A month after the unexpected blizzard moved through western South Dakota, state veterinarian Dustin Oedekoven shares the cause of death for so many cattle lost in the storm.

Cattle losses initially estimated by the state veterinarian to be between 10,000 and 20,000 was later increased to as many as 30,000 cattle.

A month after the storm moved through South Dakota, Oedekoven has verified 13,977 cattle deaths and says most of the livestock died of congestive heart failure brought on by stress.

Cows in the cold weather likely got hypothermic, forcing their cardiovascular systems to work overtime. Oedekoven says the cows then suffered from hypertension, or high blood pressure in their lungs.

“It actually caused pulmonary edema and basically caused those lungs to fill with water or fluid,” Oedekoven told the Rapid City Journal.

Most of the cattle died on dry land, however some wandered in the snow and ended up in waterways or stock dams. Others were buried in snow banks.