What Is West Nile Virus (WNV)? What Causes West Nile Virus?

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Main Category: Infectious Diseases / Bacteria / Viruses
Also Included In: Tropical Diseases
Article Date: 06 May 2010 - 0:00 PDT

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West Nile Virus, also known as WNV, is a virus of the family Flaviviridae which is spread by the bite of infected mosquitoes. The Flaviviridae virus family also includes Japanese encephalitis, yellow fever and dengue fever.

West Nile Virus mainly affects birds - but can also infect other animals, including horses, dogs, cats, bats, skunks, squirrels, chipmunks and domestic rabbits. Humans may also be infected.

Most infected humans either experience no signs or symptoms, or just a skin rash and headache. A small percentage of infected individuals, however, may develop a life-threatening illness which may include encephalitis (inflammation of the brain), myelitis (inflammation of the spinal cord), or meningitis (inflammation of tissues surrounding the brain and spinal cord). Experts say approximately 1% of infected humans develop these serious complications. Those at highest risk for serious illness are the elderly and those with lowered immune systems.

West Nile Virus (WNV) is found in both tropical and temperate regions. WNV outbreaks occur in Egypt, Israel, Pakistan, France, the northern Mediterranean and some parts of Eastern Europe. In 1996 there was a major outbreak in Budapest, Hungary. In 1999 it appeared in New York and has since been found in all 48 US contiguous states. More recently it has been found in Canada, Mexico, Central America and the West Indies.

According to the National Health Service (NHS), UK, the WNV spread in mainland Europe and North America has made many experts fear that it is only a question of time before it reaches the United Kingdom. So far, there has only been one reported case in the UK - an tourist who arrived home after becoming infected while on vacation in Portugal.

A person who is exposed to mosquitoes in areas where West Nile virus exists has a higher risk of becoming infected. This risk can be significantly reduced by using mosquito repellent, wearing clothing that covers the skin, and taking some other preventive measures.

According to the CDC (Centers for Disease Control and Prevention), USA, over 15,000 people in the United States have tested positive for WNV since 1999 - including over 500 deaths. Authorities say the real number of infected people is much higher, because those with either no symptoms or very mild ones would not have sought medical help.

A bird-mosquito life cycle maintains West Nile virus in the environment. According to the U.S. Geological Survey's National Wildlife Health Center, American crows (Corvus brachyrhynchos) and other corvids (e.g., blue jays) seem more susceptible to fatal infection - nobody knows why. The death of these birds has allowed many local health departments to utilize dead birds as an indicator of the virus emergence or re-emergence in their areas.

Thousands of horses have been infected in the USA since 1999. Some regions report that over 30% of infected horses die from the infection or are euthanized due to poor prognosis.

West Nile virus is most commonly spread to humans from infected mosquitoes. However, it may also spread through blood transfusions and organ transplants (rare). Also, evidence indicates that a baby might become infected during pregnancy, at birth or through breast milk.

According to Medilexicon's medical dictionary:

West Nile virus is "a mosquito-borne virus of the family Flaviviridae. Human infection is usually subclinical but can lead to fatal encephalitis, particularly in old people."

What are the signs and symptoms of West Nile virus (WNV)?

A symptom is something the patient senses and describes, while a sign is something other people, such as the doctor notice. For example, drowsiness may be a symptom while dilated pupils may be a sign.

West Nile virus has three different possible effects on humans:

• Asymptomatic infection - there are no signs or symptoms. About 80% of cases.
• West Nile Fever (a mild febrile syndrome) - signs and symptoms are mild. About 20% of cases.
• Neuroinvasive disease - the infection causes complications in the nervous system (brain and spine). About 1% of cases.

West Nile fever (mild febrile syndrome) - this stage has an incubation period of 2 to 8 days (time from initial infection to the appearance of symptoms). Mild signs and symptoms may include:

• Backache
• Diaphoresis - excessive sweating
• Diarrhea
• Drowsiness
• Elevated body temperature (fever)
• Headache
• Nausea
• Skin rash
• Vomiting
• Muscle aches
• Loss of appetite
• Lymphadenopathy - swollen lymph nodes (glands)

These symptoms resolve within 7 to 10 days. Fatigue may linger for several weeks, while lymphadenopathy may take up to a couple of months.

West Nile fever (mild febrile syndrome) - approximately 1% of infected individuals develop more serious neurological infections, including:

• Encephalitis - inflammation of the brain
• Meningitis - inflammation of tissues surrounding the brain and spinal cord
• Myelitis (West Nile poliomyelitis) - inflammation of the spinal cord
• Acute flaccid paralysis - sudden weakness in the arms, legs and breathing muscles

Signs and symptoms may include:

• Coma
• Confusion
• Convulsions
• Disorientation
• High fever
• Muscle jerking
• Pain
• Parkinson's disease-like symptoms
• Partial paralysis
• Poor coordination
• Severe headache
• Stiff neck
• Stupor
• Sudden weakness
• Tremors

What are the risk factors for West Nile virus?

A risk factor is something which increases the likelihood of developing a condition or disease. For example, obesity significantly raises the risk of developing diabetes type 2. Therefore, obesity is a risk factor for diabetes type 2.

Seasons - in temperate regions WNV starts to appear in early spring. Infections peak in late summer and early fall (autumn). In tropical and some sub-tropical areas the risk of infection is there all year round.

Geographical area - living in or visiting an area known to have West Nile virus. In the USA today this includes all 48 contiguous states (the states in mainland USA that touch each other - Alaska and Hawaii are not contiguous states).

Time outdoors - people who spend more time outdoors run a higher risk of being bitten by an infected mosquito.

What are the causes of West Nile virus (WNV)?

The mosquito-bird cycle - the virus is transmitted through mosquito vectors.

• The birds are amplifying hosts - they develop sufficient viral levels to transmit the infection to other biting mosquitoes.
• The mosquitoes bite an infected bird and become infected themselves. The virus enters their bloodstream and circulates for a few days, and then settles in the salivary glands.
• The infected mosquitoes go on to infect other birds. In the USA the American robin and the American crow are common carriers.
• The infected mosquitoes bite animals and humans; virus enters the victim's bloodstream.

Mosquito species that infect animals and humans in the USA:

• Eastern USA - Culex pipiens
• Southeast USA - Culex quinquefasciatus
• Midwest and West USA - Culex tarsalis

Mammals - the virus does not multiply as readily in mammals as it does in birds. Experts believe that mosquitoes can infect mammals, but cannot be infected by mammals. Mammals are known as dead-end infections.

Why do mosquitoes bite? Only the female mosquito bites, the male doesn't. The female needs a blood meal so that she has the required protein to produce eggs. During her lifetime, a female mosquito lays multiple batches of eggs - requiring a blood meal to produce each batch. Mosquitoes vary in their preferences for blood; some prefer birds, others prefer mammals - some are generalists, they like both.

The female mosquito inserts a proboscis - a needle-like, slender, tubular feeding and sucking organ - under the skin of the mammal or bird, sucking blood into her abdomen. She will continue feeding until her abdomen is full, unless the animals brushes her away or smacks her.

We do not know what the exact mechanism of the illness is. Experts say the virus enters the human's bloodstream, reproduces and sometimes crosses the blood-brain barrier and enters the brain. If it enters the brain, brain tissue becomes inflamed.

Other possible routes of transmission:

• Blood transfusions - as donated blood is now screened for WNV the risk of becoming infected is much lower.
• Organ transplants - donated organs are not yet screened for WNV.
• Pregnancy - studies indicate that the baby may become infected if the mother is infected.
• Breast feeding - if the mother is infected, she might transmit the infection to her baby through her breast milk - authorities in many countries say the risk is so small that mothers should not be put off breast feeding.
• Laboratory work - people in laboratories where WNV R&D and surveillance is being done can become infected.

How is West Nile virus diagnosed?

Diagnosis procedures may vary according to geographical area. In the UK, where WNV has not yet arrived, the doctor will ask the patient about their medical history and whether they have been travelling recently. The patient's skin will also be examined to see whether there are any signs of mosquito bites. It is likely that in countries and areas where WNV is common, the doctors will consider WNV as a possibility more rapidly.

A doctor will ask the patient about symptoms and carry out a physical examination. There may be no specific findings on a physical exam to diagnose WVN infection. The following diagnostic tests may be ordered:

• Blood test - this may reveal higher-than-normal levels of antibodies to the WVN. A complete blood count may be done.
• CT (computed tomography) scan of the head - the CT scanner uses digital geometry processing to generate a 3-dimensional (3-D) image of the inside of an object. The 3-D image is made after many 2-dimensional (2-D) X-ray images are taken around a single axis of rotation - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3-D image. It is a painless procedure. It can reveal brain inflammation and swelling (not always).
• MRI (magnetic resonance imaging) scan of the head - uses a magnetic field and radio waves to create detailed images of the body. It can reveal brain inflammation and swelling (not always).
• Lumbar puncture (spinal tap) - to diagnose meningitis. Some cerebrospinal fluid that surrounds the brain and spinal cord is extracted using a needle which is inserted between the lower vertebrae of the spine. If lab results show an elevated white cell count it means there is an infection.

What are the treatment options for West Nile Virus?

The majority of patients make a full recovery from WNV without medical treatment. Many with complications, such as encephalitis may only require supportive therapy with intravenous fluids, breathing support (by ventilator) and pain relief. Supportive therapy means helping the body fight the illness on its own, and not treating the causes of the illness directly.

Prevention of West Nile virus (WVN)

There is currently no licensed vaccine available to protect humans against WNV. Scientists are working on one.

The best method of prevention is to avoid being bitten by mosquitoes. If you live or travel to an area where dengue exists, there a number of ways to avoid being bitten:

• Clothing - your chances of being bitten are significantly reduced if you expose as little skin as possible. When in an area with mosquitoes, be sure to wear long trousers/pants, long sleeved shirts, and socks. For further protection, tuck your pant legs into your shoes or socks. Wear a hat.
• Mosquito repellants - be sure to use one with at least 10% concentration of DEET, you will need a higher concentration the longer you need the protection, avoid using DEET on young children.
• Use mosquito traps and nets - studies have shown that the risk of being bitten by mosquitoes is considerably reduced if you use a mosquito net when you go to sleep. Untreated nets are significantly less effective because the mosquito can bite the host through the net if the person is standing next to it. Also, even tiny holes in the netting are usually enough for the mosquito to find a way in. Nets that have been treated with insecticide are much more protective. Not only does the insecticide kill the mosquito and other insects, it is also a repellent - fewer mosquitoes are likely to enter the room(s).
• Smell - Avoid wearing heavily scented soaps and perfumes.
• Windows - use structural barriers, such as window screens or netting.
• Camping - if you are camping, treat clothes, shoes and camping gear with permethrin. There are clothes which have been treated with permethrin.
• Certain times of day - try to avoid being outside at dawn, dusk and early evening.
• Stagnant water - the Aedes mosquito prefers to breed in clean, stagnant water. It is important to frequently check and remove stagnant water in your home/premises.
  
  
  • Turn pails (buckets) and watering cans over; store them under shelter so water cannot accumulate in them.
  • Remove the water from plant pot plates. To remove mosquito eggs, clean and scrub them thoroughly. Ideally, do not use plant pot plates.
  • Loosen soil from potted plants. This will prevent puddles from developing on the surface of hard soil.
  • Make sure scupper drains are not blocked; do not place potted plants and other objects over the scupper drains.
  • Gully traps that are rarely used should be covered; replace gully traps with non-perforated ones, and install anti-mosquito valves.
  • Do not place receptacles under or on top of any air-conditioning unit.
  • Flower vases - change the water every other day. When you do so, scrub the inside of the vase thoroughly and rinse it out.
  • Leaves - make sure leaves are not blocking anything which may result in the accumulation of puddles or stagnant water.

Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today

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