West Nile virus (WNV) is a microorganism (virus) transmitted by mosquitoes and, more rarely, ticks. It is commonly found in Africa, West Asia, the Middle East, and the Caribbean.
Most humans who contract the virus have been bitten by an infected mosquito, often the common house mosquito (Culex pipiens). This mosquito, found in most U.S. states, is a known vector of WNV. (A “vector” is an animal host that transmits an infectious disease).
This disease can infect humans, birds, mosquitoes, horses, and some other mammals. West Nile virus infection can cause West Nile fever (WNF) and West Nile Neuroinvasive Disease (WNND).
West Nile virus was discovered in 1937 in the West Nile district of Uganda. West Nile emerged in the United States for the first time in the New York City area in Aug. 1999. There were 62 confirmed human cases and seven deaths during this outbreak, creating widespread concern.
In 2012, the CDC confirmed an outbreak of 1,118 cases of West Nile virus, the highest number of reported cases since the virus was first detected in the U.S. Of those, 56% of patients from 47 states developed neuroinvasive disease. Approximately 75% of the cases were reported from five states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma), and almost half of all cases were reported from Texas, many in the Dallas area.
According to the CDC, since 1999 more than 50,000 people have been reported with West Nile virus in the U.S. More than 2,300 of those people died. Cases of West Nile infection spiked in 2018, with 25% more cases reported that year than average.
As of August 2019, every state but Hawaii has reported West Nile virus infections according to the Centers for Disease Control. More than 2,600 cases were reported in 2018, resulting in 167 deaths. More than 1,600 of these cases had potential to infect the nervous system, marking them as more serious and potentially deadly. Indeed, 10% of the patients reported to the CDC with neuroinvasive WNV died in 2018.
Birds are vectors (intermediate carriers) of the virus. As such they are important to the virus's life cycle and transmission cycle. For female mosquitos that carry WNV, a typical blood meal comes from birds. When mosquitoes feed on the blood of an infected bird, the virus can infect these mosquitoes.
Birds make ideal vectors for WNV. That's because they can circulate high levels of the infection in their blood. The infected birds themselves may or may not show signs of illness. Crows and Jays are some of the species most likely to die from infection, whereas most other bird species survive.
Almost all people infected with West Nile virus contract the disease through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood. To date, more than 300 different species of bird have been found to carry the virus in the U.S. However, there is no evidence the virus can be spread directly from a bird to a human.
The virus eventually finds its way into the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.
It is important to remember that West Nile virus is not contagious and cannot be transmitted from person to person. A person cannot get the virus, for example, from touching or kissing someone who has the disease. Likewise, a health care worker who has treated someone with the disease cannot spread the infection to others.
Humans are called "dead-end" hosts for the virus, meaning we can be infected but our immune systems prevent the virus from multiplying enough to be passed back to mosquitoes and then to other hosts. Horses can also catch West Nile virus, but are also dead-end hosts and cannot infect humans.
In a tiny number of cases, WNV has also been spread through these unusual means:
- Blood transfusion
- Organ transplant
- Pregnancy (mother to baby)
- Laboratory exposure
Pregnant mothers have a very low risk of spreading WNV to their newborns. However, only a small number of cases have been studied so far. That means it is not yet possible to determine what percentage of WNV infections during pregnancy result in infection of the fetus or medical problems in newborns. One small study of infected mothers found that all 17 children were born healthy. In fact, there has been only one documented instance of a newborn becoming infected during pregnancy.
Even so, concerns remain that mother-to-child WNV transmission can occur with possible adverse health effects. For this reason, pregnant women should take precautions to reduce their risk for WNV and other mosquito-borne infections by avoiding mosquitoes. They can do this by following three safety precautions:
- Stay away from mosquitos and mosquito-prone areas
- Wear mosquito-protective clothing, including long sleeves
- Use repellents that have DEET. FDA-registered repellents with DEET are deemed safe for pregnant women.
There is no evidence a person can get West Nile virus by handling infected birds, whether the birds are alive or dead. However, it is a good idea in general to avoid skin contact when handling dead animals, including dead birds. Gloves or double plastic bags should be used to remove and dispose of carcasses.
Some states monitor WNV through a dead bird surveillance program. To find out if such monitoring happens in your area, check with your local vector control program.
There is no evidence the virus can be transmitted to humans who eat infected birds or animals. In keeping with overall public-health practice, and due to the risk of known food-borne pathogens, you should always follow safety precautions to ensure your meat is fully cooked.
If you live in or visit areas where active cases have been identified, you are at risk of developing West Nile viral infection. But remember—4 out of 5 infected people show no symptoms. Those most at risk of severe symptoms, including neurological disease, are people 50 years of age or older.
Children appear to be at low risk for symptomatic WNV, The American Academy of Pediatrics says. However, they still face some risk.
The risk of infection is highest during mosquito season, when mosquitos are most active. When temperatures freeze, mosquito season comes to an end in most places.
Cases of West Nile virus infection occur primarily in the late summer or early fall in temperate areas of the world. In southern climates where temperatures are milder, though, the threat of infection may last all year.
About 70% to 80% of those infected have no symptoms. About 20% to 30% of infected people develop West Nile fever with mild symptoms, including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.
A tiny percentage of infected people develop West Nile Neuroinvasive Disease. Symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It can be deadly. About one in 150 people infected with the West Nile virus will develop this more severe form of WNV disease.
It usually takes three to 14 days to develop symptoms following an infectious bite. Symptoms will generally last a few days, although even some healthy people report the illness lasting several weeks. When the infection develops into West Nile Neuroinvasive Disease (WNND), serious symptoms can last for months.
When someone is infected with West Nile virus (WNV), they will typically have one of three outcomes: no symptoms, West Nile fever, or severe West Nile Neuroinvasive Disease. WNND can cause serious symptoms such as meningitis and encephalitis, but it occurs in less than 1% of those infected. If you develop a high fever with severe headache, consult your doctor immediately or go to an Emergency Department.
Following transmission by an infected mosquito, West Nile virus can multiply in the blood system and cross the blood-brain barrier to reach the brain. When this happens, it causes West Nile Neuroinvasive Disease. During WNND the virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.
Why some people develop WNND and not others remains unclear. A weak immune response may be responsible, though. Some researchers believe the virus develops into WNND when a person's immune system fails to adequately control the virus, allowing the infection to develop deeper into the nervous system.
The diagnosis of West Nile virus infection is confirmed with a blood or cerebrospinal fluid test. There is no specific treatment for West Nile virus infection. Mild illness does not require therapy other than medication to reduce fever and pain.
When West Nile infects the brain, intensive supportive therapy is needed. Anti-inflammatory medications, intravenous fluids, and intensive medical monitoring may be required in severe cases. There is no specific antibiotic or antidote used to treat this viral infection.
Is There a West Nile Virus Vaccine?
There is no current vaccine to prevent the virus. A vaccine for horses was first introduced in 2005, and four other horse vaccines have been released since then. Several US laboratories are currently using various approaches to research and develop a human vaccine, though to date none have moved past the trial phase of development.
West Nile virus is closely related to Japanese encephalitis virus and St. Louis encephalitis virus. These viruses are found in the US in the southeast, west, and Midwest. These are all mosquito-borne viruses, and have a similar life cycle in birds and mosquitoes to WNV. They are even found in some of the same mosquitos that carry WNV.
A major difference is that St. Louis encephalitis is "silent" in birds, generally not killing them, so there is usually no warning before a human case occurs. With West Nile virus (at least the American strain), some birds, particularly jays and crows, become ill or die from the disease and therefore offer an early warning to researchers.
Limiting mosquito habitats and breeding grounds helps limit your exposure to them. Mosquitoes lay their eggs in standing water. So, dispose of all cans, plastic containers, rubber tires, or any other possible water-holding containers from nearby property.
At least once or twice a week, empty water from flowerpots, pet food and water dishes, bird baths, swimming pool covers, buckets, barrels, and cans. Drain water from pool covers. Check for clogged rain gutters and clean them out, too. By doing so, you will help control the habitats where mosquito larvae grow.
Here are some steps you can take to protect yourself and your family from mosquito bites:
Apply insect repellent to exposed skin. An effective repellent contains 20%-30% DEET (N,N-diethyl-meta-toluamide). Although DEET is approved for children, some caution is warranted. Consider lower DEET concentrations if children will be outdoors for less than two hours. (High DEET concentrations last longer). Use only once per day, and do not treat babies less than 2 months old.
When weather permits, wear long-sleeved shirts and long pants whenever you are outdoors. Spray clothing with repellents containing permethrin (not to be applied on skin) or another EPA-registered repellent since mosquitoes may bite through thin clothing.
- Place mosquito netting over infant carriers when you are outdoors with infants.
- Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.
- Install or repair screens on window and door so that mosquitoes cannot get indoors.
- Remove standing water from your yard. Encourage neighbors to drain standing water sources in their yards, too.
There are three major steps a community can take to protect itself from this virus. First, a community can monitor its bird population. This includes surveying sick birds and birds that have died of disease.
Second, the community can watch out for stagnant water, particularly if it is nutrient-laden. Stagnant water allows mosquitoes to lay their eggs.
Third, widespread mosquito-control efforts may be warranted. These include the use of spraying and larvicide. However, even with rigorous surveillance, spraying, and larvicide, the virus may still continue to infect people.
Insecticides are commonly used when whole populations of mosquitoes need to be controlled or eradicated. Mosquito insecticides come in two broad categories: larvicides, used in killing immature mosquitoes, and adulticides, which target adults.
Larvicides are pesticides used to kill immature mosquitoes. They can change the growth and development of mosquito larvae in several ways. They include insect growth regulators, surface films, bacterial insecticides, and organophosphates like Diazinon.
Larvicides are applied directly to water sources that hold mosquito eggs or larvae. When used properly, larvicides can help to reduce the overall mosquito population by limiting the number of new mosquitoes.
Adulticides are products used to kill adult mosquitoes. Adulticides can be applied from handheld sprayers, truck-mounted sprayers, or airplanes. Adulticides, when used properly, can have an immediate impact to reduce the number of adult mosquitoes in an area, with the goal of reducing the number of mosquitoes that can bite people and possibly transmit West Nile virus.
Most infected people eventually recover. That means they have developed a natural immunity to the infection. It is assumed that immunity from West Nile virus (WNV) will be lifelong. However, it is possible that immunity may wane in later years.
When dealing with West Nile virus, mosquito bite prevention is your best bet. Fighting mosquito bites reduces your risk of getting West Nile virus, along with other illnesses that mosquitoes can carry.
IMAGES PROVIDED BY:
- Image Source: CDC - P.E. Rollin / Cynthia Goldsmith
- CDC - K.S. Littig
CDC - Graham Heid, Dr. Harry D. Pratt
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