Sheep farming

Nematodirus mass hatch warning as weather warms up

Sheep farmers are being urged to monitor the Nematodirus risk on their farm with hatching expected to start within the next week

Warm spring weather has finally arrived and will bring with it a mass hatch of the infective larval stage of Nematodirus. These will have overwintered on pastures waiting for temperatures to regularly exceed 10oC.

Nematodirus is the most commonly diagnosed disease in growing lambs in May, according to the Animal and Plant Health Agency (APHA).

Farmers involved in the Parasite Watch scheme, run by Zoetis, expect to start picking up egg counts in the next couple of weeks, with results reported in almost real-time at

The 18 farms involved in the scheme, have faecal samples taken every two weeks to detect major stomach worms and Nematodirus. Information is then uploaded within hours of the test result coming back, giving a real-time picture as to what is happening on the ground.

Symptoms and risks

Nematodirus can strike very quickly in lambs under three months old that don’t have immunity to the parasite. This manifests with the sudden onset of diarrhea resulting in dull, depressed lambs who stop suckling and lose condition.

When left untreated, death can occur rapidly due to dehydration. Those that do survive will often take 2-3 months longer to reach acceptable market weights.

Risk factors include:

  • Lambs grazing pasture that carried lambs last spring
  • Lambs younger than 3-months old that don’t have immunity to the parasite, but are consuming grass (generally, 6 –12 weeks of age)
  • A sudden cold snap followed by a period of warm weather
  • Lambs under other stresses such as triplets, fostered, on young or older ewes.


It’s very important to not only keep an eye out for Nematodirus, but also be aware of which other worms may be challenging stock on the farm. If you’ve got a mixed infestation, then you need to be confident the drug you are using will treat against both Nematodirus and other stomach worms. Traditionally, the treatment advice for Nematodirus is to use a white wormer, which is fine as long as there’s not a mixed worm burden. If there is a mixed worm burden and you use a white wormer, which you may have resistance to in the other worms, it could become a dangerous and unprofitable situation.

When a wormer is 60-90% effective there often won’t be any visual resistance issues in your lambs, but you won’t be maximising their growth potential. Additionally, you will also be allowing resistance to build up on your farm, it is therefore vital to find out which worms are on your farm, which drugs are working and which are not in order to protect the farm for future generations.

For further information please see the product’s SPC or contact Zoetis UK Limited, Walton Oaks, Dorking Road, Tadworth, Surrey KT20 7NS. Customer Support: 0845 300 8034.

Use medicines responsibly ( Date of preparation: March 2018. AH269/18