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Breeding for Worm Resistance - A Technical Update

Breeding for resistance to parasites


Early in 2020 I was asked to provide SCOPS with an update on our work on breeding for worm resistance. Here is a summary of the work provided, which draws on the original work by SCOPS with an update from Signet. 

Please do go to the SCOPS website for more information


Selecting sheep for resistance to parasites

There are variations between individual sheep within a flock in the strength of their acquired resistance to parasites. Part of this variation is genetic, and it is therefore theoretically possible to selectively breed for sheep that are more resistant to internal parasites. In flocks that have undergone selection for low FEC, lambs developed stronger acquired resistance and had lower FECs and lower worm burdens than lambs in unselected flocks. Adult ewes in selected flocks had a smaller rise in FEC during the PPRI and their lambs had lower FECs at weaning. However, work is still on-going because there is a cost to the animal in mounting the immune response, which can result in reduced performance.

It is important to note that such selection programmes do not confer significant advantage over lambs in unselected flocks until they are 4–5 months of age or more. This means that as a tool for worm control, genetic resistance will be of the greatest benefit when applied to breeds involved in producing ewes rather than terminal sires. A flock of ewes that has been sired by rams that are more resistant to worms will cause less pasture contamination with worm eggs at all times of the year, including at lambing. This reduction in contamination will provide substantial benefits to their lambs.

This selection for parasite resistance can only really be done effectively in flocks breeding rams. Even those commercial sheep producers who breed their own ewe replacements cannot achieve any significant genetic improvement in their flocks by ewe selection if the sires they use to breed ewe replacements come from another flock. If such producers wish to improve the genetic resistance of their flocks they must either breed rams themselves or buy rams from a breeder who has been selecting for resistance to worms.


Breeding for Worm Resistance in the UK

Selection for low FEC is practiced in some ram-breeding flocks in the UK. Over the last 20 years, nearly 30,000 lambs have been sampled and their data included within Signet’s breeding evaluations. Data has been obtained from 13 different breeds over this period, however in the last 5 years over 75% of the samples received have come from Lleyn breeders, with significant numbers of samples also provided by Exlana and Romney breeders.

Progress has been challenging. The collection of FEC samples is time consuming, expensive and comes at a cost to the enterprise in terms of lamb performance. At a breed level genetic change is slow but heading in the right direction. Within dedicated flocks faster progress is being observed, in part through the culling of poor performing breeding lines – rather than selection for highly resistant stock.


How much is variation between sheep under genetic control?

Since 2000, AHDB has supported both the collection of FEC phenotypes and the analysis of this data. One of the great disappointments to many involved is the relatively low heritability values currently obtained from the large datasets collected. While earlier work in experimental flocks often suggested that an animal’s genes might explain 30-40% of the variation observed between individuals, the current heritability values within the Lleyn dataset are only 0.07 (s.e. 0.02) (Strongyles) and 0.21 (s.e. 0.03) (Nematodirus) respectively; i.e. only 7% and 21% of the variation between animals can be explained by their genes.

The genetic correlation between the two egg counts (Strongyles and Nematodirus) is 0.49, indicating that selection for either species would reduce some of the challenge posed by the other. Genetic correlations with other performance traits are low/negligible.


Why is the heritability of FEC so low?

In any analysis, it is right to question the data quality and analysis models used for assessing heritability. Within the Lleyn dataset, the contemporary groups are large – with lambs believed to be treated the same throughout their life and for the most part lambs are under a significant worm challenge.

One weakness of the current approach in the UK might be the wide window of ages at which lambs are able to be sampled. Restricting the dataset to lambs sampled at a certain point in the year or age, may yield positive benefits in lifting the heritability of the trait – though it would reduce the size of the dataset. Obtaining clearer advice on the optimum age to sample lambs may help in the future, though this is extremely likely to be season dependant. It is known that FEC heritabilities increase with lamb age (to a point). In the future fitting age within the analysis model may also be advisable, where measurement date is known.

The second challenge that breeders face is the relatively low repeatability of the trait, suggesting that multiple measurements would give a more accurate overall prediction of egg output. This is a particular concern where faecal matter is extremely liquid (often in the most challenged individuals) and the obvious dilution that arises can create bias. Recording the faecal consistency of the sample may help, with a different protocol in place for very watery faeces.

Despite these challenges, there have been improvements in FEC measurement technology over time and FEC counts remain the “gold standard” in determining parasitic burden in the live animal.


Breeding for Worm Resistance Overseas

Evidence from selection in Romney sheep in New Zealand indicates that substantial and useful improvement can be made over a 10-year period, with selected flocks requiring substantially fewer anthelmintic treatments. Selection for low FEC does not appear to lead to significant correlated responses in resilience to parasites (as opposed to resistance).

Experiments have produced conflicting results about the existence of correlated responses between low FEC and production traits such as growth rate. Research has indicated that lambs with low FEC’s do not always perform better than the lambs with high FEC’s and lambs which perform well when left undrenched do not always have low FEC’s. What is clear, however, is that if selection for low FEC is pursued as the only or dominant trait, then the opportunity to continue selection for other traits is foregone. Breeders are advised to combine moderate selection pressure for low FEC with continued selection for production traits, such as litter size, maternal ability, growth rate and fat depth.


Are we sampling the right species?

One potential challenge in interpreting FEC counts is getting an accurate picture of the worm species that are creating the challenge, bearing in mind that pathogenicity can vary between strains of worm.

James McGoldrick, Veterinary Diagnostic Services, University of Glasgow very kindly assisted AHDB with a small study in 2017, looking at the worm species hatched from 13 mob samples taken from Signet clients.

This very quick look at worm speciation highlights the variation seen between flocks in the proportion of Teladorsagia circumcincta, one of more challenging Strongyles species, compared to the less pathogenic Cooperia curticei. 


Speciation provided by University of Glasgow (Funded by AHDB)



Average FEC S Count of Group

Average FEC S Count of Group

% T. circumcincta

% C. curticei

% oesophagostomum



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New phenotypes for assessing worm resistance 

Research at Glasgow University has shown the antibody responses against the larval stage of Teladorsagia circumcincta, (an important member of the Strongyles family) can be used as a biological marker for host response to infection.

This potential phenotype provides a new way to identify genetic differences between sheep in their resistance to worm challenge. High levels of IgA have been shown to regulate both worm growth and fecundity – leading to a decrease in egg output. These differences would be even more useful in a breeding programme if they can be converted into breeding values.

Unfortunately we can’t directly measure the amount of IgA in the intestine mucosa, but it can be detected in saliva and serum. The potential advantages of assessing IgA levels in saliva include the ease of sampling, sampling cost and the fact that sheep that have been treated with anthelmintic can’t be sampled for FEC, but it can for saliva IgA.

In 2013, the Performance Recorded Lleyn Breeders received support from AHDB to collect Saliva IgA measurements and in recent years these measurements have been converted into EBVs (Estimated Breeding Values) to enable breeders to make more informed breeding decisions.

Once again, the heritability value obtained in this commercial dataset is lower than those values obtained in research environments, with a heritability of 0.11 (s.e. 0.02) observed in the latest Lleyn dataset (10,110 lambs with an IgA phenotype, of which 8,754 also had a FEC measurement).

Importantly, a genetic correlation is observed between Saliva IgA and Strongyles (FEC S) of -0.34, showing that selection for an enhanced Saliva IgA response will lead to a reduction in Strongyles egg counts, though this correlation is somewhat weaker than reported in experimental datasets.

Studies at Harper Adams (Phillips and Cullimore, 2019) using high and low genetic merit Lleyn sires for FEC & Saliva IgA response have shown that a lamb’s sire does have an important influence on these measures, thus demonstrating the value in breeding for these traits. However their work repeatedly raised questions about the repeatability of both FEC and Saliva IgA measurements for lambs sampled 24 hours apart and at greater intervals – with considerable re-ranking of individuals observed between days. Again, the low repeatability of Saliva IgA may explain – at least in part - the lower than anticipated heritability value observed for this trait.

One extremely promising finding from the Harper Adams study was the much higher repeatability value obtained for blood serum IgA levels (Phillips, SBRT presentation 2019). The use of this measurement to predict worm resistance merits further investigation, not least because blood serum IgA levels tend to be the measurements that have yielded the greatest success under experimental conditions. The technical challenge for breeders in measuring this trait is the need for either veterinary assistance to collect blood samples or veterinary training to enable breeders to sample their own sheep. 


Where can I find breeding information to find genetically resistance rams?

Breeding values for a series of worm resistance traits are currently published on the Signet website – Breeding values are provided for a number of breeds that have been engaged in recording FEC at some point in the past, even when the current number of lambs being sampled is low.

Signet currently publishes breeding values for:

  • FEC S (Strongyles FEC)
  • FEC N (Nematodirus FEC)
  • FEC Combined – A single breeding value combining FEC S and FEC N
  • Saliva IgA – Based on Saliva IgA measures

Breeders are collecting Serum IgA measures, but these are not currently used within breeding evaluations, whilst the dataset increases.

Advice to commercial ram buyers:

  • If you are interested in the trait, find a flock actively recording FEC and IgA
  • Use the “Accuracy Values” published alongside each EBV as a guide to the degree of recording taking place
  • Remember for FEC EBVs, a negative value (less eggs) is a positive attribute.
  • Those commercial producers that will benefit most are those retaining female replacements in the flock.
  • By using “EBV Search” on the Signet website, producers can generate lists showing those sheep within a breed with the best breeding values for FEC and IgA (


Advice to ram breeders considering breeding for worm resistance

Advice for breeders wishing to assess this trait within their own flocks can be found online

Flocks considering sampling their flocks are advised to take a mob sample. Ensure lambs are experiencing a challenge. Only when mob samples are over 400 eggs per gramme should individual lambs be sampled.

To ensure that meaningful FEC measurements are collected

  • The lambs must not have been treated for worms within at least the last 4 weeks.
  • Lambs should have graze ‘dirty’ pastures for at least the last 4 weeks.
  • Don’t sample lambs given ‘long-acting’ or ‘persistent’ drenches.
  • Ensure lambs have been under the same management throughout their life
  • Samples should only be sent once lambs are over 18 weeks of age

How many lambs should I sample?

  • The most accurate breeding values will be obtained in flocks that sample all their lambs.
  • Flocks sampling a subset of their lambs should consider sampling one sex and an absolute minimum of 15 lambs.
  • Ideally at least 5 lambs should be sampled by each sire.
  • Sample a cross section of the flock, not just the “best” lambs.