Nematodirus Warning: PGE Risk in Lambs Is Rising

Back in late May, we shared some thoughts on parasitic gastroenteritis (PGE) risk in lambs. At the time, it looked like things might stay relatively calm. The annual Nematodirus hatch was early, lambs born in March and April were still too young to be hit hard, and the weather was dry enough to keep outbreaks at bay.

But that was then.

Over the past few weeks, there’s been a clear shift—and not in a good way.

What’s Changed Since May?

We're now hearing more and more reports of significant PGE outbreaks in lambs. Vets in the field are flagging it, and we’re seeing it firsthand in lab faecal samples and post-mortem cases—some showing acute Nematodirus infections alongside general PGE.

So what’s driving the change? The weather, of course.

The Perfect Storm for Worms

After weeks of hot, dry conditions, recent spells of heavy rainfall have created the ideal environment for worm larvae. Moisture brings larvae to life, and a wet grass surface helps them migrate up the stem—right into the grazing zone.

What’s especially tricky is that after a long dry spell, lambs can ingest large numbers of immature worms, leading to clinical disease before you see a spike in faecal egg counts (FECs). This is particularly true with Nematodirus, which makes it harder to rely on FECs alone.

That’s why it's crucial to look beyond the numbers—keep an eye on body condition, scouring, and performance as well.

Summer and Autumn Nematodirus: No Longer Rare

Summer and autumn infections from Nematodirus aren’t brand new—but this year, they’re more common than usual.

A study from the SEFARI Gateway consortium in Scotland sheds some light on why. Their research shows that while some Nematodirus eggs still require a chilling period to hatch, many do not. In fact, on some farms, up to 87% of eggs were capable of hatching without any prior cold exposure. Translation? Hatching can happen year-round, not just in spring.

Farmers agree: in a recent survey, half reported seeing clinical signs of Nematodirus infections outside of spring—15% in autumn, 3% in winter, and 32% in summer.

The Bigger Picture: Long-Term Impact on Lambs

Outbreaks of PGE at this stage of the season can do more than cause short-term issues—they can have a lasting impact on lamb growth and performance well into the autumn. So managing the risk now is vital if you want to finish the season strong.

Key Takeaways for Farmers Right Now

Here are our top messages for sheep farmers this summer:

• Closely monitor lamb performance. Don’t rely on FECs alone—watch for signs like scouring and poor growth.
• Be proactive, not reactive. If you’re unsure, test. Don’t guess.
• Zero tolerance for scouring. If you see it, act fast.
• Regular FEC monitoring is a must. Especially after weather shifts.

PGE and Nematodirus are always lurking—but with the right vigilance, we can stay ahead of the game this season.

Posted by SRUC Veterinary Services on 25/07/2025