Irish sheep producers  hoping that natural immunity might help protect their sheep from Schmallenberg virus may have had their hopes dashed.

Results from a study by Dr Ian Nanjiani of Westpoint Veterinary Group will give local flock owners further cause for concern as the deadly virus spreads across the UK and Ireland.

Dr Nanjiani examined 10 sheep flocks in parts of the north, midlands and south of England where Schmallenberg disease is already a huge problem.

These flocks had either confirmed outbreaks of Schmallenberg during the past two years or were located adjacent to flocks where Schmallenberg was confirmed or, in the case of two flocks in Cumbria, had no known exposure to the virus.

A random sample of 60 breeding females was blood tested in each of the 10 flocks to assess the level of susceptibility and resistance to Schmallenberg.  The results dispel the perception that once a flock is infected with Schmallenberg up to 100 per cent of animals will have antibodies and are therefore naturally immune to the virus.

The study found that the number of sheep with antibodies ranged from 8.5 per cent to almost 75 per cent across the 10 flocks.  In two flocks in Kent, less than 30 per cent of the sheep tested had antibodies to the virus.  This is despite the fact that Schmallenberg has been rampant in Kent for the past two years.

Dr Nanjiani also measured the level of antibodies in different categories of breeding sheep within each flock. This revealed that only one flock had significantly higher levels of antibodies in ewes that in ewe lambs thus further demolishing the false hope that ewes exposed to the disease acquire natural immunity.

Also, in one of the flocks, the level of antibodies was significantly higher in barren than in pregnant ewes, raising questions about the role of the Schmallenberg virus in reduced fertility and early embryonic loss.

“This work was conducted in regions with different historical exposures to Schmallenberg, “explained Dr Nanjiani.

“Despite a small sample size the results do show that flocks with Schmallenberg-positive offspring can still have a significant population of susceptible animals after two seasons of Schmallenberg exposure. Vaccination remains the only way of controlling the problem.”

Schmallenberg virus is spread by midges and the risk period for ewes is day 20 to day 80 of pregnancy. Some cross-channel farmers closest to the continent had foetal deformities in up to 50 per cent of lambs conceived last spring. The same story unfolded on many farms in Cork and the South East of Ireland.