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Key Features

Essentials of Diagnosis

  • Fever, malaise, stiff neck, sore throat, and vomiting 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

General Considerations

  • Caused by arthropod-borne viruses

  • The mosquito-borne pathogens include

    • Three togaviruses (causing Western, Eastern, and Venezuelan equine encephalitis)

    • Four flaviviruses (causing West Nile fever, St. Louis encephalitis, Japanese encephalitis, and Murray Valley encephalitis)

    • Bunyaviruses (the California serogroup of viruses, including the La Crosse virus)

  • The tick-borne causes of encephalitis include

    • The flavivirus of the 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 2017, 47 states reported cases to the CDC

      • 67% were classified as neuroinvasive disease

      • 33% were classified as non-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 bite of 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

  • 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

      • Black race

      • Diabetes mellitus

      • Chronic kidney disease

      • Hepatitis C virus infection

Clinical Findings

Symptoms and Signs

  • 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 only 10% of the cases, and of those, about 10% progress to neuroinvasive disease including meningitis, encephalitis, and flaccid paralysis

  • Symptoms include acute febrile illness, a nonpruritic maculopapular rash is variably present

  • Meningitis is indistinguishable from other viral meningitis

  • West Nile virus encephalitis presents with fever and altered mental status

  • West Nile virus can also present as Guillain-Barré syndrome with radiculopathy

  • Other signs include tremors, seizures, cranial nerve palsies, and pathologic reflexes

Differential Diagnosis

  • Mild forms of encephalitis must be differentiated from

    • Aseptic meningitis

    • Lymphocytic choriomeningitis

    • Nonparalytic poliomyelitis

  • Severe forms of arbovirus encephalitides are to be differentiated from

    • Other causes of ...

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