The deadly Schmallenberg virus has claimed large number of lambs in some Pembrokeshire flocks, with one farmer losing nearly half of all the progeny produced by his commercial ewes.

The Animal and Plant Health Agency (APHA) has confirmed a spike in cases in sheep and cattle this year, particularly in early lambing sheep flocks.

Will Evans’ commercial ewes are among these, with 97 dead lambs with deformities typical of Schmallenberg.

Most were one of a twin, with the surviving lamb born healthy.

Mr Evans said lambing had been a very stressful period with these losses combining with weeks of heavy rain delaying turnout which added to the financial strain and increased disease burden associated with housing.

“Losing nearly half of the commercial lambs to Schmallenberg has been a real blow,’’ he said.

The sharp rise in cases this year is thought to be the result of warm and wet weather in late autumn presenting ideal conditions for biting midges, which transmit the disease.

Many have been in early lambing ewes, which were tupped when the midges were at their most active.

The most susceptible stage for foetal deformities is days 25-50 of gestation in sheep, with older foetuses able to clear the virus.

Mr Evans, whose lambing period starts in January, said he may have to rethink his entire lambing system, and forego higher prices for new season lamb, by tupping later when there is less of a risk.

“If I did delay the start of lambing it would need to be March because this year I was still getting cases of Schmallenberg in February.

“It would make it very tricky because March is already very busy because that’s when the suckler cows start calving.’’

Once ewes have been infected they become immune to the virus so future lambings are unlikely to be affected but replacements - ewe lambs and yearlings lambing for the first time – would have no immunity.

Mr Evans lambs around 100 yearlings a year.

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The true extent of infection in Pembrokeshire is unclear because farmers don’t always involve their vets as a ewe is often able to deliver a lamb with Schmallenberg without veterinary assistance.

Mike John, of Fenton Veterinary Practice, Haverfordwest, has heard anecdotal reports of large numbers of cases in some sheep flocks and cattle herds but his practice had only seen small outbreaks on clients’ farms.

“We have had sporadic cases but not on the levels we have heard farmers discussing, when they have spoken of big losses during lambing or terrible scanning results which happens when there is infection,’’ he said.

“We’ve also heard of flocks that have delivered fewer lambs than they have been expecting from their scanning results because of the loss of pregnancies.’’

The last significant wave of Schmallenberg happened in 2016-17, when APHA confirmed 141 sheep flocks and 76 cattle herds in Wales, England and Scotland had tested positive for the virus.

A vaccine is currently not commercially available, but Mr John said one could become available this summer.

“If it is approved for use in the UK, as seems likely, the primary course will involve two shots four weeks apart, which would need to be given to the ewes prior to tupping,’’ he said.