19 February 2015, 13:42 GMT
"Seventeen people have died of the mysterious disease since it broke out early this week in Ode-Irele town," the government spokesman for Ondo state, Kayode Akinmade, told AFP by telephone.
Laboratory tests have so far ruled out Ebola or any other virus, Akinmade added.
Common symptoms of the disease, which killed the victims within 24 hours of their falling ill, include headache, weight loss and blindness before death, World Health Organization spokesman Tarik Jasarevic told AFP in an email.
Health officials and experts from the government and aid agencies, as well as WHO epidemiologists, have already arrived in Ode-Irele.
Jasarevic said blood and urine samples had been taken from two victims and cerebrospinal fluid from another.
"All samples have been sent to Lagos University Teaching Hospital this morning, and results are still pending. Investigations are still ongoing," he added.