West Nile fever is an infection by the West Nile virus, which is typically spread by mosquitoes. In about 80% of infections people have few or no symptoms. About 20% of people develop a fever, headache, vomiting, or a rash. In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures. Recovery may take weeks to months. The risk of death among those in whom the nervous system is affected is about 10%. West Nile virus (WNV) is usually spread by mosquitoes that become infected when they feed on infected birds. Birds often carry the disease. Rarely the virus is spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding, but it otherwise does not spread directly between people. Risks for severe disease include being very young, over 60 years old, having a weak immune system, and having other health problems. Diagnosis is typically based on symptoms and blood tests. There is no human vaccine. The best way to reduce the risk of infection is to avoid mosquito bites. Mosquito populations may be reduced by eliminating standing pools of water, such as in old tires, buckets, gutters, and swimming pools. When mosquitoes cannot be avoided, mosquito repellent, window screens, and mosquito nets reduce the likelihood of being bitten. There is no specific treatment for the disease; pain medications may reduce symptoms. The virus was discovered in Uganda in 1937, and was first detected in North America in 1999. WNV has occurred in Europe, Africa, Asia, Australia, and North America. In the United States thousands of cases are reported a year, with most occurring in August and September. It can occur in outbreaks of disease. Severe disease may also occur in horses, for which a vaccine is available. A surveillance system in birds is useful for early detection of a potential human outbreak.
Signs and symptoms About 80% of those infected with West Nile virus (WNV) show no symptoms and go unreported. About 20% of infected people develop symptoms. These vary in severity, and begin 3 to 14 days after being bitten. Most people with mild symptoms of WNV recover completely, though fatigue and weakness may last for weeks or months. Symptoms may range from mild, such as fever, to severe, such as paralysis and meningitis. A severe infection can last weeks and can, rarely, cause permanent brain damage. Death may ensue if the central nervous system is affected. Medical conditions such as cancer and diabetes, and age over 60 years, increase the risk of developing severe symptoms. Headache can be a prominent symptom of WNV fever, meningitis, encephalitis, meningoencephalitis, and it may or may not be present in poliomyelitis-like syndrome. Thus, headache is not a useful indicator of neuroinvasive disease.
West Nile fever (WNF), which occurs in 20 percent of cases, is a febrile syndrome that causes flu-like symptoms. Most characterizations of WNF describe it as a mild, acute syndrome lasting 3 to 6 days after symptom onset. Systematic follow-up studies of patients with WNF have not been done, so this information is largely anecdotal. Possible symptoms include high fever, headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, drowsiness, pain in the joints and flu-like symptoms. There may be gastrointestinal symptoms including nausea, vomiting, loss of appetite, and diarrhea. Fewer than one-third of patients develop a rash. West Nile neuroinvasive disease (WNND), which occurs in less than 1 percent of cases, is when the virus infects the central nervous system resulting in meningitis, encephalitis, meningoencephalitis or a poliomyelitis-like syndrome. Many patients with WNND have normal neuroimaging studies, although abnormalities may be present in various cerebral areas including the basal ganglia, thalamus, cerebellum, and brainstem. West Nile virus encephalitis (WNE) is the most common neuroinvasive manifestation of WNND. WNE presents with similar symptoms to other viral encephalitis with fever, headaches, and altered mental status. A prominent finding in WNE is muscular weakness (30 to 50 percent of patients with encephalitis), often with lower motor neuron symptoms, flaccid paralysis, and hyporeflexia with no sensory abnormalities. West Nile meningitis (WNM) usually involves fever, headache, stiff neck and pleocytosis, an increase of white blood cells in cerebrospinal fluid. Changes in consciousness are not usually seen and are mild when present. West Nile meningoencephalitis is inflammation of both the brain (encephalitis) and meninges (meningitis). West Nile poliomyelitis (WNP), an acute flaccid paralysis syndrome associated with WNV infection, is less common than WNM or WNE. This syndrome is generally characterized by the acute onset of asymmetric limb weakness or paralysis in the absence of sensory loss. Pain sometimes precedes the paralysis. The paralysis can occur in the absence of fever, headache, or other common symptoms associated with WNV infection. Involvement of respiratory muscles, leading to acute respiratory failure, sometimes occurs. West-Nile reversible paralysis, Like WNP, the weakness or paralysis is asymmetric. Reported cases have been noted to have an initial preservation of deep tendon reflexes, which is not expected for a pure anterior horn involvement. Disconnect of upper motor neuron influences on the anterior horn cells possibly by myelitis or glutamate excitotoxicity have been suggested as mechanisms. The prognosis for recovery is excellent. Nonneurologic complications of WNV infection that may rarely occur include fulminant hepatitis, pancreatitis, myocarditis, rhabdomyolysis, orchitis, nephritis, optic neuritis and cardiac dysrhythmias and hemorrhagic fever with coagulopathy. Chorioretinitis may also be more common than previously thought. Skin manifestations, specifically rashes, are common; however, there are few detailed descriptions in case reports, and few images are available. Punctate erythematous, macular, and papular eruptions, most pronounced on the extremities have been observed in WNV cases and in some cases histopathologic findings have shown a sparse superficial perivascular lymphocytic infiltrate, a manifestation commonly seen in viral exanthems. A literature review provides support that this punctate rash is a common cutaneous presentation of WNV infection.
Cause
Virology
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