Published: Aug 15, 2012 7:03 PM EDT
Updated: Aug 16, 2012 6:30 AM EDT

LAKEWOOD, Colo. (AP) - Two more cows have died from anthrax exposure in northeast Colorado, expanding the first outbreak of the disease in the state in 31 years to three ranches.

Last week, 60 cows died on a Logan County ranch, where anthrax was positively identified in one animal. Officials say it's likely they all died of the disease.

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

State Veterinarian Keith Roehr said all three ranches involved share fences and 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 and no human infection has been reported, Roehr said.

Anthrax kills livestock within hours of infection and can decimate herds if animals are not quickly treated, he said.

Anthrax is caused by a bacterium that forms in spores and can lie dormant in soil for decades until ingested. Humans get anthrax most commonly through direct contact with infected animals usually when spores get into a cut or abrasion on the skin. Without treatment it can be fatal, but early treatment with antibiotics is very effective.

The personnel on the three affected ranches are working with their doctors and public health officials and are being given antibiotics as a precaution, Roehr said.

Agriculture officials are investigating the incidents and the Northeast Colorado Health Department is trying to ensure that anyone at risk for a possible exposure receives treatment.

All of the initially infected carcasses have been incinerated. Fire kills anthrax spores so burning is the best means to dispose of the carcasses, state agriculture officials said.

"At this point we really don't anticipate any health threat for the residents in northeast Colorado in regards to anthrax or from the smoke or plume from the incineration of the carcasses," Tony Cappello, the district public health administrator for the Northeast Colorado Health Department, said in a statement.

It's unclear why anthrax suddenly has surfaced after no reported cases in Colorado in more than three decades, Roehr said. The three ranches are in the South Platte River Valley, where floods followed by drought are common and the region has alkaline soil types, characteristics favorable to anthrax spores, he said.