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West Nile virus is a specific virus that is causing an illness that mainly affects birds, humans and horses. It is being transmitted through mosquito bites. There is no specific treatment for West Nile Virus infection or a vaccine to prevent it.

This condition is very strange because most people with West Nile virus have no symptoms and never know that they've been infected, while others, especially older people or people with underlying medical conditions, can develop very serious condition which is leading to inflammation of the brain or inflammation and infection of the membranes surrounding the brain and spinal cord. West Nile Virus has spread rapidly across North America, affecting thousands of birds, horses, and humans, since it was discovered in the Western hemisphere.

Incidence and spreading

According to the U.S. Centers for Disease Control and Prevention (CDC), over 15,000 people in the U.S. have tested positive for WNV infection since 1999, and 500 people died from this disease. Many more have likely been infected with WNV, but have experienced mild or no symptoms.

The West Nile virus was first detected in the Western Hemisphere in 1999 and has since rapidly spread across the North American continent into all 48 continental states, seven Canadian provinces, and throughout Mexico. In addition, WNV activity has been detected in Puerto Rico, the Dominican Republic, Jamaica, Guadeloupe and El Salvador.

Transmission of the virus

West Nile Virus is transmitted to humans only through mosquito bites. Mosquitoes become infected when they feed on infected birds that have high levels of this virus in their blood. Infected mosquitoes can then transmit the virus when they feed on humans or other animals. West Nile Virus cannot be trensferred from person to person and there is no evidence that a person can get infected by handling live or dead infected birds.

Symptoms of West Nile fever

Although most people experience no symptoms what so ever, there are some symptoms that could be considered typical for the West Nile fever. The most common signs and symptoms of West Nile fever include:

  • Nausea, vomiting and diarrhea
  • Skin rash
  • Swollen lymph glands
  • Muscle aches
  • Backache
  • Fever
  • Headache
  • Lack of appetite

In less than 1 percent of the infected people, the virus causes a more serious neurological infection, such as:

  • inflammation of the brain - encephalitis,
  • inflammation of the brain and surrounding membranes - meningo-encephalitis, 
  • infection and inflammation of the membranes surrounding the brain and spinal cord (meningitis),
  • paralysis

Signs and symptoms of these diseases also include:

  • High fever
  • Stupor or coma
  • Tremors or muscle jerking
  • Signs and symptoms of Parkinson's disease
  • Severe headache
  • Stiff neck
  • Disorientation or confusion
  • Lack of coordination
  • Convulsions

Mechanism of the condition

The exact mechanism of the disease is yet unknown. However, most experts from this field believe that the West Nile virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood-brain barrier. Once the virus crosses that barrier and infects the brain, an inflammatory response occurs and symptoms arise.
The virus transmission occurs mostly during warm weather, when mosquito populations are active. The incubation period ranges from 3 to 14 days.
In rare cases, it can be spread through other routes, including:

Organ transplantation and blood transfusion
Some people have developed West Nile virus after receiving a transplanted organ or blood products. Although donated organs are not yet screened for West Nile virus, blood donor screening for West Nile is the routine now.

Congenital infection
In 2002 there was a case of a woman in New York who contracted West Nile virus in the last trimester of her pregnancy The baby who she gave birth to five weeks later was also infected with the West Nile virus.

Infection via breastfeeding is possible but experts at the Centers for Disease Control and Prevention claim that such cases are extremely rare and should not affect any woman's decision to breast-feed her baby.

Laboratory infection
Some laboratory workers involved in West Nile surveillance and research have contracted the disease from infected animals.

Risk factors for developing West Nile fever

Overall risk of contracting West Nile virus depends on several factors:

Time of year
West Nile virus follows a seasonal pattern that begins in late spring, with the peak time for infection occurring in late summer and early fall.

Geographic region
Every visit to the country where mosquito-borne viruses are common, especially the East Coast and Midwest, increases the risk of West Nile virus infection.
Working outdoors puts a person at a greater chance of being bitten by an infected mosquito.
The course of the disease varies significantly. Some patients recover in two weeks and some develop serious health problems. Among those more likely to develop severe or fatal infections are:

  • Older adults
  • Pregnant women
  • People with immune systems weakened by HIV/AIDS, long-term steroid use, chemotherapy drugs or anti-rejection drugs following transplant surgery

Diagnosis of West Nile Virus infection

Anyone who has symptoms of severe illness such as mental status changes, high fever, neck stiffness, sensitivity to light, or confusion should go to the hospital's emergency department immediately.

Diagnosing West Nile virus infection is not complicated and is generally done through a combination of observing signs and symptoms along with specialized molecular biologic testing for the virus itself. Doctor can confirm the presence of West Nile virus in patient’s body by analyzing a sample of blood or the cerebrospinal fluid.

Signs of the disease include:

  • A rising level of antibodies to the West Nile virus.
  • A positive ribonucleic acid test for the West Nile virus.

Confirmatory diagnosis of West Nile virus infection is also done by a DNA test called polymerase chain reaction (PCR) or viral culture of fluid around the spinal cord.

Other tests:

Lumbar puncture (spinal tap)
The most common way to diagnose meningitis, as one of the possible complications of West Nile Virus infection, is to analyze the cerebrospinal fluid surrounding the brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to extract a sample of fluid for laboratory analysis.
Brain imaging
In some, but not all cases, a computerized tomography (CT) or magnetic resonance imaging (MRI) scan can reveal brain inflammation and swelling.

The bottom line is that the most accurate way to diagnose this infection is serology, a test to detect the presence of antibodies against West Nile virus in CSF Or serum. This is considered the gold standard for diagnosis.

Treatment of West Nile fever

Most people recover from West Nile virus without treatment.

Even those who develop encephalitis or meningitis may only need therapy with intravenous fluids and pain relievers. Statistically, a person's risk of contracting West Nile is low, and less than 1% of those infected develop serious illness from the virus. Scientists are currently investigating interferon therapy, a type of immune cell therapy, as a treatment for encephalitis caused by West Nile virus.

Prevention of infection

Efforts to detect and contain the virus include:

  • Sampling of mosquito and bird populations for West Nile virus
  • Increased surveillance of animals and humans for infection
  • Eliminating mosquito-breeding areas
  • Increased physician awareness and reporting of the virus so that its spread can be tracked
  • Conducting public-awareness campaigns to let people know how to reduce their risk of exposure to the virus

What can an individual do to reduce the risk of the infection? Here are some tips that showed beneficial:

  • Eliminate standing water in your yard. 
  • Unclog roof gutters.
  • Empty unused swimming pools.
  • Change water in birdbaths at least once a week.
  • Remove all old unused things that might hold water and serve as a breeding ground for mosquitoes.
  • Watch for sick or dying birds and report them to your local health department.

There are also several other prevention measures. Some of those are:

  • Avoiding unnecessary outdoor activity when mosquitoes are most prevalent
  • Wearing long-sleeved shirts and long pants 
  • Applying mosquito repellent with a 10 percent to 30 percent concentration of deet to your skin and clothing. 


A vaccine which protects horses from West Nile already exists but no such vaccine is yet available for humans.