West Nile virus
The term West Nile virus (WNV) has been used to describe both a specific flavivirus and the disease conditions caused by that virus. Other commonly used names for conditions caused by the disease include
- West Nile disease
- West Nile fever
- West Nile neuroinvasive disease
- West Nile encephalitis (WNE)
- West Nile meningitis (WNM)
- West Nile polimyelitis (WNP), also called acute flaccid paralysis
The big picture
- While West Nile virus infections can be lethal, approximately 80% of people who get infected show no symptoms.
- About 20% of people who get infected show symptoms of West Nile fever and recover completely.
- Fewer than 1% (the WHO estimates approximately one out of 150 people) of people who get infected develop West Nile neuroinvasive disease, an all-encompassing term for severe neurological conditions including WNE, WNM, WNP and related conditions.
- 2,374 cases of WNV disease in the US were reported to the CDC in 2013, including 114 deaths. In Virginia, there were 121 cases reported.
Preliminary numbers for West Nile virus neuroinvasive disease Incidence by state, 2013 (as of January 7, 2014). Includes data for WNE, WNM and WNP; does not include data for non-neuroinvasive West Nile fever. The legend indicates number of cases per 100,000 population (CDC).
How do people get infected with WNV?
West Nile virus is normally a bird virus. The virus cycles between birds and mosquitoes. Birds get infected by being bitten by an infected mosquito. The virus multiplies to a high concentration in the blood of birds. Mosquitoes get infected by biting infected birds. Because birds are the organisms in which the virus multiplies enough to be transmitted back to mosquitoes, we call them the amplification host.
Infected mosquitoes can also pass the virus on to mammals, including people. This is a one-way transfer. That is, mosquitoes can infect these hosts, but cannot become infected by them. For this reason, people and other mammalian hosts are called "dead-end hosts".
Infected mosquitoes can also pass the virus on to mammals, including people. This is a one-way transfer. That is, mosquitoes can infect these hosts, but cannot become infected by them. For this reason, people and other mammalian hosts are called "dead-end hosts".
Transmission cycle of West Nile virus. Infected mosquitoes bite and infect birds, and infected birds that develop a high concentration of virus in their blood pass the virus back to mosquitoes when bitten again. In contrast, infected mosquitoes can infect other mammals, including people, but these "dead-end hosts" cannot pass the virus back to mosquitoes. Click on the image for a larger view. Copyright, Anthony L. Palombella, 2014.
Why is this important to know? Suppose a person becomes infected with WNV while visiting another part of the world. Upon returning home, that person will not be able to pass the virus to mosquitoes and spread the disease to other people.
In rare cases, WNV has been transmitted to people via organ transplantation, blood transfusions and handling of laboratory cultures. In addition, mothers can pass on the virus during pregnancy, birth or breastfeeding. Again, these are rare occurrences.
In rare cases, WNV has been transmitted to people via organ transplantation, blood transfusions and handling of laboratory cultures. In addition, mothers can pass on the virus during pregnancy, birth or breastfeeding. Again, these are rare occurrences.
Can you get WNV from handling live or dead infected birds or mammals?
The short answer is no. The methods of transmission described above are the only known ways to become infected with WNV, with mosquito bites being by far the most common way. Having said that, there are other infections you can get and parasites you can acquire from handling wild animals (dead or alive), so it's a good idea to handle them with care or stay away from them entirely. If you wish to dispose of a dead bird or small mammal on your property, the CDC recommends wearing gloves, double-bagging the animal in plastic bags, and disposing of it in a garbage can.
Likewise, WNV cannot be transmitted by consuming the meat of an infected bird or mammal. Again, there are a wide variety of other infections that can be gotten from improperly prepared meat, and all food should always be handled safely.
Likewise, WNV cannot be transmitted by consuming the meat of an infected bird or mammal. Again, there are a wide variety of other infections that can be gotten from improperly prepared meat, and all food should always be handled safely.
What are the symptoms of WNV infection?
As noted above, about 80% of people who become infected by WNV never develop any symptoms (1). Those who do develop symptoms fall into several categories:
- West Nile fever. About 20% of infected people have a fever, rash, headache, nausea, vomiting, diarrhea, muscle and joint aches, and fatigue. The majority of these individuals make a complete recovery.
- West Nile virus neuroinvasive disease. About 1 in 150 infected individuals develop inflammation of the brain (encephalitis) and/or the tissues surrounding the brain and spinal cord (meningitis). Symptoms (not all of which may be present) include severe headache, high fever, neck stiffness, seizures, coma, and paralysis (acute flaccid paralysis as seen in poliomyelitis).
How are WNV diseases treated?
There is currently no vaccine to prevent WNV infection, and no specific treatment. In most cases, over-the-counter medications are used to relieve pain and fever symptoms. Only the most severe cases require hospitalization to keep the patient hydrated and comfortable. The mortality rate for WNV infections ranges from 3% to 15%.
Who is most at risk for WNV infection?
Anyone at risk of being bitten by mosquitoes has some risk of WNV infection although people over 50 have an increased risk of developing neuroinvasive symptoms. People who have a weakened immune system, and people with cancer, high blood pressure, diabetes or certain other diseases also have an increased risk of developing more severe symptoms once infected.
In the US, WNV infections have occurred in each of the contiguous 48 states. No cases have been reported from Alaska and Hawaii.
In the US, WNV infections have occurred in each of the contiguous 48 states. No cases have been reported from Alaska and Hawaii.
Which mosquitoes and birds carry WNV?
According to the CDC, there are 65 species of mosquito in the US in which WNV has been observed, but not all of these are equally able to transmit the virus to humans. Species of the genus Culex are particularly efficient vectors, as are the Asian tiger mosquito (Aedes albopictus) and some other species of the genus Aedes. Of these, the Asian tiger mosquito is of particular importance in Virginia because it bites in the daytime (when more people are out and about) and because it is such a common mosquito. See the Virginia species page for more information about mosquito vectors in Virginia.
WNV has been detected in over 300 species of bird in the US. This includes many common species in Virginia. For example:
WNV has been detected in over 300 species of bird in the US. This includes many common species in Virginia. For example:
- American crow
- American robin
- Blue jay
- Carolina wren
- European starling
- Eastern bluebird
- Northern cardinal, Virginia's state bird
- Sparrows, many species
What can I do to protect myself from being infected with WNV?
The most effective ways to prevent infection with WNV is to minimize your exposure to mosquito bites. See the mosquito safety page for more information.
Where can I learn more about WNV?
- Detailed information about West Nile virus is available from the CDC and WHO.
- Research papers and professional review articles can be accessed through PubMed.gov, including over 2,000 papers available as full-text downloads.
- West Nile virus was first observed in 1937 in the West Nile region of Uganda and didn't appear in the US until 1999. A good review of the introduction and surprisingly quick spread of WNV in the US is available here.
Notes
- How do the CDC and WHO come up with numbers of people with no symptoms? Researchers travel to areas where WNV is endemic (regularly present) or epidemic (a sudden uncommon outbreak) and examine large random parts of the population for infection, with or without symptoms.