Farmers are urged to watch for nematodirus risk factors in lambs, as a spell of warm weather could spark a mass hatch of over-wintered parasites.

Cases have already been reported in the south west, Northamptonshire and Wales and, as weather improves, it will move north, says independent sheep consultant Lesley Stubbings.

“Farmers in the south must start taking notice of risk factors due to rising temperatures. High risks are lambs six to 10 weeks of age, grazing on pasture that saw nematodirus last season, in temperatures above 10C.

“Farmers must consult their vet with regard to local risks and treat those lambs at risk before nematodirus affects growth.”

Temperatures rise

Areas seeing rising temperatures should be considering treatment in the next two to three weeks, says Gloucestershire vet Cath Tudor.

“The cold April means nematodirus won’t gradually hatch, so sustained temperatures above 10C will see mass hatch. When this coincides with lambs starting to eat significant amounts of grass, mortality may be as high as 5%.

“Once lambs have ingested larvae, dark, watery scours followed by dehydration will occur 10 days later. Following symptoms, death can occur in days.

“Lambs 6-10 weeks old eating increasing amounts of grass are most susceptible due to poor immunity. Lambs under stresses such as triplets, or those fostered or infected with coccidiosis, could be at increased risk.”

High risk treatment

Farmers cannot have a “wait and see” policy, adds Salisbury vet Peter Vencer. “Lambs at high risk or presenting symptoms should be treated with a white (benzimidazole) drench, as nematodirus has no known resistance to benzimidazole. A second drench may be needed three weeks later.

“Faecal egg counts can be useful as, although it is immature larvae that cause damage, any adult eggs will indicate significant numbers of larvae.”

The risk of nematodirus this year is a cause for concern, says Devon farmer Peter Baber. “In the past nematodirus has rarely been a problem. We lamb later, so normally miss the peak period. However, with temperatures only beginning to warm now, lambs will be at peak susceptibility.

“I am vigilantly checking for any ill-thrift in lambs, particularly those with scours. If there are any showing signs I won’t hesitate to drench. The rise in temperature over the past week means I am planning on drenching in the next two weeks.”

Risk factors

  • Pastures with previous nematodirus
  • Cold weather followed by warm
  • Lambs 6-10 weeks old
  • Weaker lambs