MEXICO CITY (AFP) -- Mexican health authorities said Monday they have confirmed four cases of West Nile virus in the northern part of the country, the first cases so far discovered in Mexico.

One of the cases of the mosquito-borne virus was found in Sonora state and the other three in Chihuahua, said Oscar Velazquez, director of the National Center for Epidemiological Monitoring.

Velazquez recommended residents in affected area remove sources of standing water in which mosquitoes breed.

Chihuahua borders on the United States, where the virus killed 284 of the 4,156 people infected in 2002 and is currently experiencing a resurgence.

West Nile virus is generally transmitted by a mosquito bite. Five days after being bitten an infected person develops a light fever, which lasts three to six days.

In one out of 150 cases, the virus can cause a meningoencephalitis, resulting in death. The elderly and those with weak immune systems are most vulnerable.