Four farms on the Devon and Somerset border are being investigated after a suspected botulism outbreak.
It is believed that up to 40 cattle have died as a result of the infection in the Hemyock area.
Steps are being taken to contain the illness, which affects the muscles and causes lameness and poor coordination in the animals.
A spokeswoman for Defra said the source of the outbreak appears to have been poultry litter sold to the farms.
Isolated outbreak
"We believe the poultry litter was sold to a number of farms in the area for manure and that it contained dead chicken carcasses," she said.
"The Food Standards Agency (FSA) has ensured that there is no risk to human health in this particular instance, because no milk from the infected cows went into the human food chain."
John Dole, from the National Farmers Union, said he believed 40 cows had died so far.
"Accidents do happen, even with the best will in the world, but I would assess it's a totally isolated outbreak," he said.
'Precautionary measure'
The Food Standards Agency said on Thursday it was aware of "several incidents involving suspected botulism in cattle in Devon".
A spokesman went on: "There have been no reports that meat or milk from cattle affected by botulism can cause human disease, but as a precautionary measure meat and milk from the farms where outbreaks have been reported has been withheld from the food chain.
"The FSA is continuing its investigations in conjunction with the Veterinary Laboratories Agency."