Risk for West Nile Virus elevated to ‘moderate’ status after second batch of mosquitoes tests positive

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MANCHESTER, NH – The City of Manchester Health Department (MHD) announced today that an additional batch of mosquitoes collected in Manchester on September 7, 2021, has tested positive for West Nile Virus (WNV). This is the second positive batch of mosquitoes to test positive in Manchester this season. Per New Hampshire Department of Health and Human Services guidance, the level of risk for Manchester has been elevated to moderate. 

The MHD conducts routine mosquito surveillance (trapping and testing) every year until the first, killing frost. As with each season, if there is evidence of ongoing mosquito-borne illness, and the risk level moves to ‘high’, the Department will facilitate spraying for adult mosquitoes as allowed by its “Special Permit” on-file, with the Department of Agriculture. According to Anna Thomas, MPH, Manchester’s Public Health Director, “It is important for residents and visitors to take steps to prevent mosquito bites, as WNV and other arboviruses can be transmitted to humans from the bite of an infected mosquito. From this point in the season, until there is a mosquito-killing frost, the risk of infection by mosquito-borne viruses may increase. We are recommending that all residents use an effective mosquito repellant that contains 30% DEET, avoid being outdoors at dawn and dusk, and remove standing water from around the home, where mosquitos reproduce, to limit the risks of catching these infections.”

West Nile Virus is transmitted through the bite of an infected mosquito. WNV was first identified in New Hampshire in August of 2000. Symptoms usually appear within a week after being bitten. Symptoms can include flu-like illness including fever, muscle aches, headaches, and fatigue. Many people may not develop any symptoms, or only develop very mild symptoms. A very small percentage of individuals infected with WNV can go on to develop more serious central nervous system disease, including meningitis or encephalitis. If you or someone you know is experiencing flu-like symptoms, including fever and headache, contact your local medical provider.

Manchester residents and businesses can call the Manchester Health Department with concerns and questions about WNV and EEE. The phone number is 624-6466 and ask to speak to the Environmental Health Branch. Prevention guidelines for WNV and other arboviruses can be found below. Fact Sheets on West Nile virus and other arboviruses are available on the DHHS website at www.dhhs.nh.gov. For more information, please visit the Centers for Disease Control and Prevention website at www.cdc.gov.


  1. Eliminate standing water and other mosquito breeding locations around your property. Please do not attempt to drain or alter natural waterbodies for mosquito control, since the management of ponds and wetlands is regulated by the Department of Environmental Services and any planned alterations will require a permit before work may begin. In warm weather, mosquitoes can breed in any puddle that lasts more than four days.
  • Remove old tires from your property. 
  • Dispose of tin cans, plastic containers, ceramic pots, or other containers. Do not overlook containers that have become overgrown by aquatic vegetation. 
  • Drill holes in the bottom of recycling containers that are left outside. 
  • Make sure roof gutters are clean and draining properly. 
  • Clean and chlorinate swimming pools and hot tubs. If not in use, keep empty and covered and keep covers free of standing water. 
  • Aerate garden ponds or stock them with fish. 
  • Turn over wheelbarrows and change water in birdbaths at least twice weekly. 
  • Turn over plastic wading pools when not in use. 
  • Remind or help neighbors to eliminate breeding sites on their properties.  
  1. Be aware of where mosquitoes live and breed. Keep them from entering your home.
  • Mosquitoes lay their eggs in standing water. Weeds, tall grass, and bushes provide an outdoor home for adult mosquitoes, including several species commonly associated with West Nile virus and Eastern Equine Encephalitis. 
  • Mosquitoes can enter homes through unscreened windows or doors or broken screens. Make sure that doors and windows have tight-fitting screens. Repair or replace all screens in your home that have tears or holes. 
  • Resting mosquitoes can often be flushed from indoor resting sites by using sweeping motions under beds, behind bedside tables, etc. and once in flight, exterminated prior to sleeping at night.
  1. Protect yourself from mosquito bites.
  • If outside during evening, nighttime, and dawn hours when mosquitoes are most active and likely to bite, children and adults should wear protective clothing such as long pants, long-sleeved shirts, and socks. 
  • Consider the use of an effective insect repellent, such as one containing DEET. A repellent containing 30% or less DEET (N,N-diethyl-methyl-meta-toluamide) for children and adults. Use DEET according to the manufacturer’s directions. Children should not apply DEET to themselves. Repellents that contain Picaridin, oil of lemon eucalyptus, or IR3535 have also been determined to be effective. 
  • Vitamin B, ultrasonic devices, incense, and bug zappers have not been shown to be effective in preventing mosquito bites.