President George H.W. Bush and wife Barbara attend the premiere of HBO's new documentary on his life. (Charles Krupa/AP)

Former president George H.W. Bush remains in a Houston hospital with a rising fever, his office confirms.

Spokesman Jim McGrath said in a statement Wednesday evening that doctors put Bush in the intensive care unit Sunday and on a liquid diet. As of Thursday morning, there was no update on Bush's condition, but doctors are cautiously optimistic.