By Eirish Sison
The news has been abuzz with reports of an especially virulent and long-lasting outbreak of West Nile virus, a mosquito-transmitted infection that has impacted over 2,500 Americans in the year 2012 alone. Researchers have found that West Nile virus (WNV) is a seasonal epidemic in the U.S. that regularly flares up in the summer and stretches into the fall. According to the Centers for Disease Control and Prevention (CDC), researchers have found that WNV is a seasonal epidemic in the U.S. that regularly flares up in the summer and stretches into the fall. The CDC has also reported that the 2012 outbreak is the worst in the U.S. since 2003. Scientists believe that it has been especially bad due to the relatively mild temperatures last winter, and to this year’s hot, dry summer with occasional rainfall which provided the stagnant water where mosquitoes could breed.
To help you get the important facts on WNV, we conducted an email interview with Angela Rasmussen, Ph.D., infectious disease expert, and post-doctoral fellow at the University of Washington School of Medicine Microbiology Department.
Dr. Angela Rasmussen:
West Nile virus is transmitted by mosquitoes, which contract WNV by feeding on infected birds. When a person is infected through a mosquito bite, the virus creates copies of itself in immune cells in the skin called Langerhans cells. Infected Langerhans cells migrate to your lymph nodes, where the virus infects other types of white blood cells and replicates itself. The virus then circulates in the blood and the lymphatic system until the immune system clears the infection, which takes about a week. In patients who develop more serious disease, the virus enters central nervous system tissues (the brain and spinal cord), leading to neurological disease symptoms.
The majority of people infected with WNV will have no symptoms at all, and no infection of the central nervous system. However, about 20 percent of people who get WNV will develop West Nile fever, a condition that can cause a fever, fatigue, headache, body aches, skin rash and swollen, sore lymph nodes.
About one in 150 people who get WNV develop an even more severe disease caused by infection of the brain and other parts of the central nervous system. Symptoms include a high fever, severe headache, tremors, stupor, muscle weakness, disorientation and confusion. This can progress to muscle paralysis, convulsions and seizures, coma and death.
Currently, there are no antiviral drugs or vaccines that can effectively treat or prevent WNV infection. Most people with mild disease can treat symptoms like fever and aches with analgesics such as acetaminophen (Tylenol) or ibuprofen (Advil). However, patients with severe disease can be treated with supportive care in a hospital; treatments include IV fluids, nutritional support and assistance with breathing. Patients who develop symptoms that may be WNV should see their doctor, and those who also experience severe headaches, neck pain and/or confusion should seek immediate medical attention.
The best way to prevent WNV infection is to avoid mosquito bites. People going outside should use an EPA-registered insect repellent recommended by the CDC. These include DEET, Picaridin, oil of lemon eucalyptus, PMD (synthetic oil of lemon eucalyptus), or IR3535, and these ingredients should be no less than 10% of the product formulation. Additionally, clothing, shoes and camping equipment containing permethrin effectively repels mosquitoes, and can be used along with a topical insect repellent.
People can also control mosquito populations around their homes by eliminating sources of standing water, which mosquitoes use to breed. Some common sources of standing water include buckets and barrels, pet dishes, "kiddie pools," garden ponds, water features, empty flower pots, used tires and bird baths. Change water in birdbaths and pet dishes frequently, and turn buckets, barrels, pools, and the like upside down or on their sides to prevent rainwater from collecting in them. Alternatively, place these items in a sheltered place such as a garage, carport or shed.
Approximately one in 150 people infected with WNV will develop WNV encephalitis, meningitis or poliomyelitis (flaccid limb paralysis). People of all ages can develop these symptoms, but older patients and those with compromised immune systems are at the highest risk. The only known way to prevent severe WNV is to avoid WNV infection altogether: Practice mosquito control as described above. However, patients can reduce complications and boost their odds of survival by seeking medical support immediately after developing symptoms — like severe headache, neck pain and confusion — that indicate infection has spread to the central nervous system.
The current outbreak is expected to end over the next month, as temperatures become cooler. There were fewer cases reported during the first week of October than there were during late summer. The outbreak should taper off with the first hard freeze, which will kill off most of the mosquitoes. However, if unseasonably warm temperatures persist and there is a significant amount of rain, the outbreak may last until the weather becomes very cold. Long term, the U.S. can expect hot, rainy summers to support large outbreaks of WNV nationwide — particularly in mid-Atlantic and southern states that experience considerable heat and humidity.
Eirish Sison is a health writer based in the San Francisco Bay Area.
Published October 9, 2012.