(Reuters) - Doctors in Houston have decided to keep former U.S. President George H.W. Bush in the hospital for a fourth night since his bout with shortness of breath, though his condition has improved and he "remains in high spirits," his spokesman said on Friday.

Bush, 90, was taken by ambulance to Houston Methodist Hospital on Tuesday night and admitted for observation after experiencing breathing difficulties earlier that evening, according to his spokesman, Jim McGrath.

There has been no word on whether the nation's 41st president is suffering from any particular ailment. McGrath said on Wednesday that Bush's prognosis was positive and that he was being kept hospitalized as a precaution.

On Thursday, Bush received a Christmas Day visit from his wife, Barbara, as well as his son Neil and daughter-in-law, Maria Bush, and was described as being in "great spirits."

"President Bush remains in high spirits and continues to make progress, but he will remain at the Houston Methodist Hospital this evening," McGrath said in an updated message on Friday.

Bush, who was president from 1989 to 1993, was admitted to the same hospital in November 2012 for treatment of bronchitis and related ailments. He was so ill at one point that he was believed to be near death but ultimately recuperated and was allowed to go home after seven weeks.

The former Republican president suffers from Parkinson's disease and cannot use his legs. But he celebrated his 90th birthday on June 12 by skydiving near Kennebunkport, Maine, with the Army's Golden Knight parachute team.

His eldest son, former President George W. Bush, this year published a best-selling book about his father, titled "41 - A Portrait of My Father."