Sen. Tim Johnson (D-S.D.) is resting comfortably following gallbladder surgery Tuesday morning.

Doctors at Avera McKennan Hospital and University Health Center in Sioux Falls originally thought a severe fever Johnson was suffering from was related to a reaction to an antibiotic, but they now believe his gallbladder was the culprit.

“The surgery went exceedingly well,” said Dr. Donald J. Wingert, Johnson’s surgeon.

“The Senator is resting comfortably. All of our findings point to the gallbladder as the primary problem, but it is still possible that he is having some lingering effects from the antibiotic drug reaction. We will keep the Senator in the hospital a few more days for observation,” Wingert added.

“Tim is in good hands and in good spirits. We want to thank everyone for their prayers and support,” Johnson’s wife, Barb, said in a statement.

Johnson, 63, suffered a stroke related to an arteriovenous malformation in 2006.