The Mississippi Department of Health said in a news release that the new cases occurred in adults.
This year's three cases have been reported in Leflore and Coahoma counties.
One human case of LaCrosse encephalitis has been reported in Montgomery County. That mosquito-borne illness is similar to West Nile virus.
Symptoms of West Nile virus may include fever, headache, nausea, vomiting, a rash, muscle weakness or swollen lymph nodes.
In a small number of cases, infection can result in encephalitis or meningitis, which can lead to paralysis, coma and possibly death.