“The prevalence in western Victoria has increased rapidly over the last three to four years and Animal Health Australia graphs suggest that 32 per cent of sheep properties in Victoria are infected with OJD,” Dr Hucker said at the Pfizer Animal Health seminar.

“The area from Melbourne through to the South Australian border - western Victoria – is where probably the bulk of these infected properties are,” he said.

Sheep movements were undoubtedly the main reason it has spread and all producers should vaccinate against the disease, he said.

Pfizer Animal Health sales representative Dr Steve Cotton said even at $2.20 per dose given to replacement ewes lambs at 4-16 weeks-old and the ewes were kept till six-years-old the annual cost would 36 cents. The Gudair vaccine was available from veterinarians, DPI and from stock and station agents, he said.

Hamilton farm business adviser Dr Graham Lean said vaccination in a fine wool Merino flock was profitable at a range of wool prices but at average prices the profitability was about by 60 cents a dry sheep equivalent “clean profit”.

“That equates to about 5pc long-term net profit for wool flocks.”

In prime lamb flocks, vaccination on average could increase net profits by 10pc and was profitable at low meat prices. The break-even mortality rate to cover vaccination costs was 1.4pc in wool flocks and 0.4pc in prime lamb flocks.

Mr Lean said average death rates in OJD-endemic flocks had been shown to be 6.8pc and up to 14pc over 15-20 years. But early vaccination of the whole flock - including Merino wethers, but not terminal lambs - could limit annual losses to 2pc, declining to lower levels over 20 years.

There was also potential for animal welfare concerns by not taking action and the “non-stop grind” of OJD sheep deaths was depressing for staff, he said. Dr Lean said current diagnostic tools for OJD were too slow and not accurate enough to identify the disease.