A virus spreading through a zoo in Chippewa Falls, Wis., has killed two more bison, WQOW-TV reports.
The Irvine Park Zoo has now lost four of the animals to the virus, which officials believe was spread by sheep that once shared a fence-line with the bison.
The Leader-Telegram says the virus -- known as malignant catarrhal fever -- causes an inflammation of the mucus membrane, and has no cure.
The paper says the first bison died on Sept. 20 and another died the following day.
The zoo remains open to the public, but officials are asking patrons to keep their distance from the bison.
Officials say the Bison become stressed when there are a lot of people around, which makes it more difficult for zoo staff to lead the animals to their stalls.