© Stephen StockwellA mystery disease is killing cattle in the west of New South Wales
Hay district veterinarian, Colin Peake, says the illness was first detected in October last year.
A similar disease hit the Western Division in 2006 and again in 2009.
Two farms near Hay and one at Mossgiel have been affected.
Mr Peake says he is perplexed by the deaths with tests returning inconclusive results.
"There's a lot of diseases that have been ruled out, even lead poisoning has been ruled out," he said.
He says farmers should report any signs of the disease as soon as possible.
The fatal disease is most commonly affecting calves that are dying close to water, possibly because of fever.
Mr Peake says farmers have reported losses of more than half of their herds and symptoms include drooling and diarrhoea before dying.
"A lot of the cattle are either dead near a water course or water tanks or troughs," he said.
"I think they've got a fever or they're very hot, they're trying to get to somewhere where they can cool down."
Mr Peake says if the illness persists for more than two weeks it spreads to older cows.
Farmers should report any signs of the illness as it is more easily identified in sick cattle or shortly after death.
The Department of Primary Industries is investigating the disease and says it is developing a questionnaire to examine the extent of the disease.
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