Health authorities have issued a warning on Japanese encephalitis, a brain infection caused by a mosquito-borne virus.

The Korea Disease Control and Prevention Agency(KDCA) issued the warning on Thursday after the species of mosquitoes that transmit the virus was collected in Busan earlier this week. 

The agency issues a warning when the daily average number of mosquitoes that can carry the virus collected in two days is over 500 and that number is at least half of the mosquitoes collected overall.

According to the agency, nearly 86 percent of the mosquito samples trapped in Busan earlier this week were of the variety that could carry the virus that can cause brain inflammation.

The warning comes around two weeks later than last year. The nation sees about 20 cases a year of Japanese encephalitis on average, with 20 percent of patients aged 40 or older.