Standing water can be treated with EPA-approved larvicides to kill mosquitoes before they become adults.
Anton Sattler, with Bismarck Burleigh Public Health, says they set an all-time record for mosquito trap counts with 16,776 reported in mid-July.
Limit outdoor activities during dawn and dusk, when mosquitoes are most active.
DCHHS said there is now no vaccine or specific treatment for West Nile virus.
The Health Unit is urging the public to take steps to prevent mosquito bites.
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The state only had four cases until then, but reported 14 positive cases last week.
The mosquitos get the virus by feeding on infected birds.
Cover children's playpens or baby carriages with netting when outdoors.
Clean your gutters and flush birdbaths or fountains once a week. In North America, that means that mosquito season - extending from the late spring through the fall - is West Nile season. Wearing light-coloured, long-sleeved shirts, pants, socks and a hat is recommended, as is using insect repellent containing DEET or Icaridin.
The county's Health Unit reports that mosquitoes taken from a trap in the southeastern part of Mitchell have tested positive for West Nile virus. Of the 26 Dallas County residents infected past year, two died from the mosquito-borne illness.