Arbovirus Encephalitides

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 (CSF) pressure and protein are often increased with lymphocytic pleocytosis

General Considerations

• Caused by arthropod-borne viruses

• The mosquito-borne pathogens include

  • Togaviruses (most of which are of the genus Alphavirus: Western, Eastern, and Venezuelan equine encephalitis, chikungunya, and Mayaro virus)

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

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

• The tick-borne causes of encephalitis include

  • The flaviviruses 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 2020, 713 cases were reported to the CDC from 45 US states and the District of Columbia

  • 78% of cases in 2020 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

    • The virus cannot be transmitted from infected humans and other mammals to other biting mosquitoes

    • However, new data suggest that white-tailed deer may play a role in West Nile virus and other arboviral circulation

    • 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

CLINICAL FINDINGS

Symptoms and Signs

• West Nile virus 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 middle-aged persons

  • Frank encephalopathy is seen in older adults

• 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 and altered mental status

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

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

Differential Diagnosis

• Mild forms of encephalitis must be differentiated from

  • Aseptic ...