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Arboviruses, Rabies Virus, & Ebola Virus

Chapter Contents

• Arboviruses
• Important properties of Arboviruses
• Transmission of Arboviruses
• Clinical findings & Epidemiology of Arboviruses
• Important Arboviruses that cause disease in the United States
  • West Nile Virus
  • Eastern Equine Encephalitis Virus
  • Western Equine Encephalitis Virus
  • St. Louis Encephalitis Virus
  • California Encephalitis Virus
  • Colorado Tick Fever Virus
• Important Arboviruses that cause disease primarily outside the United States
  • Yellow Fever Virus
  • Dengue Virus
  • Zika Virus
  • Chikungunya Virus
  • Rabies Virus
  • Ebola Virus
• Self-assessment questions
• Summaries of organisms
• Practice questions: USMLE & course examinations

Arboviruses

• Definition: Arbovirus is an acronym for arthropod-borne virus, signifying that these viruses are transmitted primarily by arthropods such as mosquitoes and ticks.
• Diversity: More than 600 different arboviruses exist, categorized based on either the diseases they cause or the geographic area they were first isolated.
• Comparison with Roboviruses: Roboviruses are rodent-borne viruses transmitted directly from rodents to humans, with transmission occurring via inhalation of dried rodent excrement. Examples include Sin Nombre virus (a hantavirus) and Whitewater Arroyo virus (an arenavirus).

Important Properties of Arboviruses

• Classification: Most arboviruses fall into three major families:
  1. Togaviruses
  • Characterized by a size of 70 nm, with an icosahedral nucleocapsid enclosed in an envelope.
  • Contains a single-stranded positive-polarity RNA genome.
  • Grouped into two subfamilies: Alphaviruses and Rubiviruses. The primary arboviral Alphaviruses include Eastern and Western Equine Encephalitis viruses.
  1. Flaviviruses
  • Size ranges from 40-50 nm with similar structural characteristics to togaviruses.
  • Common examples include West Nile virus, Yellow fever virus, and Dengue virus.
  1. Bunyaviruses
  • Characterized by a helical nucleocapsid and a segmented RNA genome (three segments: large (L), medium (M), and small (S)). Examples include California Encephalitis virus.
• Other groups include reoviruses and rhabdoviruses.

Transmission of Arboviruses

• Life Cycle: Arboviruses replicate in both vertebrate hosts and blood-sucking arthropod vectors.
  • Viremia: The virus must reach the bloodstream of the vertebrate host in high titers, so it can be ingested by the arthropods during a blood meal.
  • Extrinsic Incubation Period: The time required for the virus to replicate in the vector, typically 7-14 days.
• Vertical Transmission: Some arboviruses can be passed from mother ticks to offspring without requiring an intermediate vertebrate host.
• Hosts in Transmission: Humans are typically dead-end hosts, meaning their viremia is too low to sustain the virus within the transmission cycle. However, in certain instances (Yellow fever, Dengue), human hosts can contain high levels of the virus.
• Symptomatology: Arboviruses generally do not cause disease in their natural arthropod or vertebrate hosts but can cause illness in incidental dead-end hosts like humans.

Clinical Findings & Epidemiology of Arboviruses

• Most human arboviral infections are asymptomatic. Symptomatic infections mainly manifest as acute febrile illnesses or neuroinvasive diseases (encephalitis, meningitis).
• Severity ranges from mild to fatal infections.
• The typical clinical presentations include:
  1. Encephalitis
  2. Hemorrhagic fever
  3. Febrile illness with systemic symptoms (e.g., myalgia, arthralgia).
• Epidemiology: Arboviral diseases are more prevalent in tropical regions but can also occur in temperate climates. Outbreaks often occur at the interface between human communities and natural habitats.

Important Arboviruses that Cause Disease in the United States

West Nile Virus (WNV)

• The most prevalent cause of neuroinvasive arboviral disease in the U.S.
• Outbreaks: First notable outbreak recorded in New York City in 1999, causing 27 confirmed cases and 23 probable cases, with 5 deaths.
• Transmission: Transmitted via Culex mosquitoes; wild birds are the primary reservoirs.
• Incidence: Data from 2012 shows 3142 reported cases and 134 deaths.
• Clinical Picture: Encephalitis occurs in about 1% of infections; approximately 20% of infections exhibit fever and headache without encephalitis (West Nile fever), while 80% are asymptomatic.
• Diagnosis: Identified by detecting IgM antibodies or through PCR.
• Prevention: No antiviral agent or vaccine for humans; blood banks screen donated blood for WNV presence.

Eastern Equine Encephalitis (EEE) Virus

• Causes severe disease (50% fatality rate).
• Primarily transmitted by swamp mosquitoes among wild birds.
• Incidence of human cases typically ranges from 0-4 per year, with notable outbreaks possible.
• Clinical presentation includes sudden severe headaches, nausea, fever, leading possibly to confusion or coma.

Western Equine Encephalitis (WEE) Virus

• More common than EEE, but less severe overall.
• Incidence ranges 5-20 cases yearly, with a mortality rate of approximately 2%.
• Transmitted by Culex mosquitoes primarily among birds.

St. Louis Encephalitis (SLE) Virus

• Found broadly across southern, central, and western U.S.; typically causes 10-30 cases annually.
• Known to cause encephalitis cases, primarily also transmitted by Culex mosquitoes.

California Encephalitis (CE) Virus

• Most common strain is La Crosse virus.
• Commonly transmitted by Aedes triseriatus mosquitoes.
• Generally causes symptoms resembling mild viral meningitis.

Colorado Tick Fever Virus (CTF)

• Distinct as a reovirus transmitted by Dermacentor andersoni ticks.
• Often identified in rodents of the Rocky Mountains.
• Typical clinical presentation: mild flu-like symptoms, and diagnosis is primarily serological.

Summary of Arboviral Diseases in the U.S.