Abuja, March 4 : The death toll from Nigeria's Lassa fever outbreak since the beginning of the year had risen to 104, local health authorities said. In its latest update on the outbreak which has spread to 88 local government areas across 22 states in the country, the Nigeria Centre for Disease Control and Prevention (NCDC) on Friday said the death toll climbed to 104 as of February 26. A total of 636 confirmed cases have been so far recorded, the NCDC said, noting with the latest death toll, the case fatality rate had also risen from 16 per cent earlier recorded to 16.4 per cent, Xinhua news agency reported. According to the World Health Organisation, Lassa fever is caused by the Lassa virus. Humans usually contract the virus through exposure to food or household items contaminated by the urine or faeces of infected Mastomys rats. The disease is endemic in the rodent population in parts of West Africa. In some cases, Lassa fever has similar symptoms to malaria, appearing between one and three weeks after exposure to the virus. In mild cases, the disease causes fever, fatigue, weakness and headache. Last year, the death toll of Lassa fever in Nigeria topped 170 from nearly 1,000 cases amid intensified measures by the government to reduce infections. /IANS