West Nile virus is in the continental U.S., with per year.
Author
Daniel Pastula
Professor of Neurology, Medicine (Infectious Diseases), and Epidemiology, University of Colorado Anschutz Medical Campus
Every August, West Nile season ramps up, primarily in parts of the Southern and Western United States.
The Conversation U.S. asked neurologist and medical epidemiologist to explain what researchers know about West Nile and how to protect yourself from contracting the disease.
What is West Nile virus?
is a virus that was in the West Nile region of Uganda in 1937 and subsequently caused outbreaks throughout Africa, the Mediterranean region and parts of Europe.
It , probably from an infected bird or mosquito. The virus has since spread and become endemic throughout the continental United States and other parts of North America.
How is West Nile virus transmitted?
West Nile virus is primarily transmitted to humans by mosquito bites. West Nile virus between various birds such as crows, robins and blue jays and primarily , which are very common across the U.S.
The mosquitoes become infected by feeding on infected birds with high levels of virus. Then the infected mosquitoes pass the virus to other birds by feeding on them.
Sometimes, an infected mosquito will bite a human, horse or other mammal, transmitting the virus to them. However, these animals do not develop a high enough viral level in their blood to keep the cycle going. In other words, they are “dead-end” hosts.
Rarely, West Nile virus is , including through accidental laboratory exposures, blood transfusion or organ transplantation. Very rarely, it may be transmitted during pregnancy, delivery or through breastfeeding.
When and where does it show up, and is it seasonal?
In the continental U.S., most human cases of West Nile virus disease occur from , or until the first hard freeze or snowfall. This is commonly called “West Nile season.” By this time of year, mosquitoes have had enough time to emerge, feed on infected birds and become infected themselves.
In more southern areas such as Arizona, Florida and Texas, mosquito transmission may occur earlier than July and later than September. For example, mosquitoes infected with West Nile virus were
While human cases of West Nile virus disease have occurred in the , the majority of cases over the past two decades have been in the . This likely has to do with the types of mosquitoes, birds, bird migration patterns, temperature, rainfall and irrigation practices in these areas.
How dangerous is West Nile?
Approximately have no symptoms or very mild ones.
But , people infected with West Nile virus can develop a sudden flu-like illness with symptoms such as high fever, headache, muscle and joint aches, stomach problems such as nausea, vomiting and diarrhea, rash and fatigue within two to 14 days after being bitten by a West Nile-carrying mosquito.
Most people recover from this “West Nile fever,” .
In about 1% of cases, however, the virus can invade the covering of the brain, causing what is called ; the brain itself, resulting in ; or the spinal cord, causing a . When the virus invades the nervous system like this, it is called “.”
Approximately 10% of West Nile neuroinvasive disease cases are fatal, particularly in cases where the brain and spinal cord are directly infected. Many survivors are left with such as weakness, movement disorders or cognitive issues to varying degrees.
is higher in people who are 60 or older, those who are immunocompromised and those with conditions such as cancer, diabetes or kidney failure.
Are there any treatments or vaccines?
While , there are currently no proven antiviral treatments for West Nile virus disease. Treatment remains largely focused on addressing symptoms.
Some patients may require over-the-counter medications for fever and headaches, fluids for dehydration, anti-nausea medications for nausea or vomiting, and rest. Some may require hospitalization and treatment for severe dehydration, organ dysfunction, seizures or brain swelling.
There are currently no West Nile vaccines licensed for use in humans, though .
How can you help prevent it?
Generally speaking, if you prevent mosquito bites, you prevent West Nile virus infection. There are several things you can do to :
First of all, use an on exposed skin when you are outside in an area with mosquitoes. These are safe and effective when used as directed.
You can also wear long-sleeved shirts and pants when feasible. It is much harder – or even impossible, depending on the fabric – for mosquitoes to bite through clothing. You can also , an insecticide, which further repels mosquitoes.
When possible, avoid being outside between dusk and dawn. This is when the West Nile-carrying mosquitoes are most active.
It’s also a good idea to get rid of any unneeded standing water in places such as birdbaths or kiddie pools around your property, as mosquitoes like to breed in standing water. When indoors, be sure that any unscreened windows are closed to keep mosquitoes outside. These simple precautions can greatly reduce the risk of getting severely ill.
In addition to these precautions for individuals, robust public health and vector control programs help to monitor and control the spread of West Nile virus by warning area residents of the presence of the virus and reducing infected mosquito populations when necessary.
Daniel Pastula does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
