. It is therefore unsurprising to learn that the daily output of faeces of such a sheep may contain hundreds of thousands of roundworm eggs, thus amplifying the worm burden on that area of pasture. In the establishment and spread of worm infection in a population – sheep themselves play a major role.
An important factor when striving for sustainable worm control is the exploitation of the variation that exists in worm resistance between sheep. In some individuals, the roundworm populations in the gut will grow and reproduce relatively unopposed, allowing successful reproduction and the shedding of vast numbers of eggs in faecal matter – these individuals will significantly amplify the worm burden on pasture. In other individuals, the growth and reproduction of worms in the gut is less successful, and hence fewer eggs are shed. It is these animals that are more resistant to roundworm infection, and that will lead to decreased worm burdens on pasture. This variation in resistance has a genetic component; therefore allowing producers to selectively breed for animals that are genetically more resistant to worms. However – if you wish to breed for this trait, you must first measure it.
The current approach to measuring worm resistance in sheep is the measurement of Faecal Egg Counts (FEC). In order to obtain a FEC sample, lambs are exposed to a worm ‘challenge’ at around 18-21 weeks of age. During this ‘challenge’, lambs must be exposed to worms on ‘dirty’ pasture, and cannot be treated with anthelmintic drench. A faecal sample is then extracted from the worm infected lambs, and sent away for analysis (which simply involves counting the number of eggs in a specified quantity of faecal matter using a microscope). This information is then used in breeding evaluations.
Where selection based on FEC EBVs have been implemented over a number of generations this has led to reduced pasture contamination and a reduction in the need to treat animals. Furthermore, research has shown that ewe lambs with improved genetic resistance as lambs also have reduced FECs around lambing time and less increase in FEC post-lambing. This in turn reduces pasture contamination for their offspring which results in a favourable cycle whereby pasture contamination is reduced.However, uptake of the technology has been relatively low, with lack of awareness amongst commercial producers, concerns over the level of worm challenge required and complications of sample collection being quoted as barriers to uptake in the pedigree sheep breeding sector.
AHDB Beef and Lamb Farm Innovation Grant Tests a New Approach to Assessing Resistance
AHDB Beef and Lamb Farm Innovation Grant (FIG) funding was requested to develop a method of improving detection and uptake of genetic resistance to roundworms in the UK national sheep flock. A proposal was made by the Performance Recorded Lleyn Breeders group, in conjunction with Glasgow Veterinary School and KN Consulting, to test for parasite specific antibody levels in saliva as a novel indicator of worm resistance in sheep. Early research indicates that as opposed to counting the number of worm eggs in the faeces of sheep, measuring the levels of IgA (an antibody specific to particular roundworm species) in saliva may provide a more accurate indicator of resistance. IgA severely affects roundworm reproductive success in the gut. Therefore if an animal is producing higher levels of IgA – it is likely to be shedding less worm eggs (and is more resistant). As IgA is produced in the gut, and mouth, it was proposed that the collection and analysis of a saliva sample from the mouth of sheep would be a good indicator of levels of IgA production.
Measuring IgA in Saliva
Approximately 3000 saliva and FEC samples were collected across 12 Lleyn flocks from June to December 2014. The equipment required for sample collection is easily available and consists of a dental swab which is used to absorb the saliva and a pair of forceps which are used to manoeuvre and hold the swab. Finally, a plastic container is needed to hold the swab after saliva collection.
The procedure for saliva sampling was as follows:
1) Put the dental swabs in a clean and dry container next to you to be easily picked up by the forceps as you should try not to touch them.
2) Make sure the lamb is restrained
3) Use the forceps to clamp a swab at one end and gently insert it into the side of the lamb’s mouth.
4) Turn and twist the swab around the mouth and over the tongue collecting saliva for around 10 seconds.
5) Using the forceps place the swab into a tube, and label the tube with the lamb’s tag number.
6) Package and send the saliva samples for analysis.
Producers found saliva sampling to be simpler, and therefore quicker than FEC sampling.
Saliva IgA levels are a reflection of worm challenge over a period of months whereas a faecal egg count only measures the current level of adult worm infection in an animal. This means that the actual level of worm infection at the point of saliva sample collection is less important than when relying on a faecal sample and does not need an adult worm infection to the be present. It is still important that animals are exposed to sufficient worm challenge over a season to mount an effective immune response but saliva sample collection is easier to integrate into flock management practices.
It is likely that these factors combined may make saliva testing for IgA a more attractive procedure to breeders than FEC sampling.
Preliminary analysis of IgA levels has yielded interesting results. The expected relationship between FEC and IgA levels was confirmed – animals with higher IgA levels tended to have lower FEC values (and were therefore perceived as being more resistant to roundworm infection). IgA measurements have been used to produce preliminary EBVs. There were no unfavourable correlations found between IgA and production traits. This indicates that selection for resistance using IgA levels would not lead to any reduction in animal performance.
This EBLEX funded study indicates that IgA could offer itself as a feasible alternative, or improvement to the current FEC system. Additional research is required to predict the response to selection using salivary IgA and to incorporate salivary IgA into selection indices. Further work is planned for next year.
The Performance Recorded Lleyn Group intend to collect a similar number of samples again in 2015, making this one of the biggest datasets of this kind anywhere in the world.
For more information about FEC or saliva testing, contact Catherine Nakielny
[email protected] Tel : 01558 685 132
For information about the Performance Recorded Lleyn Breeders Group please contact George Cullimore
[email protected] Tel : 01225 448 757