A package containing West Nile virus (WNV) exploded on March 18 at a Federal Express building in Columbus, Ohio, near the Port Columbus International Airport, the Associated Press reported. Fifty workers were evacuated.

The shoebox-size sealed package probably burst because of dry ice used to preserve tissue samples from a bird with the virus. The package was being sent from the Ohio Department of Health to a researcher at the University of Texas, and it held brain and kidney tissue from a bird that had tested positive for the virus, according to Jay Carey, spokesman for the health department.

Samples were frozen, so the live virus was unlikely to become airborne. Officials said the only people at risk for infection were those with open wounds who came in direct contact with the sample material. Workers were allowed back into the building after four hours