A professor at the University of Washington Medical School who moved to Botswana to help alleviate a shortage of doctors there, was killed when a crocodile dragged him from a canoe, his family and colleagues said.

Richard Root (68) was on a wildlife tour of the Limpopo River in remote north-eastern Botswana with his wife, Rita O'Boyle, on Sunday when it happened. The couple had been visiting a clinic in the area.

A nationally known expert in infectious disease and the former chief of medicine at Harborview Medical Centre in Seattle, Root went to Botswana to train health care workers to deal with HIV/Aids. Botswana's rate of HIV infection is at about 40%.

The move and his marriage last year had given him a new purpose in life after some difficult years, which included having bypass surgery, suffering with depression and caring for his previous wife until she died in 2001 of a neuromuscular disorder.

Root's son David Root, a Seattle architect, said he had spoken with his father on Saturday, and that he was happy about his work at Botswana's Princess Marina Hospital in Gaborone.

"Dad had gone through hell and had to take stock of his life," Root told the Seattle Post-Intelligencer.

Another son, Richard Root of Los Angeles, said his father, who had worked as a doctor in Iran in the 1970s, now wanted to dedicate himself to helping Africa.

Root taught at Penn in the early 1970s before moving on to Yale and then, in 1982, to Seattle. He first served as chief of medicine at the Veterans Administration Hospital, then took over the same position at Harborview in 1991.

He was former president of the American Federation of Clinical Research; editor in chief of a textbook, Clinical Infectious Diseases; and, from 1986 to 1991, he directed the National Institutes of Health's Aids Advisory Committee.

Steve Gluckman, medical director of the Botswana programme, said Root was in a lead canoe with the tour guides when the crocodile rose out of the water and grabbed him. He was not seen again. The tour guides were wary of hippos, but there had been no reports of crocodile attacks in the area. - Sapa-AP