West Nile Virus (WNV)- An Overview
West Nile Virus (WNV) is a member of the Flaviviridae family, which includes other viruses that cause diseases such as Zika, dengue, and yellow fever. WNV is an enveloped virus, meaning that it has a lipid membrane derived from the host cell that surrounds its protein shell. The protein shell, or capsid, has an icosahedral symmetry and contains the viral genome. The genome of WNV is a single-stranded, positive-sense RNA molecule that is about 11,000 nucleotides long. The RNA genome has a 5` cap and a 3` poly(A) tail, and it encodes for 10 proteins: three structural proteins and seven nonstructural proteins. The structural proteins are the capsid (C), the envelope (E), and the pre-membrane (prM) proteins. The nonstructural proteins are NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5. The structural proteins are involved in the formation and entry of new viral particles, while the nonstructural proteins are involved in the replication and processing of the viral genome.
The structure and genome of WNV are similar to other flaviviruses, but they also have some distinctive features. For example, WNV has a relatively smooth protein shell compared to other flaviviruses, which have more prominent spikes on their surface. The spikes are formed by the E protein, which mediates the attachment and fusion of the virus with the host cell membrane. The E protein also undergoes conformational changes in response to pH changes during viral entry and maturation. Another feature of WNV is that its genome has two noncoding regions at both ends, called the untranslated regions (UTRs). These regions form stem-loop structures that play important roles in the initiation and regulation of viral transcription, translation, replication, and packaging.
The structure and genome of WNV determine its biological properties and interactions with the host cell. Understanding how WNV infects and replicates in different cell types and tissues can help us develop better diagnostic tools and preventive measures against this virus.
West Nile virus (WNV) is a mosquito-borne flavivirus that can cause neurological disease and death in humans and animals. WNV is widely distributed in Africa, Europe, the Middle East, North America, and West Asia . WNV was first isolated from a febrile woman in Uganda in 1937, but few outbreaks of human disease were reported until the 1990s, when epidemics occurred in Algeria, Morocco, Tunisia, Italy, France, Romania, Israel, and Russia. The most severe outbreak of WNV disease occurred in 1996 in Bucharest, Romania, where 393 cases of encephalitis and 17 deaths were reported.
WNV was introduced into the Western Hemisphere in 1999, when an outbreak of encephalitis occurred in New York City. Since then, WNV has spread across the continental United States, Canada, Mexico, the Caribbean, and Central and South America. The largest epidemic of WNV disease in the United States occurred in 2003, when 9,862 cases and 264 deaths were reported. In 2012, another large outbreak occurred, with 5,674 cases and 286 deaths reported. As of January 2023, a total of 57,372 cases and 2,549 deaths have been reported in the United States since 1999.
The incidence and severity of WNV disease vary by geographic region, season, climate, mosquito vector abundance and competence, bird reservoir host abundance and susceptibility, human behavior and immunity, and viral strain . WNV transmission is highest during the summer and early fall months when mosquito activity is greatest . However, WNV transmission can occur year-round in some areas with warmer climates . WNV transmission is also influenced by environmental factors such as rainfall, temperature, humidity, vegetation, and land use that affect mosquito breeding habitats and bird distributions .
WNV circulates mainly between birds and mosquitoes in an enzootic cycle that maintains the virus in nature . Culex mosquitoes are the primary vectors of WNV transmission to humans and other mammals . Humans are incidental or dead-end hosts of WNV infection because they usually do not develop high enough levels of viremia to infect mosquitoes . However, human-to-human transmission can occur through blood transfusion, organ transplantation, intrauterine infection, breastfeeding, and laboratory exposure .
Most people (80%) infected with WNV are asymptomatic and do not develop any clinical signs or symptoms. About 20% of infected people develop a mild febrile illness called West Nile fever that lasts for a few days to weeks. The symptoms include headache, body aches, fever, joint pain, vomiting, diarrhea, rash, and swollen lymph glands. About 1% of infected people develop a severe neuroinvasive disease that affects the central nervous system (CNS), such as meningitis (inflammation of the membranes around the brain and spinal cord), encephalitis (inflammation of the brain), or acute flaccid paralysis (weakness or paralysis of limbs or respiratory muscles). The symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and death. The case-fatality rate for neuroinvasive disease ranges from 3% to 15%, depending on age and underlying medical conditions. The risk of developing severe disease is higher among older adults (>60 years), immunocompromised persons (e.g., transplant recipients or cancer patients), and persons with certain chronic medical conditions (e.g., diabetes or hypertension).
There is no specific treatment or vaccine for WNV infection in humans. The diagnosis is based on clinical presentation and laboratory confirmation by detecting WNV-specific IgM antibodies in serum or cerebrospinal fluid (CSF), neutralizing antibodies by plaque-reduction neutralization tests (PRNTs), viral RNA by reverse transcription-polymerase chain reaction (RT-PCR), or viral antigens by immunohistochemistry (IHC). Supportive care is given to patients with symptomatic infection. Prevention and control measures include reducing exposure to mosquito bites by using repellents, wearing protective clothing, and avoiding outdoor activities during peak mosquito activity periods; controlling mosquito populations by eliminating breeding sites, applying larvicides or adulticides, and implementing surveillance programs; screening blood donors and organ donors for WNV infection; and educating the public about the risk factors and signs and symptoms of WNV disease.
The West Nile virus is mainly transmitted to humans and animals by the bite of an infected mosquito. Mosquitoes become infected when they feed on birds that carry the virus. The infected mosquitoes can then spread the virus to other birds, animals, and humans. The primary mosquito vector for the West Nile virus is the genus Culex, which is most active from dusk to dawn. However, other mosquito species, such as Aedes, can also transmit the virus and act as bridge vectors between birds and mammals.
In rare cases, the West Nile virus can also be transmitted through other routes, such as:
- Exposure in a laboratory setting
- Blood transfusion and organ transplantation
- Mother to baby during pregnancy, delivery, or breastfeeding
The risk of transmission through these routes is very low, as blood donors and organ donors are screened for the virus, and infection during pregnancy or breastfeeding is uncommon.
The West Nile virus is not transmitted by:
- Coughing, sneezing, or touching
- Touching live animals
- Handling live or dead infected birds (although gloves and plastic bags should be used for disposal)
- Eating infected animals (as long as the meat is cooked properly)
The replication process of West Nile Virus (WNV) involves several steps that take place in the cytoplasm of the infected cells . The steps are as follows:
- Attachment/Adsorption: The virus attaches to the host cell surface through an unknown receptor, although some potential candidates have been suggested, such as DC-SIGN, mannose receptor, and glycosaminoglycans .
- Penetration: The virus enters the host cell by receptor-mediated endocytosis and is transported to the endosomal compartment .
- Uncoating: The acidic environment of the endosome triggers conformational changes in the envelope protein (E) of the virus, leading to the fusion of the viral and endosomal membranes. This releases the nucleocapsid, which contains the viral RNA genome, into the cytoplasm .
- Biosynthesis: The viral RNA serves as both mRNA and template for translation and replication. The RNA is translated into a single polyprotein that is cleaved by viral and host proteases into three structural proteins (capsid, pre-membrane, and envelope) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5). The structural proteins are involved in viral assembly and entry, while the non-structural proteins have various roles in viral replication, modulation of host cell functions, and evasion of host immune responses . The viral replication takes place in specific membrane structures derived from the endoplasmic reticulum (ER), called vesicle packets (VP) and convoluted membranes (CM) .
- Assembly: The viral assembly occurs at the ER membrane, where the capsid protein binds to the viral RNA and forms the nucleocapsid. The nucleocapsid then buds into the ER lumen and acquires the envelope containing the pre-membrane and envelope proteins. This results in an immature virion that has 60 trimeric spikes on its surface .
- Maturation: The immature virion travels through the secretory pathway and reaches the Golgi apparatus, where it undergoes a maturation process. The maturation involves the cleavage of the pre-membrane protein by a host protease called furin, which converts it into a membrane-anchored protein called M. This causes a rearrangement of the envelope protein into 90 antiparallel dimers that lie flat on the viral surface. The mature virion has a smooth appearance and is more infectious than the immature one .
- Release: The mature virion exits the cell by exocytosis through vesicles that fuse with the plasma membrane. The released virion can then infect new cells or be transmitted to another host .
The exact mechanism of pathogenesis of the virus in humans has been difficult to understand due to the differences between the viral strains as well as infrequent laboratory-confirmed human infections. Most of the knowledge regarding WNV pathogenesis comes from the study of animal models infected under controlled laboratory conditions.
The WNV enters the human host from an infected mosquito through its saliva during a blood meal and gets deposited in the blood and skin tissue . The virus then infects the neighboring dendritic cells like Langerhans cells and then travels to the lymph nodes . It multiplies in the tissues and causes a transient viremia that typically lasts for a few days . Anti-WNV IgM antibodies are produced against the virus that deplete the viral load during the viremia . Subsequently, the virus infects multiple organs in the body like the spleen, liver, and kidneys . The virus can be detected in the urine of a patient with encephalitis and the viral infection in the kidneys 8 days after the onset of the symptoms .
The virus is able to cross the blood-brain barrier (BBB) and therefore causes neurological disorders . It may however pass into the CNS without disrupting the BBB. The WNV pathogenesis is also associated with the host’s immune response . The virus induces inflammatory cytokines and chemokines that recruit immune cells to the site of infection. However, excessive inflammation can also cause tissue damage and neuronal death. The virus also evades the host immune system by various mechanisms such as downregulating major histocompatibility complex (MHC) class I molecules, inhibiting interferon signaling, and suppressing antigen presentation .
The outcome of WNV infection depends on a balance between virulence, innate and adaptive immunity, and viral evasion. Factors such as age, genetic background, immune status, and viral strain can influence this balance and determine whether an infection is asymptomatic, mild, or severe .
The clinical manifestations of West Nile virus (WNV) infection vary depending on the severity and the involvement of the central nervous system (CNS). Most people infected with WNV (about 80%) do not develop any symptoms and remain asymptomatic. However, some people (about 20%) develop a febrile illness, called West Nile fever, and a few people (about 1 in 150 to 250) develop a severe neuroinvasive disease, such as West Nile encephalitis, meningitis, or poliomyelitis .
West Nile fever
The usual presentation of West Nile fever is a self-limited febrile illness that is indistinguishable from other viral infections. The incubation period is typically 3 to 14 days, but may be longer in immunocompromised patients. The common symptoms of West Nile fever include :
- Body aches
- Joint pain
- Excessive sweating
- Occasional skin rash (on the trunk of the body)
- Swollen lymph glands
Most people with West Nile fever recover completely within a few days to weeks, but some may experience fatigue and weakness for months .
The neuroinvasive disease caused by WNV infection can affect the brain, spinal cord, or peripheral nerves. The most common forms of neuroinvasive disease are West Nile encephalitis, meningitis, and poliomyelitis .
West Nile encephalitis
West Nile encephalitis is the inflammation of the brain due to WNV infection. It is the most common neuroinvasive manifestation of WNV infection. It can occur in people of any age, but is more severe and fatal in older adults and those with underlying medical conditions . The symptoms of West Nile encephalitis include :
- High fever
- Neck stiffness
- Muscle weakness
- Vision loss
- Numbness and paralysis
West Nile meningitis
West Nile meningitis is the inflammation of the membranes that surround the brain and spinal cord due to WNV infection. It can occur alone or in combination with encephalitis. It is more common in younger adults and children than in older adults . The symptoms of West Nile meningitis include :
- Neck stiffness
- Photophobia (sensitivity to light)
West Nile poliomyelitis
West Nile poliomyelitis is the inflammation of the spinal cord or peripheral nerves due to WNV infection. It can result in acute flaccid paralysis, which is a sudden onset of weakness or loss of muscle tone in one or more limbs. It can also affect the muscles involved in breathing, swallowing, or speaking . The symptoms of West Nile poliomyelitis include :
- Neck stiffness
- Muscle pain
- Asymmetric weakness or paralysis of limbs
- Respiratory failure
In rare cases, WNV infection can cause other extra-neurological manifestations, such as myocarditis (inflammation of the heart muscle), pancreatitis (inflammation of the pancreas), hepatitis (inflammation of the liver), or uveitis (inflammation of the eye) . These manifestations may be associated with more severe outcomes and higher mortality rates.
The diagnosis of West Nile virus infection is mainly based on the detection of specific antibodies in the blood or cerebrospinal fluid (CSF) of the patient. These antibodies are proteins that show the body`s immune response to the virus. The most commonly used test is the IgM-capture enzyme-linked immunosorbent assay (ELISA), which can identify IgM antibodies that appear early in the infection and last for a few weeks or months . However, this test may also give positive results for past infections or cross-reactions with other flaviviruses, such as dengue or Zika . Therefore, a confirmatory test called plaque-reduction neutralization test (PRNT) is performed in reference laboratories to measure the level of neutralizing antibodies that can block the virus from infecting cells . PRNT can also distinguish between acute and past infections by comparing the antibody titers in two blood samples collected 2 to 3 weeks apart .
In some cases, other tests may be used to diagnose West Nile virus infection, such as viral culture, reverse transcriptase-polymerase chain reaction (RT-PCR), or immunohistochemistry (IHC) . These tests can directly detect the presence of the virus or its genetic material or antigens in the serum, CSF, or tissue specimens . However, these tests are less sensitive and specific than antibody tests and may give negative results even if the patient is infected . Therefore, they are usually performed only in early stages of infection or in severe cases with neurological complications .
If you suspect that you or a family member may have West Nile virus infection, you should consult your healthcare provider as soon as possible. They can order the appropriate tests and provide you with supportive treatment and care .
There is no specific treatment for West Nile virus infection. Antibiotics do not work against viruses. Most people with mild symptoms recover on their own within a few days or weeks. They may benefit from rest, fluids, and over-the-counter pain relievers to ease their discomfort.
People with severe symptoms, such as encephalitis or meningitis, may need to be hospitalized and receive supportive care. This may include intravenous fluids, pain medication, anti-inflammatory drugs, and nursing care. Some people may also need breathing support with a ventilator if they have respiratory failure.
There is no vaccine or antiviral drug available for West Nile virus infection in humans. However, some experimental treatments are being tested in clinical trials. For example, high doses of ribavirin and interferon-alpha 2b have shown some efficacy against the virus in laboratory studies, but their safety and effectiveness in humans are still unknown.
The best way to prevent West Nile virus infection is to avoid mosquito bites. This can be done by using insect repellents, wearing long-sleeved clothing and pants, installing window screens, and eliminating mosquito breeding sites around the home. People who live in areas where the virus is active should also avoid outdoor activities during dawn and dusk, when mosquitoes are most active.
People who have had West Nile virus infection should not donate blood or organs for at least four months after their illness, as they may still carry the virus in their blood or tissues. Pregnant women who have been infected with the virus should consult their healthcare providers about the possible risks to their unborn babies.
There is no vaccine to prevent WNV infection in humans. Therefore, the best way to prevent WNV is to protect yourself from mosquito bites. Mosquitoes bite during the day and night and can spread WNV to people and animals. Here are some prevention and control measures that you can take to reduce your risk of WNV infection:
Use insect repellent: Apply an Environmental Protection Agency (EPA)-registered insect repellent with one of the following active ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone. Follow the product label instructions and reapply as directed. Do not use products containing OLE or PMD on children under 3 years old. Do not spray repellent on the skin under clothing or on cuts, irritated skin, or eyes. If you are also using sunscreen, apply it before applying insect repellent.
Wear long-sleeved shirts and long pants: Cover as much of your skin as possible when you are outdoors, especially during dawn and dusk when mosquitoes are most active. You can also treat your clothing and gear with 0.5% permethrin, an insecticide that kills or repels mosquitoes. You can buy permethrin-treated clothing and gear or treat them yourself following the product instructions. Do not use permethrin directly on your skin.
Use screens on windows and doors: Repair any holes in the screens to keep mosquitoes out of your home. You can also use air conditioning if available.
Eliminate mosquito breeding sites: Mosquitoes lay eggs in or near water. Once a week, empty and scrub, turn over, cover, or throw out any items that hold water, such as tires, buckets, planters, toys, pools, birdbaths, flowerpots, or trash containers. Check inside and outside your home for any potential mosquito habitats.
Avoid blood transfusions and organ transplants from infected donors: WNV can be transmitted through blood transfusions and organ transplants from infected donors. Although screening tests are used to reduce this risk, they are not 100% effective. If you need a blood transfusion or an organ transplant, ask your doctor about the safety measures in place to prevent WNV transmission.
Avoid contact with infected animals: WNV can infect birds, horses, and other animals. If you find a dead bird, do not touch it with bare hands. Contact your local health department to report the bird and follow their instructions for disposal. If you own a horse or other animal, consult your veterinarian about WNV vaccines and prevention strategies.
Avoid mother-to-child transmission: WNV can be transmitted from a pregnant or breastfeeding woman to her baby. Although this is very rare, it can cause serious complications for the baby. If you are pregnant or breastfeeding and think you may have been exposed to WNV, talk to your doctor about testing and treatment options.
By following these prevention and control measures, you can reduce your risk of getting infected with WNV and protect yourself and others from this potentially serious disease.
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