An outbreak of West Nile virus occurs when the virus widely circulates among mosquitoes, leading to an increased risk of human infection. This mosquito-borne disease is the most commonly reported in the continental United States. Its presence in local mosquito populations prompts public health concern and action.
How a West Nile Virus Outbreak Develops
West Nile virus maintains itself in nature through a cycle involving mosquitoes and birds. Culex mosquitoes become infected when they feed on birds carrying the virus. Birds act as amplifying hosts, allowing the virus to multiply to high levels in their bloodstream, which then allows mosquitoes to pick up the infection.
Once infected, the virus replicates within the mosquito, and after about a week, it can transmit the virus to other birds, humans, or other mammals through its bite. Humans and horses are “dead-end” hosts because the virus does not multiply to high enough levels in their blood to be passed back to biting mosquitoes. Warmer temperatures accelerate mosquito development, increase biting rates, and speed up virus replication within the mosquito. Increased rainfall creates more standing water, providing additional breeding grounds for mosquitoes and potentially leading to larger populations and higher transmission risk.
Identifying Infection in Humans
Most people infected with West Nile virus (70% to 80%) do not develop any symptoms. For those who become ill, symptoms typically appear 2 to 14 days after a mosquito bite, though this period can extend to several weeks in individuals with compromised immune systems. About one in five infected people develop West Nile fever, a milder illness with symptoms such as fever, headache, body aches, joint pain, vomiting, diarrhea, or a rash. Most individuals with these milder symptoms recover completely, though fatigue and weakness may persist for weeks or months.
Less than 1% of infected individuals develop a more serious illness affecting the central nervous system, known as neuroinvasive disease. This severe form can manifest as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). Symptoms of neuroinvasive disease include high fever, severe headache, neck stiffness, disorientation, stupor, tremors, convulsions, muscle weakness, vision loss, numbness, or paralysis. Individuals over 60, those with weakened immune systems, organ transplant recipients, and people with underlying conditions like diabetes or high blood pressure face a higher risk of developing these severe complications.
Community-Level Control Measures
Public health agencies implement community-level measures to monitor and control West Nile virus outbreaks. Surveillance is a primary tool, involving trapping and testing mosquito populations to detect the virus and assess its prevalence. Monitoring reports of dead birds, particularly crows and jays, also serves as an early indicator of virus activity, as these species are susceptible to West Nile virus.
Once virus activity is detected, public health departments employ mosquito control methods. Larviciding involves treating standing water sources, such as storm drains or neglected swimming pools, with insecticides that target mosquito larvae, preventing them from developing into biting adults. Adulticiding, or spraying to kill adult mosquitoes, is used when there is a significant risk of human infection, often conducted during peak mosquito activity times like dusk or dawn. These coordinated efforts aim to reduce mosquito populations and interrupt the transmission cycle on a broader scale.
Individual and Household Protection Strategies
Protecting oneself and one’s household from West Nile virus involves personal and environmental actions. When outdoors, use an EPA-registered insect repellent containing ingredients like DEET, picaridin, or oil of lemon eucalyptus. Apply repellents to exposed skin and clothing, following label instructions, to deter mosquito bites. Wearing long-sleeved shirts and long pants, especially during dawn and dusk when mosquitoes are most active, provides a physical barrier.
Preventing mosquitoes from breeding around the home is equally important. This involves the “dump and drain” strategy: regularly empty, scrub, or turn over items that can hold standing water. Common breeding sites include birdbaths, pet dishes, flowerpots, buckets, barrels, and clogged rain gutters. Changing water in birdbaths and pet dishes weekly, and drilling drainage holes in tire swings, removes potential habitats for mosquito larvae, reducing mosquito populations in residential areas.