How to use and go beyond worm egg counts in sheep

Gut worms are one of the most significant challenges to the health and production of grazed livestock – particularly sheep.

Over the past century, drenches and injectable wormers have proved an effective weapon.

Until relatively recently, the industry advice was for a “suppressive” strategy. This meant worming at-risk livestock every 28 days during the grazing period.

See also: Can sheep be ‘worm tolerant’ and what are the implications?

However, as worms have evolved to resist the wormers, vets and farmers have had to refine their practices. The worm egg count, or faecal egg count (FEC), has become a cornerstone of this refinement.

Quantifying the burden within a group of sheep or cattle can be a useful guide to whether they need treatment or not.

Likewise, they can tell us if a wormer is effective, by using either a post-drench check or an FEC reduction test.

They are being used increasingly to identify both animals with resistance and resilience to worms to help breeding decisions.

Nonetheless, the egg count has its limitations.

First, while a composite sample will quickly tell you the average status of the group, it will also mask the variation between individuals, which is often significant.

This means that if – based on a group average – you decide not to treat, there are probably some individuals who would benefit from treatment.

And if you decide to treat, there are likely some animals who are being treated unnecessarily. Testing individuals would be impractical for most commercial flocks and herds.

Second, because FECs pick up eggs, they require the infectious juvenile stages to develop into adults. This takes time.

A mass hatch can cause issues before egg production and, therefore, before detection has happened via an egg count. Most classically, this happens with Nematodirus species.

Third, although egg counts are reliable across all ages of sheep, in cattle by nine to 12 months of age, they lose much of their diagnostic reliability.

As a result, cattle with high worm burdens can often have relatively low egg counts.

Other tools used alongside egg counts, include:

Targeted selective treatment

Any strategy that leaves a proportion of a cohort untreated, based on rational criteria, qualifies for targeted selective treatment (TST).

Growth rates can be used as a criterion, with any lambs or calves not hitting a target threshold being wormed.

More readily, body condition can be checked by hand – for instance, when choosing which ewes to worm around lambing.

Litter size is another criterion for treatment, such as not treating single-bearing ewes.

No one criterion is perfect, so have a thorough conversation with your vet before going ahead.

TST strategies can reduce wormer use by roughly 50%, with little to no impact on animal health or performance.

TST is best suited to good operators who integrate it with other tools. Suppressive treatment may still occasionally be needed.

Alternative tests

Egg counts reflect several different species of gut worm, each of which has its own seasonality and resistance profiles.

Larval culture is a rare and underused tool to add detail when tackling a resistance issue, as it can tell which species are present and in what proportions.

As for cattle, the use of pepsinogen (a marker of gut damage) can be more helpful than egg counts in mature age groups.

In practice, especially for Nematodirus species, egg counts are integrated with weather data, grazing history, clinical signs, and other cases in the area.

A negative egg count does not necessarily preclude treatment. If you have any questions regarding worm treatment on your farm, please speak to your vet.