The district has had one spike in facial eczema spores and continued hot still weather will bring another, Wanganui Veterinary Services general manager Tom Dinwiddie says.
Meatworks are already finding this season's lambs with clinical signs of the disease, including liver damage.
There was a sudden spike in facial eczema spores in mid-February, when the days and nights were hot, still and humid. The highest reading came from the Whangaehu Valley river flats - 140,000 spores per gram of pasture.
"That is very, very dangerous," Mr Dinwiddie said.
Average readings across the district are now at 60,000 to 70,000 spores, with the highest now at Maxwell, 90,000. Vets in Hunterville and Taihape have also reporting high readings.
The spore counts are highest from January to May, and can spike in March and April. The final few weeks of March are often the worst.
It's been a tough season for livestock in other ways too, with extra hot days and a lot of flystrike.
Facial eczema is a disease caused by the spores of fungi that feed on decaying vegetation. It affects sheep, cattle and deer.
The spores contain a toxin that stop the animals' livers functioning. The outward signs include lower production, skin irritation and peeling and animals avoiding the sun. Severely affected animals can die.
The spores are more common when weather is warm, damp and still.
The disease can be financially devastating to farmers - and hugely stressful. It was particularly bad in 2016.
Farmers are remembering that, Mr Dinwiddie said. Many have taken steps to prevent it - by spraying fungicide on pasture, moving stock to pasture with low spore counts, selling stock early or giving animals the zinc capsules that will protect them for six weeks.
"I think every farmer should have a plan, and keep monitoring the spore levels," he said.