West Nile Virus

Are Wetlands A Threat?

Healthy wetlands are not uncontrolled breeding grounds for mosquitoes. Healthy wetlands sustain numerous species of mosquito-eating fish, amphibians, insects and birds, all of which help limit mosquito populations.

The principal mosquito carrier of West Nile virus on the East Coast, Culex pipiens, does not prefer to reproduce in most wetlands. These species reach greatest numbers in large urban centers, breeding easily in artificial containers – birdbaths, discarded tires, buckets  - and in human created environments, such as clogged gutters, animal waste lagoons and sewage effluent,  Adapted to polluted habitats, these Culex species generally avoid swamps and salt marshes altogether.

Damaged or degraded wetlands can provide ideal habitat for some mosquito species that carry West Nile.  Excess nutrients in contaminated waters can spur microbial growth and cause harmful algal blooms, which feed mosquito larvae.  Filling or draining wetlands may also increase mosquito predators.

Sometimes, even healthy wetlands may harbor large numbers of mosquito species that carry WNV.  Unlike Culex pipiens, Culex tarsalis, the major WNV vector in western states prefers to breed in clean water.  Therefore, it may be necessary to us appropriate mosquito control measures to prevent WNV disease transmission.

   As of Sept. 2021 - 4 cases of this disease have been identified in the Boston suburb area.

Disease Transmission: 

Mosquitoes are the primary vectors of West Nile Disease, meaning they carry the vius from host to host.  While nectar is their primary food source, females take blood in order to develop their eggs.  Mosquito activity is reduced in colder months, but the virus may still persist in dormant mosquitoes and eggs that survive winter.

The Centers for Disease Control and Prevention (CDC) reports that 43 mosquito species in the U.S. have tested positive for West Nile virus.  The most common carriers are the House mosquito on the East coast and Culex tarsalis in the West.  Because it readily feeds on humans, Culex salinarius is also an important vector.

Since mosquitoes primarily infect birds, unuaual bird deaths may signal a West Nile virus outbreak and should be reported to appropriate authorities.  Based on 2001 and 2002 data, the CDC reports that counties that report WNV-infected dead birds early in the transmission season are more likely to report subsequent disease cases in humans. 

Protection of Home and Community:
  1. Eliminate stagnant water – tires, garden pots and birdbaths, etc.
  2. Protect wetlands from pollution – runoff from farms, lawns and roads with buffers
  3. Check stormwater systems – catchments are properly designed and maintained
  4. Install screens.
  5. Consider using pesticides
  • How many people have become infected with WNV? 
    • In 2003 9,858 human cases were recorded and 262 deaths attributed  to WNV nationwide.
  • Will draining filling a wetland prevent WNV? 
    • This is not a necessary or appropriate way to control mosquitoes or WNV.  
  • How far do mosquitoes travel?
    • Many of the mosquitoes that transmit WNV have very short flight ranges.  Therefore, eliminating backyard mosquito habitat can help to control mosquito populations.