Nematodirus warning as mass hatch may loom

Sheep farmers should be alert for nematodirus as dry weather followed by wet conditions could lead to a mass hatch of infectious larvae.

So far nematodirus has not been too much of a problem this season except for small pockets across the country, said independent sheep consultant Lesley Stubbings.

But because of dry ground conditions the larvae haven’t been hatching gradually which would normally help lambs develop immunity. Instead, when it rains, a mass hatching of larvae could occur which would be a problem in areas with slightly later lambing flocks, said Ms Stubbings.

“Four to six-week-old lambs just starting to graze would be most at risk if there was a mass hatch and pasture infectivity was high, as they wouldn’t have the immunity against the worms.”

One of the control methods Ms Stubbings suggested was to avoid grazing lambs on “high risk pasture” such as land that carried ewes and lambs the previous year. When this is not possible then a risk assessment should be carried out based on farm and pasture history, forecasts and the age of the lambs during the risk period, she said.

“Nematodirus is always there and eggs deposited during one spring/summer will develop in to infective larvae within a few weeks, but generally don’t hatch until the following spring.”

And the concern is that the performance of young lambs that receive an early season check in growth due to nematodirus may be compromised for the rest of the grazing season, with lambs taking longer to reach a market weight as a result, added parasitologist David Bartley of Moredun Research Institute.

“Nematodirus can be a major threat to productivity and survival in young lambs. Most losses in the UK occur in the spring when lambs start to graze pastures contaminated with parasite larvae.”

However, because it is the immature larvae that cause most of the damage to the lambs, faecal egg counts don’t always give an accurate indication of the problem, said Novartis Animal Health vet adviser, Thomas Tiley.

“Severe disease can be caused by immature worm larvae even before eggs appear in the faeces of the lambs, so planning treatments around egg counts can be risky.”

Instead Mr Tiley said farmers needed to assess the risk and in high risk areas to offer a total of three treatments with an effective anthelmintic three weeks apart. A white drench can also be used for nematodirus control as there is little or no resistance known. He also recommended ideally performing a drench test 10-14 days later to check the medication had worked.

But on farms with a reduced risk of nematodirus, only two treatments may be necessary. “Next month will likely be the peak time for nematodirus given recent weather conditions. But speak to your vet and develop a specific strategy for your own farm. Coccidiosis can also present similar symptoms to nematodirus and this is also something you need to consider and discuss with your vet.

Sustainable control of parasites: key risk factors

The Sustainable Control of Parasites key risk factors to consider

• Lambs grazing pasture that carried lambs the previous spring

• A sudden, late cold snap which is followed by a period of warm weather

• Lambs that are old enough to be eating significant amounts of grass (6-12 weeks olds)

• Groups where there is also likely to be a challenge from coccidiosis

• Lambs that are under other stresses eg triplets, fostered on young or older ewes