The Colorado Department of Agriculture says the additional cows were on two separate adjacent ranches in Logan County. Both died from the disease.
No human infections have been reported. Ranch workers are taking antibiotics as a precaution. So are some of the 25 CSU faculty, staff and students who were working in a vet school lab when veterinarians did a necropsy on a dead cow, finding the anthrax.
The school says their risk of infection is low but all have been notified they were exposed and are being advised on how to watch for symptoms.
"This is a very isolated incidence. It happens like it usually happens where people that came in contact with the cattle got infected themselves," 9NEWS Health Reporter Dr. John Torres said. "Human to human transmission hasn't happened and that's when we start getting worried about it and it hasn't happened at this point."
State Veterinarian Keith Roehr says all of the ranches involved share fences. He says 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, which are now under quarantine to prevent the cattle from entering the food supply.
Anthrax is caused by a bacterium that forms spores. It can lie dormant in soil for decades until ingested.
The 1,100 remaining cattle on one of the properties have all been vaccinated.
This is the first time anthrax has been found on land in Colorado in more than 30 years.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)