By Tim Cocks

KAMPALA, Dec 4 (Reuters) - At least 75 Ugandans are now suspected to have contracted a previously unknown strain of the lethal Ebola virus, although the death toll in the east African country remains at 18, an official said on Tuesday.

Uganda confirmed on Sunday it had 58 cases of the virus, which causes symptoms including fever, vomiting, diarrhoea and bleeding. The infected include medical staff looking after Ebola patients, deputy health minister Dr. Emmanuel Otaala said.

"Since the outbreak started back in August, we have registered 75 suspected cases," he told reporters.

Otaala said all patients were in Uganda's western Bundibugyo district, bordering Democratic Republic of Congo, except one nurse being treated in the Ugandan capital Kampala who contracted it in Bundibugyo.

Otaala denied claims in the local press that the government knew of the outbreak but concealed it to avoid putting off the 53 heads of government and thousands of delegates -- including Britain's Queen Elizabeth -- who came to Kampala 11 days ago for the Commonwealth Heads of Government Meeting (CHOGM).

The government announced the Ebola outbreak four days after the summit ended, fuelling speculation of a cover-up.

"We did not keep quiet after learning that we were dealing with Ebola. The medical fraternity would not go to the expense of leaving people to die...because we are hosting CHOGM," Otaala said.

Genetic analysis of samples taken from some victims shows this virus is a previously unrecorded type of Ebola, making it the fifth strain, U.S. and Ugandan health officials say.

The World Health Organisation says it is concerned about the way the virus keeps mutating.

Victims of Ebola -- known to infect humans, chimpanzees and gorillas -- often die of blood loss, but the fever and dehydration can also kill. Otaala said this strain was less lethal than previous ones identified, which normally kill 50 to 90 percent of those infected.

"We have so far registered 18 deaths ... this is a 25 percent (death rate). That means we are dealing with a milder form of Ebola," he said.

Uganda was last hit by an epidemic of Ebola in 2000, when 425 people caught it and just over half died.

This year, an outbreak in Congo -- where some of the first recorded cases in 1976 gave the virus its name after the country's Ebola river -- infected up to 264 people, killing 187.

(Editing by Sophie Walker)

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