(CNN)A mysterious affliction has killed as many as 18 people in southwestern Nigeria, leaving health officials scrambling to determine its cause. The cases have all occurred in Nigeria's Ondo state since April 13, health officials said Sunday. Dr. Dayo Adeyanju, Ondo's state commissioner for health, said 18 people have died and five people are being treated.  Those affected are between the ages of 25 and 60.  The disease does not appear to be contagious, he said. Symptoms include headaches, blurred vision, loss of sight and unconsciousness, Adeyanju said. Some suspect it may be the result of locally brewed alcohol. The World Health Organization, earlier Sunday, reported at least 13 people were killed and that there were 18 total cases. Those numbers were reported by Adeyanju released somewhat different totals. 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, WHO spokesman Tarik Jasarevic said.
18 dead and 5 being treated, Nigeria says .
Locally brewed alcohol is suspected .
Some patients have died within hours .