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Essentials of Diagnosis
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Acute febrile illness; rash may be present; stiff neck progressing to stupor, coma, and convulsions
Upper motor neuron lesion signs: exaggerated deep tendon reflexes, absent superficial reflexes, and spastic paralysis
Cerebrospinal fluid opening pressure and protein are often increased with lymphocytic pleocytosis
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General Considerations
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Caused by arthropod-borne viruses
The mosquito-borne pathogens include
Togaviruses (most of which are of the genus Alphavirus: Western, Eastern, Venezuelan equine encephalitis, chikungunya, and Mayaro virus)
Flaviviruses (causing West Nile fever, St. Louis encephalitis, Japanese encephalitis, and Murray Valley encephalitis; dengue, Zika, yellow fever can present with encephalitis)
Bunyaviruses (the California serogroup of viruses, including the Jamestown Canyon and the La Crosse viruses)
The tick-borne causes of encephalitis include
The flavivirus of Powassan encephalitis (North America)
Tick-borne encephalitis virus of Europe and Asia
The Colorado tick fever reovirus
West Nile virus
Leading cause of domestically acquired arboviral disease in the United States; most cases are identified in Texas and California
In 2019, 971 cases were reported to the CDC from 43 US states and the District of Columbia
65% of cases in 2019 were classified as neuroinvasive disease
Outbreaks with West Nile infection tend to occur between mid-July and early September
Climatic factors, including elevated mean temperatures and rainfall, correlate with increased West Nile infection
Transmission
Mosquitos infected by biting infected birds can infect people and other mammals
However, the virus cannot be transmitted from infected humans and other mammals to other biting mosquitoes
Human-to-human transmission is usually related to blood transfusion and organ transplantation
Since 2003, all blood donations in the United States are screened with nucleic acid amplification assays for West Nile virus
Eastern equine encephalitis and Powassan encephalitis virus have also been shown to be transmitted by organ transplantation
La Crosse virus
Average number of neuroinvasive cases reported annually is 63 cases
Most cases are reported from Midwestern states
Occur from late spring to early fall
Less severe forms of California serogroup viruses include Jamestown Canyon virus and California encephalitis virus
Risk factors for development of neuroinvasive disease and increased mortality include
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Incubation period is 2–14 days
Disease manifestations are strongly age-dependent
Acute febrile syndrome and mild neurologic symptoms are more common in the young
Aseptic meningitis and poliomyelitis-like syndromes are seen in the middle aged
Frank encephalopathy is seen in the elderly
The infection is symptomatic in about 20% of cases, and of those, less than 1% progress to neuroinvasive disease including meningitis, encephalitis, and flaccid paralysis
Symptoms include acute febrile illness and a nonpruritic maculopapular rash (variably present)
Meningitis is indistinguishable from other viral meningitis
West Nile virus encephalitis presents with fever ...