Ukraine preidential candidate Viktor Yushchenko left a Vienna clinic on Sunday and headed home, as officials reopened an investigation into his apparent poisoning.

A criminal probe into the poisoning was reopened after doctors at the Vienna hospital -- where Yushchenko was treated for a mysterious illness in September -- confirmed that he had been poisoned with dioxin.

The doctors did not say they were certain the poisoning was deliberate, but did say tests showed the poison was taken orally, and could easily have been added to something Yushchenko ate or drank.

The poisoning left Yushchenko, often described as having movie-star good looks, with a disfigured and partially paralyzed face.

Doctors say dioxin levels in his liver have returned to normal. Although his skin condition is unsightly, it is reversible, they added. But it could take more than two years for his skin to return to normal.

At a brief news conference on Sunday, Yushchenko said he was happy to be alive.

"They've spent many days and nights with me and I am very happy to be alive in this world today," Yushchenko said. "I thank these people for this."

He is returning to the campaign trail, where he is battling Russian-backed Prime Minister Viktor Yanukovych in a repeat of the Nov. 21 presidential runoff. A new vote was called for Dec. 26 following allegations of voter fraud.

"I am very happy that we were able to mobilize the Ukrainian community to stand up for its rights," he said.

Yushchenko didn't mention his previous claims that he was poisoned by Ukrainian authorities. But his wife said she wasn't surprised by the diagnosis.

"We had received threats before it happened and we continued to receive threats because I think there are many people who consider my husband and the changes he would bring to Ukraine of great threat to them personally," said Kateryna Chumachenko-Yushchenko.

Yushchenko checked back into the clinic on Friday to get an official medical conclusion about what's been ailing him since he attended a banquet in September -- one attended by the head of Ukraine's internal security service.

Yushchenko, 50, began vomiting the next day. A few weeks later, the dramatic changes in his face became apparent.

Medical experts have said Yushchenko's disfiguring condition appeared to be "chloracne," which is commonly linked to dioxin poisoning.

"There were high concentrations of dioxin, most likely orally administered. There is no doubt," said Dr. Michael Zimpfer, head of the private Rudolfinerhaus clinic, told reporters at a news conference.

Chloracne is similar to acne, and it caused by exposure to toxic chemicals. It usually results from occupational exposure to fungicides, insecticides and herbicides.

Last month, a Ukrainian parliamentary commission that investigated Yushchenko's illness said a forensic medical exam found no traces of "any biological weapons" in his system. They reportedly checked his blood, nails, hair and urine.

With files The Associated Press