West Nile fever

West Nile fever
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West Nile virus
SpecialtyInfectious disease
SymptomsNone, fever, headache, vomiting, rash[1]
ComplicationsEncephalitis, meningitis[1]
Usual onset2 to 14 days post exposure[1]
DurationWeeks to months[1]
CausesWest Nile virus spread by mosquito[1]
Diagnostic methodBased on symptoms and blood tests[1]
PreventionReducing mosquitoes, preventing mosquito bites[1]
TreatmentSupportive care (pain medication)[1]
Prognosis10% risk of death among those seriously affected[1]

West Nile fever is a viral infection typically spread by mosquitoes.[1] In about 75% of infections people have few or no symptoms.[1] About 20% of people develop a fever, headache, vomiting, or a rash.[1] In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures.[1] Recovery may take weeks to months.[1] The risk of death among those in whom the nervous system is affected is about 10%.[1]

West Nile virus (WNV) is typically spread by infected mosquitoes.[1] Mosquitoes become infected when they feed on infected birds.[1] Rarely the virus is spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding.[1] It otherwise does not spread directly between people.[2] Risks for severe disease include age over 60 and other health problems.[1] Diagnosis is typically based on symptoms and blood tests.[1]

There is no human vaccine.[1] The best method to reduce the risk of infections is avoiding mosquito bites.[1] This may be done by eliminating standing pools of water, such as in old tires, buckets, gutters, and swimming pools.[1] Mosquito repellent, window screens, mosquito nets, and avoiding areas where mosquitoes occur may also be useful.[1][2] While there is no specific treatment, pain medications may be useful.[1]

WNV has occurred in Europe, Africa, Asia, Australia, and North America.[1] In the United States thousands of cases are reported a year, with most occurring in August and September.[3] It can occur in outbreaks of disease.[2] The virus was discovered in Uganda in 1937 and was first detected in North America in 1999.[1][2] Severe disease may also occur in horses and a vaccine for these animals is available.[2] A surveillance system in birds is useful for early detection of a potential human outbreak.[2]

Signs and symptoms

The incubation period for WNV—the amount of time from infection to symptom onset—is typically from between 2 and 15 days. 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.[4] Most characterizations of WNF generally 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. In addition to a high fever, headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, drowsiness, pain in the joints and flu-like symptoms. Gastrointestinal symptoms that may occur include 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.[5] 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.[5]
  • 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.[6][7]
  • West Nile meningitis (WNM) usually involves fever, headache, and stiff neck. Pleocytosis, an increase of white blood cells in cerebrospinal fluid, is also present. 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, can sometimes occur.
  • West-Nile reversible paralysis, Like WNP, the weakness or paralysis is asymmetric.[8] Reported cases have been noted to have an initial preservation of deep tendon reflexes, which is not expected for a pure anterior horn involvement.[8] Disconnect of upper motor neuron influences on the anterior horn cells possibly by myelitis or glutamate excitotoxicity have been suggested as mechanisms.[8] The prognosis for recovery is excellent.
  • Nonneurologic complications of WNV infection that may rarely occur include fulminant hepatitis, pancreatitis,[9] myocarditis, rhabdomyolysis,[10] orchitis,[11] nephritis, optic neuritis[12] and cardiac dysrhythmias and hemorrhagic fever with coagulopathy.[13] Chorioretinitis may also be more common than previously thought.[14]
  • Cutaneous manifestations specifically rashes, are not uncommon in WNV-infected patients; however, there is a paucity of detailed descriptions in case reports and there are few clinical images widely 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.[15]
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