Suffolk County health officials say West Nile virus has been detected in four mosquito samples and in two dead birds.
No humans have tested positive for West Nile this year, but Suffolk County health commissioner James Tomarken is urging residents to cooperate with efforts to contain the virus.
West Nile virus is transmitted to humans by the bite of an infected mosquito.
Most of those who are infected experience mild or no symptoms, but some develop symptoms including fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.
In rare cases the virus can be fatal.
Health officials are urging Suffolk County residents to eliminate stagnant water where mosquitoes can breed.