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West Nile Virus (WNV) is transmitted by mosquitos and causes an illness that ranges from mild to severe. The mild, flu-like version is often called West Nile fever. More severe forms of disease may be called West Nile encephalitis or West Nile meningitis, depending on where it spreads.
West Nile virus is a type of organism called a flavivirus and is similar to many other mosquito-borne viruses, including Japanese encephalitis (which is found in Asia). Researchers believe the virus is spread when a mosquito bites an infected bird and then bites a person.
West Nile virus was first identified in 1937 in the West Nile region of Uganda, in eastern Africa. It was first identified in the US in the summer of 1999 in the Queens borough of New York, NY. It caused 62 cases of encephalitis and 7 deaths that summer. Since 1999 the virus has spread throughout the continental US and as of September 2002 had been identified in 42 states.
Incidence; Risk Factors
Data from the outbreak in Queens, New York suggested that although 2.6% of the population was infected, only 1 in 5 infected people developed mild illness, and only 1 in 150 infected people developed brain inflammation (meningitis or encephalitis).
Mosquitos carry the highest amounts of virus in late August to early September, causing a spike in the disease at this time. The risk of disease decreases as the weather becomes colder and mosquitos die off.
Signs & Symptoms
Although many people are bitten by mosquitos that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all.
With more severe disease, the following symptoms can also be seen and require prompt attention: Muscle weakness, stiff neck, confusion, loss of consciousness.
Diagnosis & Tests
Signs of West Nile virus infection are similar to those of other viral infections. There is nothing that can be found on physical examination to diagnose West Nile virus infection.
Diagnostic tests that may be used if West Nile virus is suspected include the following:- CBC -- may show a normal or elevated white blood cell (WBC) count
- Lumbar puncture and cerebrospinal fluid (CSF) testing
- usually shows elevated WBC count (especially lymphocytes)
- usually shows elevated protein level
- Head CT scan -- often normal
- Head MRI scan -- may show evidence of inflammation (in about one-third of patients).
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 (a blood component). This is considered the gold standard for diagnosis.
Rarely, a sample of blood or CSF may be sent to a lab to be cultured to evaluate the presence of West Nile virus. The virus can also be identified in body fluids using a technique called polymerase chain reaction (PCR). However, these methods can provide false negative results.
Treatment & Prevention
Because this illness is not caused by bacteria, antibiotics do not help. Standard hospital care may help decrease the risk of complications in severe illness. There is no human vaccine available at present, and it is likely there will not be one for several years.
Research trials are under way to determine whether ribavirin, an antiviral drug used to treat hepatitis C, may be helpful.
Prognosis; Complications
In general, the likely outcome of a mild West Nile virus infection is excellent. For patients with severe cases of West Nile virus infection, the outlook is more guarded. West Nile encephalitis or meningitis has the potential to lead to brain damage and death. Approximately 10% of patients with brain inflammation do not survive.
Complications from mild West Nile virus infection are extremely rare but include permanent brain damage or muscle weakness (sometimes similar to polio), and death.
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Signs, symptoms & indicators of West Nile Virus:
Conditions that suggest West Nile Virus:
Risk factors for West Nile Virus:
West Nile Virus suggests the following may be present:
West Nile Virus can lead to:
KEY | | Weak or unproven link |  | | Strong or generally accepted link |  | | Proven definite or direct link |
GLOSSARY
AIDS
Acquired Immune Deficiency Syndrome. An immune system deficiency disorder that suddenly alters the body's ability to defend itself. The AIDS virus invades the T4 helper/inducer lymphocytes and multiplies, causing a breakdown in the body's immune system, eventually leading to overwhelming infection and/or cancer, with ultimate death.
Antibody (Antibodies)
A type of serum protein (globulin) synthesized by white blood cells of the lymphoid type in response to an antigenic (foreign substance) stimulus. Antibodies are complex substances formed to neutralize or destroy these antigens in the blood. Antibody activity normally fights infection but can be damaging in allergies and a group of diseases that are called autoimmune diseases.
Antiviral
Any of a number of herbs, drugs or agents capable of destroying viruses or inhibiting their growth or multiplication until the body is capable of destroying the virus itself. Most antiviral agents are members of the antimetabolite family.
Bacteria (Bacterial, Bacterium)
Microscopic germs. Some bacteria are "harmful" and can cause disease, while other "friendly" bacteria protect the body from harmful invading organisms.
Chemotherapy
A treatment of disease by any chemicals. Used most often to refer to the chemical treatments used to combat cancer cells. Chemotherapy is usually given in cycles: a treatment period followed by a recovery period, then another treatment period, and so on. Most anticancer drugs are given by injection into a blood vessel (IV); some are given by mouth. Chemotherapy is a systemic therapy, meaning that the drugs enter the bloodstream and travel throughout the body. Usually, a patient has chemotherapy as an outpatient (at the hospital, at the doctor's office, or at home). However, depending on which drugs are given and the patient's general health, a short hospital stay may be needed.
Complete Blood Count (CBC)
A set of measurements that are generally determined by specially designed machines that analyze the different components of blood in less than a minute. The values generally included are:- White blood cell count (WBC). The number of white blood cells in a volume of blood. Normal range varies slightly between laboratories but is generally between 4,300 and 10,800 cells per cubic millimeter. This can also be referred to as the leukocyte count and can be expressed in international units as 4.3 - 10.8 x 10^9 cells per liter.
- Automated white cell differential. A machine-generated percentage of the different types of white blood cells, usually split into granulocytes, lymphocytes, monocytes, eosinophils, and basophils.
- Red cell count (RBC). The number of red blood cells in a volume of blood. Normal range varies slightly between laboratories but is generally between 4.2 - 5.9 million cells per cubic millimeter. This can also be referred to as the erythrocyte count and can be expressed in international units as 4.2 - 5.9 x 10^12 cells per liter.
- Hemoglobin (Hb). The amount of hemoglobin in a volume of blood. Normal range for hemoglobin is different between the sexes and is approximately 13 - 18 gm/dl for men and 12 - 16 for women (international units 8.1 - 11.2 millimoles/liter for men, 7.4 - 9.9 for women).
- Hematocrit (Hct). The ratio of the volume of red cells to the volume of whole blood. Normal range for hematocrit is different between the sexes and is approximately 45 - 52% for men and 37 - 48% for women.
- Mean cell volume (MCV). The average volume of a red cell. This is a calculated value derived from the hematocrit and red cell count. Normal range is 86 - 98 femtoliters.
- Mean cell hemoglobin (MCH). The average amount of hemoglobin in the average red cell. This is a calculated value derived from the measurement of hemoglobin and the red cell count. Normal range is 27 - 32 picograms.
- Mean cell hemoglobin concentration (MCHC). The average concentration of hemoglobin in a given volume of red cells. This is a calculated volume derived from the hemoglobin measurement and the hematocrit. Normal range is 32 - 36%.
- Red cell distribution width (RDW). A measurement of the variability of red cell size. Higher numbers indicate greater variation in size. Normal range is 11 - 15.
- Platelet count. The number of platelets in a volume blood. Platelets play a vital role in blood clotting. Normal range varies slightly between laboratories but is in the range of 150,000 - 400,000 per cubic millimeter (150 - 400 x 10^9 per liter).
Computerised Tomography (CAT Scan, Computed Tomography, Computerized Axial Tomography, CT Scan)
A scanning procedure using X-rays and a computer to detect abnormalities of the body's organs. This technique provides cross-sectional images of body organs, which is much clearer than those provided by conventional X-rays.
Hepatitis C
Caused by an RNA flavivirus. Transmission is predominantly through broken skin on contact with infected blood or blood products, especially through needle sharing. Sexual transmission is relatively rare. Symptoms are almost always present, and very similar to those for Hepatitis B: initially flu-like, with malaise, fatigue, muscle pain and chest pain on the right side. This is followed by jaundice (slight skin yellowing), anorexia, nausea, fatigue, pale stools, dark urine and tender liver enlargement, but usually no fever.
Immune System (Immune Response, Immunity)
A complex that protects the body from disease organisms and other foreign bodies. The system includes the humoral immune response and the cell-mediated response. The immune system also protects the body from invasion by making local barriers and inflammation. The process may involve acquired immunity (the ability to learn and remember a specific infectious agent), or innate immunity (the genetically programmed system of responses that attack, digest, remove, and initiate inflammation and tissue healing).
Magnetic Resonance Imaging (MRI)
A technique used in diagnosis that combines radio waves and magnetic forces to produce detailed images of the internal structures of the body.
Protein (Proteins)
Compounds composed of hydrogen, oxygen, and nitrogen present in the body and in foods that form complex combinations of amino acids. Protein is essential for life and is used for growth and repair. Foods that supply the body with protein include animal products, grains, legumes, and vegetables. Proteins from animal sources contain the essential amino acids. Proteins are changed to amino acids in the body.
Serum
The cell-free fluid of the bloodstream. It appears in a test tube after the blood clots and is often used in expressions relating to the levels of certain compounds in the blood stream.
Spinal Tap (Lumbar Puncture)
Despite the discomfort felt by many over this procedure, spinal taps are actually very simple procedures from the doctor's standpoint. Because the fluid that coats and flows through the brain also flows down into the spinal cord, the spinal fluid reflects what is occurring in the brain. A spinal tap involves: numbing the skin over the lower part of the spine with an injection or cream; sterilizing the area; having the patient curl up on his side while being held securely; passing a needle through the skin at a large space in between two of the lower vertebrae (the needle does NOT pass through bone); slowly advancing the needle until it "pops" through the lining of the spinal canal into an area of spinal fluid (the needle does NOT go into the spinal cord where the nerves are because the spinal cord stops further up the spine: the bottom of the spine simply contains a reservoir of spinal fluid); spinal fluid will flow out through the needle and the doctor will collect it and send it to the lab. Once the needle is pulled out, the hole seals up rapidly.
Virus (Viri, Viruses)
Any of a vast group of minute structures composed of a protein coat and a core of DNA and/or RNA that reproduces in the cells of the infected host. Capable of infecting all animals and plants, causing devastating disease in immunocompromised individuals. Viruses are not affected by antibiotics, and are completely dependent upon the cells of the infected host for the ability to reproduce.
White Blood Cell (WBC, White Blood Cells)
A blood cell that does not contain hemoglobin: a blood corpuscle responsible for maintaining the body's immune surveillance system against invasion by foreign substances such as viruses or bacteria. White cells become specifically programmed against foreign invaders and work to inactivate and rid the body of a foreign substance. White blood cells are composed primarily of neutrophils, monocytes and lymphocytes. Lymphocytes are either T-cells or B-cells. T-cells (CD3 cells) are divided into T-helper (CD4 cells) and T-suppressor/cytotoxic (CD8 cells) cells.
Last updated: Apr 27, 2008
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