With the nematodirus hatch imminent, are your lambs at risk? We explore your options.
As spring arrives across North Yorkshire, one of the most serious and rapidly developing threats to young lambs is the annual hatch of Nematodirus battus.
Unlike most sheep worm species, nematodirus can strike with very little warning, and by the time clinical signs are apparent, significant damage may already be done. Understanding the risk, and using the tools available, can make all the difference.
What Makes Nematodirus Different?
The lifecycle of Nematodirus battus sets it apart from other gut worms. The infection passes directly from one season's lamb crop to the next. Larvae develop to an infective stage inside the egg over winter, and are only released when triggered by specific temperature conditions: a prolonged cold period followed by sustained warmth above 10°C.
When this temperature trigger occurs rapidly - as is common in a Yorkshire spring - it can cause a mass hatch: thousands of infective larvae appearing on pasture almost simultaneously. If this coincides with lambs aged around six weeks or older, when they are beginning to consume significant amounts of grass, the impact can be devastating, causing severe diarrhoea, rapid dehydration and death.
Which Lambs Are at Risk?
Not every group of lambs faces the same level of threat. The key risk factors to consider are:
• Grazing history: Fields grazed by lambs the previous spring will carry a reservoir of overwintered eggs. Fields that were not grazed by lambs last year present a much lower risk.
• Lamb age: Lambs under six weeks of age eat very little grass and are largely protected. The danger window opens at around six weeks and continues to around 12–16 weeks, when natural immunity begins to develop. February and early March-born lambs are typically most vulnerable during the spring hatch period.
• Ewe condition: If ewes are under nutritional stress and milk production is poor, lambs may start grazing earlier and at a younger age, increasing their exposure. This is where lambs < 6 weeks may be at risk.
• Field aspect: South-facing fields warm up more quickly and tend to experience an earlier hatch than north-facing fields on the same farm.
What to Look Out For
Nematodirosis can cause sudden illness and death before you have had the chance to act, so vigilance is essential during the risk period. Signs to watch for include:
• Profuse, watery or yellow-green diarrhoea in lambs
• Rapid weight loss and dehydration
• Lambs appearing tucked up, hunched and lethargic
• Sudden deaths in otherwise apparently healthy lambs
Important: Faecal Egg Counts (FECs) are not reliable for diagnosing nematodirosis. The damage is caused by immature larvae that do not produce eggs, so FECs will not reflect the true challenge your lambs are facing. Clinical judgement combined with the forecast tool is essential.
Using the SCOPS Nematodirus Forecast
SCOPS (Sustainable Control of Parasites in Sheep) provides a free, daily-updated Nematodirus Forecast at www.scops.org.uk/nematodirus. The interactive map draws on data from 140 Met Office weather stations across the UK and uses a traffic-light colour system to indicate the risk level at each station:
• Grey/White: Negligible risk
• Yellow: Low risk — begin assessing your lambs and fields
• Orange: Moderate risk — prepare to act
• Red: High risk — take action now for at-risk lambs
• Black: Very high risk — immediate action required
• Triangle symbol: Peak hatch has passed for that station
For farms in North Yorkshire, your most relevant forecast stations are Topcliffe and Leeming. Check the map daily during the risk period — particularly from late March through to early June — and use the 'Historic Data' option to review how the risk level has changed over previous weeks.
Adjusting the Forecast for Your Farm's Altitude
Weather stations record conditions at their specific location, and your fields may sit at a different altitude. As a rule of thumb, every 100 metres increase in altitude delays the hatch by approximately seven days. Conversely, fields below the station's altitude will see an earlier hatch.
The formula to calculate your adjusted hatch date is:
Days to adjust = (Your field's altitude in metres − Station altitude in metres) ÷ 100 × 7
A positive result means your hatch will be later than the forecast; a negative result means it will be earlier.
Practical examples for North Yorkshire farms:
• Topcliffe station sits at approximately 25m above sea level. If your fields are at 225m asl: (225 − 25) ÷ 100 × 7 = 14 days later than forecast.
• Leeming station sits at approximately 32m above sea level. If your fields are at 182m asl: (182 − 32) ÷ 100 × 7 = approximately 10.5 days later than forecast.
• For low-lying fields near the Vale of York at around 30m asl, using the Topcliffe forecast with very little adjustment needed: (30 − 25) ÷ 100 × 7 = less than 1 day difference.
Remember also that south-facing fields will warm faster and may hatch earlier than the formula alone suggests, while north-facing slopes will be later. Always consider both altitude and aspect together.
Using Parasite Mapping to Move Lambs to Safer Ground
The single most effective action you can take when the forecast indicates rising risk is to move at-risk lambs to low-risk pasture - fields that were not grazed by lambs the previous spring. This removes the challenge entirely, rather than relying solely on treatment.
To do this effectively, we recommend keeping a simple parasite map of your farm each year. Record which fields were grazed by ewes and lambs during spring, and which fields carried only older stock, hay crops or cattle. This gives you a ready-made list of safer grazing options when the forecast signals danger.
When Planning your Parasite Map, Consider:
• Fields used for aftermath grazing (post-silage or hay) are generally lower risk
• Pastures grazed only by cattle carry virtually no nematodirus risk for lambs
• Arable stubbles or newly reseeded leys are also safe options
• Higher-altitude fields may also offer a useful buffer of additional days before the hatch reaches them.
If Treatment Is Needed
Where it is not practical to move lambs, or where clinical signs are already present, treatment will be required. SCOPS recommends a white drench (1-BZ benzimidazole), which remains highly effective against nematodirus on the vast majority of UK farms. Resistance to white drenches in nematodirus is still rare, unlike in other worm species.
Please contact us at Bishopton Vets to discuss the most appropriate product and dose for your flock. We would also recommend a faecal egg count 7–10 days after treatment to confirm efficacy, and be aware that in groups with a wide spread of ages, a second treatment may be necessary as younger lambs reach the vulnerable stage.
Get Ahead of the Hatch - Talk to Us
If you are unsure about the risk to your lambs, or would like help drawing up a parasite management plan for your farm, please give us a call on 01765 602396. Our vets are happy to help you use the SCOPS forecast alongside your own farm knowledge, to keep your lambs safe this spring.