GENEVA, Nov 16 (Reuters) - A mystery illness in Angola that has made at least 370 people sick may be linked to exposure to a toxin, but lead and mercury poisoning have been ruled out, the World Health Organisation said on Friday.

Investigations are continuing into the outbreak which began in early October in Cacuaco, north of Angola's capital Luanda, WHO spokeswoman Fadela Chaib said.

The WHO said last week that four people had died and 200 had become ill for an unknown reason. Chaib said the outbreak had since grown and the new death toll was not clear.

"More and more people have fallen ill but we have not been able to identify the cause of their sickness. It might be exposure to a toxic substance, something in the environment," she told a news briefing. "But the possibility of an infectious disease is not yet excluded," she added.

The illness, whose symptoms include fever and extreme drowsiness, has mostly struck children and adolescents.

The WHO said on Friday that patients showed neurological abnormalities, suggesting a "toxicological cause". No causal agent has been identified.

"Tests for cadmium, lead, mercury and manganese have shown levels within the normal range," the United Nations agency said.

Patients recovered slowly over a few days, but many were unable to walk unaided, according to the WHO.

Samples from affected patients had been tested for 300 organic solvents, and a general drug screen covering 800 compounds had also been carried out. Tests have also been done on local food and drinking water, and results have not yet come in, the WHO said. (Reporting by Stephanie Nebehay, Editing by Laura MacInnis)

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