A mysterious illness that has left at least 18 people dead in a matter of days in Nigeria’s southwestern Ondo state has sparked fears of a new epidemic just six months after the country was declared Ebola-free. However, the World Health Organization has now said there is no reason to suspect the deaths are a result of an infectious disease like the Ebola virus, which left more than 10,000 dead in West Africa.

The "current hypothesis is cause of the event is herbicides,” WHO spokesman Gregory Hartl said in a Tweet Sunday. "Tests done so far are negative for viral and bacterial infection," he added.

Health officials have been scrambling to identify the cause of the illness that has affected at least 23 people in the town of Ode-Irele since last Monday, leaving 18 dead within 24 hours of the onset of symptoms. Victims complained of headache, blurred vision and a loss of consciousness.

The state government of Ondo set up an emergency response task force to track and monitor everyone who had come into contact with the dead to contain the illness while experts from government and aid agencies arrived to investigate the deaths. Investigators had sent samples of blood, urine and other bodily fluids to the Lagos University Teaching Hospital for tests, WHO said.

Preliminary laboratory results ruled out Ebola or any other virus, Ondo state government spokesman Kayode Akinmade told Agence France-Presse Sunday.

Nigeria’s federal government had previously banned the sale of 30 agricultural chemicals after discovering in 2008 the pesticides were causing food poisoning resulting in deaths in a number of states. The country’s food and drug agency issued a recommendation at the time warning Nigerians to stop using chemicals unless they were specifically approved by the government.