KAMPALA - Some 330 people are being monitored for possible Ebola infection in western Uganda, health authorities announced Thursday, as the national death toll hit 22.

But authorities faced hurdles probing the extent of the outbreak in Bundibugyo district, home to 250,000 people and epicentre of the disease, with many villagers unwilling to cooperate with medical detectives, they said.

"We are monitoring 330 people in the community. These are the people who had contact or were near Ebola patients," said Sam Zaramba, the director of medical services.

"Ninety-three have fitted in the diagnosis of Ebola -- including 22 dead. 38 have been admitted to health centers in Bundibugyo where isolation units have been set up," he told AFP.

Because no one can explain how the disease entered Bundibugyo -- seven years after it killed at least 170 people in northern Uganda -- it is now the job of the US Centers for Disease Control (CDC) to scour the district for clues.

Virologists are convinced that the disease erupted in September, wending its way through the district unnoticed, even though the first cases arrived in ill-equipped hospitals around the same time.

Doctors then misdiagonised it, fostering the outbreak, until samples were flown to the Atlanta-based CDC laboratory, where it was identified as a new Ebola strain with a new set of behaviour and symptoms in late November.

By then, the disease had slipped through the medical chaos, obscured by the absence of its signature haemorrhage, thus affecting isolated groups in the district and perhaps far afield, Zaramba speculated.

"It has taken us three months to diagnose that this was Ebola. It presented itself differently from the known haemorrhagic fevers," Zaramba added.

It was instead characterized by high fever, abdominal pain, diarrhoea vomiting and headache," he told AFP.

Spread by body secretions -- mainly blood -- Ebola can be fatal for up to 90 percent of its victims, often killing them in shock.

The rare disease was named after a small Democratic Republic of Congo river where it was discovered 1976 and in Sudan later the same year. Other outbreaks have been recorded in Ivory Coast, Gabon and Uganda.