GENEVA, Nov 2 (Reuters) - An outbreak of Rift Valley Fever in Sudan has struck at least 125 people, killing 60 of them, the World Health Organisation (WHO) said on Friday.

The United Nations health agency said two weeks ago that it was investigating a deadly outbreak in Sudan suspected to be yellow fever, but laboratory tests have shown it was Rift Valley Fever, WHO spokesman John Rainford said.

"There are 125 human cases and 60 deaths," he told Reuters.

More investigation was needed into the outbreak, which had erupted in White Nile, Sennar and Jazeera provinces in central and eastern Sudan, including the exact timing of the first cases, Rainford said. "Right now we don't have a clear picture."

Rift Valley Fever virus, transmitted by contact with the blood or organs of infected animals, can also be carried by mosquitoes, according to the WHO. Herders, farmers, veterinarians and slaughterhouse workers are deemed at higher risk of infection from the disease, which can devastate livestock.

While most human cases are relatively mild, a small percentage of patients develop a much more severe haemorrhagic form which can cause them to vomit blood or pass it in their faeces. Bleeding from the nose or gums can also occur.

The nearly 50 percent fatality rate was "very high" compared to the usual expectations for Rift Valley Fever, but it was likely that many more mild cases have not been detected in Sudan, according to Rainford.

"The Sudanese government has been highly cooperative and transparent in sharing information," Rainford said.

WHO officials were in the area of the outbreak and the U.S. Naval Medical Research Unit NAMRU-3 laboratory in Cairo had helped with the laboratory analysis, he added.

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