The Colorado Department of Agriculture said the additional cows were on two separate adjacent ranches in Logan County. Both died from the disease.

State Veterinarian Keith Roehr said all of the ranches involved share fences. He said the new cases likely are the result of cows grazing in an area with soil containing anthrax spores.

Neighboring herds have been vaccinated. No cows left the affected ranches so none entered the food supply.

Anthrax is caused by a bacterium that forms spores. It can lie dormant in soil for decades until ingested.

No human infections have been reported.

Ranch workers are taking antibiotics as a precaution.