The animal disease facial eczema is rampant in the Whanganui region this year and one farmer has already had to destroy 100 sheep, Wanganui Veterinary Services general manager Tom Dinwiddie says.
The worst affected areas are Westmere and Maxwell but cases are showing up far inland in the Mangamahu and Waitotara valleys.
Facial eczema is a disease caused by the spores of a fungi that grows on dead vegetation in sheltered, humid places. The spores release toxins that can cause fatal liver damage.
Red, swollen skin and an avoidance of direct sun are early signs of it. Animals will also be restless, and shake or rub their heads and bodies. This can eventually rub their skin off, making them vulnerable to fly strike.
Facial eczema is not contagious or a danger to humans. But badly affected animals cannot be eaten, or sold. This year could rival 1999, the previous worst ever year for the disease in New Zealand.
Mr Dinwiddie said some people lost 500 sheep that year - and it could happen again.
Even animals in Taihape and Raetihi - where it's usually too high and cold for the fungus - are being affected.
"We're getting eczema in places where the farmers would say that they don't get eczema."
Farmers with a lot of stock affected can be traumatised by financial loss, and also by the pain their animals suffer.
Spore counts have dropped slightly this week, with the onset of cooler weather. But they are at nearly 200,000 in Westmere and Maxwell, and they have been over 100,000 at four other places in the region - an extreme risk level.
If temperatures stay up the risk level could continue high for another three weeks, Mr Dinwiddie said.
In South Taranaki dairy cattle are starting to be affected. Taranaki Veterinary Services vet Stephen Hopkinson said the spore count there has hit a peak late in the season. Clinical signs are just starting to appear, and may be lessened by precautions farmers have already taken.
This includes spraying pasture to kill the fungus or giving animals extra zinc to fight the spore toxins. But some have been too slow, or unaware of the danger. By the time stock are visibly affected the damage has already happened, two or three weeks earlier.
Stock on pasture high in spores should be moved to a shady paddock, and fed on either unaffected pasture, hay or silage. Their livers can regenerate from up to 70 per cent damage, provided their intake of spores stops.
This year's unusually warm March increased spore numbers. What's needed now is a cold southerly change or heavy rain to kill the fungus or wash the spores into the ground.