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West Nile virus (WNV) is a mosquito-borne zoonotic arbovirus belonging to the genus ''Flavivirus'' in the family ''Flaviviridae''. It is found in temperate and tropical regions of the world. It was first identified in the West Nile subregion in the East African nation of Uganda in 1937. Prior to the mid-1990s, WNV disease occurred only sporadically and was considered a minor risk for humans, until an outbreak in Algeria in 1994, with cases of WNV-caused encephalitis, and the first large outbreak in Romania in 1996, with a high number of cases with neuroinvasive disease. WNV has now spread globally, with the first case in the Western Hemisphere being identified in New York City in 1999; over the next five years, the virus spread across the continental United States, north into Canada, and southward into the Caribbean islands and Latin America. WNV also spread to Europe, beyond the Mediterranean Basin, and a new strain of the virus was identified in Italy in 2012. WNV is occurs on an ongoing basis in Africa, Asia, Australia, the Middle East, Europe, Canada and in the United States. In 2012 the US experienced one of its worst epidemics in which 286 people died, with the state of Texas being hard hit by this virus, The main mode of WNV transmission is via various species of mosquitoes, which are the prime vector, with birds being the most commonly infected animal and serving as the prime reservoir host—especially passerines, which are of the largest order of birds, Passeriformes. WNV has been found in various species of ticks, but current research suggests they are not important vectors of the virus. WNV also infects various mammal species, including humans, and has been identified in reptilian species, including alligators and crocodiles, and also in amphibians. Not all animal species that are susceptible to WNV infection, including humans, and not all bird species develop sufficient viral levels to transmit the disease to uninfected mosquitoes, and are thus not considered major factors in WNV transmission. Approximately 80% of West Nile virus infections in humans are subclinical, which cause no symptoms.〔(【引用サイトリンク】title=West Nile Virus: What You Need to Know CDC Fact Sheet )〕 In the cases where symptoms do occur—termed West Nile fever in cases without neurological disease—the time from infection to the appearance of symptoms (incubation period) is typically between 2 and 15 days. Symptoms may include fever, headaches, fatigue, muscle pain or aches (myalgias), malaise, nausea, anorexia, vomiting, and rash. Less than 1% of the cases are severe and result in neurological disease when the central nervous system is affected. People of advanced age, the very young, or those with immunosuppression, either medically induced, such as those taking immunosupressive drugs, or due to a pre-existing medical condition such as HIV infection, are most susceptible. The specific neurological diseases that may occur are West Nile encephalitis, which causes inflammation of the brain, West Nile meningitis, which causes inflammation of the meninges, which are the protective membranes that cover the brain and spinal cord, West Nile meningoencephalitis, which causes inflammation of the brain and also the meninges surrounding it, and West Nile poliomyelitis—spinal cord inflammation, which results in a syndrome similar to polio, which may cause acute flaccid paralysis. Currently, no vaccine against WNV infection is available. The best method to reduce the rates of WNV infection is mosquito control on the part of municipalities, businesses and individual citizens to reduce breeding populations of mosquitoes in public, commercial and private areas via various means including eliminating standing pools of water where mosquitoes breed, such as in old tires, buckets, and unused swimming pools. On an individual basis, the use of personal protective measures to avoid being bitten by an infected mosquito, via the use of mosquito repellent, window screens, avoiding areas where mosquitoes are more prone to congregate, such as near marshes, and areas with heavy vegetation, and being more vigilant from dusk to dawn when mosquitoes are most active offers the best defense. In the event of being bitten by an infected mosquito, familiarity of the symptoms of WNV on the part of laypersons, physicians and allied health professions affords the best chance of receiving timely medical treatment, which may aid in reducing associated possible complications and also appropriate palliative care. ==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. 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. 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, 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. 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. * 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「West Nile virus」の詳細全文を読む スポンサード リンク
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