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For further information, see CMDT Part 32-03: Other Neurotropic Viruses

Key Features

Essentials of Diagnosis

  • 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

General Considerations

  • Caused by arthropod-borne viruses

  • The mosquito-borne pathogens include

    • Togaviruses (Western, Eastern, and Venezuelan equine encephalitis)

    • Flaviviruses (West Nile fever, St. Louis encephalitis, Japanese encephalitis, Murray Valley encephalitis, dengue, Zika, and yellow fever, and Rocio viruses)

    • Orthobunyaviruses (the California serogroup of viruses, including the Jamestown Canyon and the La Crosse virus, Oropouche, and Keystone viruses)

    • The alphaviruses (chikungunya, Venezuelan equine encephalitis virus, and Mayaro 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

    • The states reporting the most cases of neuroinvasive disease in 2018 included

      • California (154 cases)

      • Illinois (126 cases)

      • Nebraska (124 cases)

      • Texas (108 cases)

      • Pennsylvania (95 cases)

    • Among the 2647 cases reported in 2018, cases were reported to the CDC from 48 states and the District of Columbia, 1658 (63%) 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 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

  • Eastern equine encephalitis is also shown to be transmitted by organ transplantation

  • La Crosse virus

    • Continued to be the most common cause of neuroinvasive arboviral disease in children

    • More cases were reported in 2018 than in any year since 2011

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 ...

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