Continued warm, moist weather has vets worried stock will get facial eczema, Wanganui Veterinary Services owner Tom Dinwiddie says.
He is especially concerned for lifestyle block owners, who may be less aware of the disease and have fewer options for moving animals.
Facial eczema is caused by grazing animals eating the spores of a particular fungus as they graze grass. The spores produce a toxin that can eventually lead to liver damage and death.
Red, swollen skin and an avoidance of direct sun are early signs of it. The 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.
Wanganui Veterinary Services has been monitoring the number of those damaging fungus spores since early February.
Monitoring sites are at Westmere, Maxwell, Fordell, Ngaturi, and in the Turakina and Whangaehu valleys.
At first only Westmere and Maxwell had dangerously high levels. Now all the sites are up at 20,000 to 25,000.
Lambs should not be grazed for long on pasture with that many spores, and Mr Dinwiddie warned spore counts could double overnight if the weather stays warm.
At 40,000 to 50,000 spores calves are affected, and above 50,000 all stock will be – sheep, cattle, goats and especially alpacas.
The danger is worst in warm, moist, sheltered areas where there is dead vegetation and animals are grazing down to low levels.
Farmers were already taking preventive action, by spraying pasture with fungicide or giving animals zinc capsules or drenches as a remedy.
Those with other feed options can move stock to longer grass. Facial eczema is usually only a problem in warm coastal areas. In 1999 even farms in Taihape and Raetihi were affected.