Vets advise on worm strategy as data reveals huge variations

Farmers have been urged to adopt a risk-based approach to worming stock after national records revealed huge yearly and regional variations in grassland worm burdens.

The records, published by Zoetis’s Parasite Watch, showed 2019 was a consistently high year for stomach worms.

Average egg counts hovered at or above the 250 eggs per gram (epg) treatment threshold from March through into the autumn.

See also: 6 tips for a good livestock parasite control plan

In stark contrast, data for 2017 showed none of the monitored flocks had counts in lambs above the threshold level where treatment would be justified.

The highest count only reached 150 epg in July of that year. Regional data for 2019 also varied dramatically across the country.

The West Country saw three main stomach worm spikes in spring (342epg), late summer (470epg) and autumn (292epg). Nematodirus species peaked in October at 175epg.

Farms in South East England recorded high levels from March through to July with a high of 711epg in September, but nematodirus cases were rare.

In the North West, stomach worm cases failed to hit the 250epg threshold until September when they rose to 408epg, while nematodirus peaked in May (241epg).

However, the North East produced counts above the treatment threshold from March through until autumn, peaking at 505 epg in October with nematodirus cases staying below the treatment threshold.


The findings add weight to advice from vets and consultants to avoid unnecessary and over-use of wormers which increases the likelihood of product resistance.

Veterinary consultant Fiona Lovatt from Flock Health said: “Routinely treating lambs is usually both unnecessary and uneconomical, as well as adding to the pressure for wormer resistance to develop.

“This is why it is important farmers don’t base treatments simply on what they have done historically.”

Ms Lovatt added that treatment decisions should be based on faecal egg counts (FEC), records of high-risk areas, lamb production and forecasts.

Faecal egg counts

Samples for FECs should be taken from a representative group – not just individuals with dirty back-ends – at regular intervals from spring until early autumn.


The results should be recorded, dated and mapped to identify high-risk areas of the farm and establish peak times of the year.

This can help inform management strategies to reduce worm burdens or, when pasture contamination is high, to consider grazing animals elsewhere, for example.

Lamb growth rates

Monitoring lamb growth rates provides invaluable additional information. All sheep farmers should be engaged with a proactive vet who is interested in their sheep.

Weather forecasts

Worm egg counts and fluke counts are dependent on the weather, though the mechanisms are very different between them.

Watch the weather and follow forecasting services such as the Zoetis Parasite Watch Scheme which provides real-time information and alerts from March.