Most of the cases have been reported along Costa Rica's Pacific Coast, an area popular with foreign tourists. Officials said about 2,500 cases of dengue have been reported there -- eight times more than last year.
Health authorities said they have been baffled by this recent outbreak, which has occurred despite a spate of unusually dry weather. The mosquitoes which carry dengue usually proliferate in times of heavy rain.
Dengue affects between 50 and 100 million people in the tropics and subtropics each year, resulting in fever, muscle and joint ache.
The disease is caused by four strains of virus that are spread by the mosquito Aedes aegypti. There is no vaccine, so medical authorities in the region have been trying to stamp it out by focusing on mosquito control.
The illness can be fatal, developing into hemorrhagic fever which can lead to shock and internal bleeding.