West Nile Virus is a vector-bourne disease, spread to birds, humans and other mammals through mosquito bitesIt was recently discovered that West Nile Virus can spread from person to person via blood transfusions, organ transplants and possibly through mother's breast milk. West Nile Virus is prominent in the summer in North America. Infection with West Nile Virus occurs in areas were mosquitoes are present such as mountain ranges, coastal areas, and tropical areas. Although it rarely causes disease in humans in developed countries, it greatly affects 3rd world countries and animals.
Structure and Classification
West Nile Virus is a member of the Flaviviridae family of viruses. Flaviviridae is made up of single stranded plus-sense RNA viruses. This places the West Nile Virus in Class IV of the Baltimore system (Shors 76).West Nile is an enveloped virus with an icosahedral capsid. The envelope does not have any surface projections. Its genome consists of 11 kilo-bases. Like other plus-sense RNA viruses, West Nile Virus contains 5' and 3' untranslated regions. West Nile Virus however only has one open reading frame (ORF) while others in its family can have up to 10 ORFs. West Nile's ORF is cleaved by cellular and viral proteases into 10 proteins, three of which are structural. The seven non-structural proteins are NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5. These proteins are important in the passage of the virus to other hosts. They are vital for replication and assembly of viruses. Non-structural proteins are also vital to make the virus unnoticed by the host's immune system, most notably NS4B (Kramer 172).
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West Nile Virus's genome, similar to all other single strand plus-sense RNA viruses, can be used as mRNA. A structural protein, E protein (envelope) imbedded in the capsid attaches to a cell receptor on the host. Researchers are unsure which specific molecules are involved in attachment although several are theorized. The virus enters the cell via clathrin-mediated endocytosis. The internalized virus's low pH leads to fusion of the cell and viral membranes and later the entrance of viral RNA into the cell. The viral RNA enters the cytoplasm as all RNA plus sense viruses need to replicate in the cell cytoplasm. As mentioned previously the viral genome can be translated into proteins directly and cleaved by proteases to form 10 proteins. The non-structural proteins replicate the RNA genome and produce glycoprotein capsids. Structural proteins prM and E envelope replicated RNA genomes and capsids to make new West Nile viruses. The viruses are not mature yet. They bud through the endoplasmic reticulum and are transported to the Golgi bodies via the secretory pathway. prM protein is cleaved to become M protein. This causes the low pH of the Golgi and then a rearrangement of the immature virus's capsid. New mature West Nile Viruses are the result of this process. The new viruses travel to the cell membrane and exit via clathrin (Perera 12).
West Nile Virus has a lengthy incubation period, varying from 2 days to 2 weeks. As shown in rodent models, West Nile Virus is transported to the lymph nodes after replication. This leads to the virus being spread through the blood to infect tissues in the kidney and other organs. In rodent models, West Nile Virus is virtually cleared from serum and organs weeks after infection (Samuel 9350). During infection, Viremia and spread to organs result in many of the symptoms associated with West Nile Virus infection.
Many infected individuals do not show symptoms. If an infected individual shows symptoms there are four possible outcomes of the disease. The first and least severe is West Nile Fever. Humans with West Nile Fever will feel general flu symptoms. This includes fever, headache, muscle aches, swollen lymph nodes, skin rash and gastrointestinal problems. Individuals generally recover within days although some take weeks.
Serious infections include West Nile meningitis, West Nile encephalitis and acute flaccid paralysis. Damage to the neurons of the spinal cord and brain stem are the cause of symptoms in these diseases (Samuel 9350). Meningitis is inflammation of the brain and spinal cord's protective membrane known as the meninges. If infection of West Nile Virus leads to meningitis then intolerability of light, headaches and stiffness in the neck occurs. With the neck stiffness nuchal rigidity also occurs which is the inability to move the head forward. After hospitalization to treat meningitis many infected individuals require medical attention due to the damage done to the meninges. Encephalitis or inflammation of the brain may also occur after West Nile Virus infection. This leads to neurological issues including tremors and problems with consciousness and orientation. Flaccid paralysis symptoms include asymmetric paralysis of the limbs, lower back pain and occasionally bladder problems (Kramer 173).
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West Nile is highly preventable. The best preventative measure is to prevent getting bit by mosquitoes. It is important to wear insect repellant when outdoors, especially between 6pm and 6am when mosquitoes are most active. Appropriate screening on doors and windows are easy ways to keep mosquitoes out of homes. Remove standing water which is a major breeding ground for mosquitoes. This includes pet dishes, bird baths, pools and flower pots (CDC).
Vaccines are another great preventative measure. Unfortunately there is no vaccine for humans available for West Nile Virus currently. Scientists do have vaccines for other viruses in West Nile Virus's family Flaviviridae such as Yellow Fever Virus. A vaccine using chimeric viruses to deliver West Nile's antigens is currently in clinical trials. Two vaccines are currently in use for horses and mice (Kramer 177).
If West Nile Virus is not prevented there is a chance of infection. West Nile Viral Diseases do not have any specific treatment. So, if mild illness occurs treatment is not necessary. However with the severe diseases associated with West Nile Virus medical care is required. Treatment consists of suppressing symptoms which includes giving intravenous fluids, rest, and respiratory help if needed (CDC). Antiviral therapy can also be used to treat West Nile Viral Diseases. Intravenous immunoglobulin with high amounts of West Nile antibodies shows promise as an antiviral treatment however it has not been confirmed as a safe and reliable treatment (Kramer 177). Individuals who survive West Nile Virus infection may not get rid of the virus completely. According to a recent study affiliated with the University of Texas, patients who suffered from West Nile encephalitis had the virus in their blood, spinal fluid and brain several years after infection (Murray 2).
Public Health Impact
Anyone that can be bitten by a mosquito is at risk for West Nile Virus. Those especially at risk are those that go outdoors during mosquitos' peak activity or live in endemic areas. Endemic areas are worldwide minus the continents of Asia, Australia and Antarctica (Shors 212). It was also shown that individuals who live near contaminated urban streams such as in Atlanta, Georgia are at risk for infection with West Nile Virus (Vazquez-Prokopec 1388).
Infection with West Nile Virus has a great societal impact. After the confirmation that an arthropod borne virus was the cause of an outbreak of disease in Queens, New York in 1999 many health organizations went into overdrive to prevent the spread of the virus. Insecticides were spread all over the area in hopes to stop the virus. The CDC worked hard to isolate the virus from human and bird cases of disease to confirm that it was West Nile. That year close to 2000 New Yorkers were infected with West Nile with 7 dying from infection (Shors 22,23). Third-world nations that do not have the resources of New York would have suffered great numbers of human and animal fatalities. Over 200 species of animals were infected with West Nile during the outbreak in New York. This includes many animals that are important parts of countries economies such as cattle, sheep, turkeys, chickens, llamas and alpacas (Shors 24-26). These animals provide money, food, transportation, and clothing to many residents of third-world nations. Loss of these animals would be detrimental to these societies.
With proper precautions outdoors West Nile Virus infections can be prevented. If infection occurs and symptoms are present, serious diseases can occur. Scientists are currently in several clinical trials for human vaccines and antiviral treatments. Until these are confirmed and reliable prevention is the best way to control the spread of West Nile Virus.