Vets have reported Bovine Ephemeral Fever, or three-day sickness, is spreading rapidly in the Hunter and Mid North Coast regions.
The Hunter Local Land Services said laboratory testing had confirmed the disease in cattle near Wingham and Gloucester in recent days.
Cases have also been reported at Hannam Vale, Moto, Dyers Crossing, Cooloongolook, Stroud, Dungog, Gresford and Seaham.
The virus, which is transmitted by insects, causes a high fever and pain in the muscles and joints. Cattle that have previously been exposed will have developed an immunity to the disease.
Locally-bred cattle under two years old are at the highest risk.
Cattle of any age which have been brought into the area from southern and western parts of NSW or from Victoria, Tasmania or South Australia, where immunity from previous exposure is unlikely, could also be affected.
Hunter LLS district veterinarian Lyndell Stone said sick cattle were often by themselves, off their feed, seeking shade and water, shivering, drooling and could show signs of lameness.
"These signs usually last only a few days and most cattle recover uneventfully. However, reports from Queensland, where the disease was seen earlier in the season, suggest that cases this year seem to be more severe," Dr Stone said.
Some cattle, especially bulls and heavier conditioned cattle, may go down and take several days to get back on their feet, increasing the risk of secondary complications.
Pregnant cows may abort and bulls may become infertile for up to six months. Milk production can drop significantly in lactating cows.
Dr Stone said vaccines were available but protection required two doses at least two weeks apart, meaning cattle owners in or close to areas where the disease was already present had probably left it too late.
"With BEF now spreading in the region, producers are advised to monitor cattle daily and seek veterinary advice if they are concerned," Dr Stone said.
"Medication is highly effective in bringing down the fever and reducing the muscle and joint pain, improving recovery timeframes and resulting in less weight loss.
"There are several other diseases that may resemble three-day sickness requiring alternative treatment, so veterinary diagnosis is recommended."
The LLS recommends providing affected cattle with shade and water, and feed once they regain an appetite.
Cattle unable to get up after a day or two should be turned or lifted twice daily to help prevent secondary complications.