Doctors investigating mystery illness that has killed at least 13

A mysterious affliction has killed at least 13 people in southwestern Nigeria, leaving health officials scrambling to determine its cause.

The 18 cases have all occurred in Nigeria’s Ondo state since April 13, the World Health Organization said Sunday.

The symptoms common in all cases are headache, loss of sight and restlessness, said WHO spokesman Tarik Jasarevic. Death has followed in most cases within six to eight hours.

Investigators sent samples of blood, urine and spinal fluid to a university in the city of Lagos for tests, which ruled out infections from viruses or bacteria, the WHO said.

Doctors plan to carry out toxicological tests on one of the victims who died in a hospital, Jasarevic said.

The WHO said it is closely monitoring the situation and participating in meetings held by the Nigerian Ministry of Health.