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West Nile Virus

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West Nile Virus (WNV) is an illness caused by a virus carried by infected mosquitoes. Mosquitoes become infected when they feed on infected birds of the Corvid family, such as ravens, crows, jays and magpies. The mosquito can then transmit the infection to humans and animals by biting them. You cannot get the virus from another person or from an animal.

Symptoms of West Nile Virus infection

Most people who become infected with the West Nile Virus do not become ill. Only about 20 percent develop West Nile fever, which can include fever, headache and body aches, and, occasionally, swollen lymph glands and a rash on the trunk of the body. Symptoms begin 3-14 days after the bite of an infected mosquito. Mild cases last only a few days.  If you have questions about WNV symptoms, call your healthcare provider.

Only about one in 150 of those who become infected develop a severe infection (called West Nile encephalitis or meningitis). Severe infection is relatively rare and more common in people over 50. In 2005, there were 1,272 cases and 116 deaths in the United States, including 8 cases. There were no deaths in Oregon. (To put that number in context, influenza kills about 30,000 people in the United States each year.)

Symptoms of severe infection may include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. Symptoms of severe disease may last several weeks. Some brain and nerve damage may be permanent. Unfortunately, there is no effective treatment for WNV infection other than supportive therapy, such as good nursing care and hospitalization with intravenous (IV) fluids, airway management, respiratory support (ventilator) if needed and the prevention of secondary infections (pneumonia, urinary tract, etc.).

Preventing West Nile Virus

The only effective strategy against WNV is prevention. There are two basic prevention strategies: reducing the mosquito population and avoiding mosquito bites. The best way to reduce the mosquito population is to drain sources of standing water by early April. This will prevent mosquitoes from laying eggs and breeding.

  • At least once or twice per week, empty water from flowerpots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels and cans.
  • Check for clogged rain gutters and clean them out.
  • Remove discarded tires and other items such as cans, bottles and other containers that could collect water.
  • Be sure to check for containers or trash in places that may be hard to see, such as under bushes or under your home.
  • Check around faucets and air conditioner units and repair leaks and eliminate puddles.
  • Cover boats or store upside down or in a dry place.
  • Where feasible, fill all holes in the trees on your property.
  • When swimming season is over, empty and store wading pools in a dry place. Cover large pools and check frequently to ensure against mosquito breeding. Pool chemicals like chlorine at ordinary concentrations will not kill mosquito larvae. Keep pool water circulating and run filters from April through October.
  • Call the Clackamas County Vector Control District (503) 655-8394 to have mosquito larvae eating fish (Gambusia affinis) placed in your pond. This is a free service, but supplies are limited.

Avoiding Mosquito Bites

  • Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.
  • When possible, wear long-sleeved shirts and long pants outdoors.
  • Place mosquito netting over infant carriers
  • Install or repair window and door screens.
  • Apply insect repellent containing DEET (N,N-diethyl-meta-toluamide) when you're outdoors. Be sure to read and follow the manufacturer's directions for use. Alternatively, spray clothing with DEET since mosquitoes may bite through thin clothing. If you spray your clothing, there is no need to spray repellent containing DEET on the skin under your clothing. Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.

Detecting West Nile Virus in Clackamas County

Dead Corvid birds, such as crows, jays or magpies in an area may indicate that WNV is circulating between the birds and the mosquitoes in that area. The public can play an important role in helping to monitor for WNV. Clackamas County Vector Control staff can pick up specimens. To reduce the cost and increase the efficiency of our detection system, use these criteria when calling us to pick up birds for testing:

  • The birds should be Corvids (crows, jays and magpies) that have been dead less than 24 hours.
  • The birds must be part of a continuous die-out over 1-2 days.
  • Birds killed on the road, in parking lots, or deaths related to pesticide spraying or injuries from other animals will not be tested.
  • Clackamas Vector Control staff (503) 655-8394 will provide information on how to collect the specimens.
  • For wild birds other than crows or blue jays, call the Oregon Department of Fish and Wildlife at 503-657-2000.
  • For information about sick or dying domestic birds, such as poultry, call a vet first or call 503-655-8386.
  • Once WNV has been detected in Clackamas County, we will probably not need to continue bird testing.

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