Farmers in Waikato are being warned of a "sleeping giant" in the form of sub-clinical facial eczema.
The disease breaks out each summer mainly in the North Island, and can result in skin and liver damage in livestock. In some cases death can occur.
Agricultural consultant Sue Hagenson from FarmWise says low feed levels - due to a very dry March followed by an early winter chill and not enough rainfall in the region - have resulted in cows that are generally underweight.
She says skinny cows could have a sub-clinical form of facial eczema.
Sub-clinical facial eczema has no external signs and is very hard to detect.