Medical Microbiology & Immunology



Medical Microbiology & Immunology

Lecture 28 Arthropod-Borne and Other Zoonotic Viruses (Hemorrhagic Fever and Encephalitis Viruses)

1) To know the family of viruses that cause encephalitis and hemorrhagic fever.

Arboviruses (transmitted by blood sucking insects such as mosquitoes, ticks, and sandflies (arthropod-borne): Togaviruses, Flaviviruses, Bunyaviruses, Reoviruses.

Other zoonotic RNA viruses are generally transmitted by infected animals (non-arthropod borne): Rhabdoviruses, Arenaviruses, Filoviruses

See question three.

2) To describe the structure, replication and pathogenesis of zoonotic viruses.

|Virus |Structure |Replication |

|Togaviruses |ss(+)RNA, enveloped virions containing hemagglutinin and lipoproteins, 40-70 nm |Can occur in the cells of infected arthropods invertebrate|

|Flaviviruses |in external diameter |hosts |

|Bunyaviruses |Spherical, enveloped and ss(-)RNA seg. 90-100 nm in external diameter, Four |Mature by budding with smooth surfaced vesicles or near |

| |genera; bunyavirus (-) RNA, phlebovirus (+/-) RNA, nairovirus and Hantavirus (-)|Golgi region of infected cells. |

| |RNA | |

|Reoviruses |Spherical, uneveloped, ds RNA seg. About 80 nm in diameter. | |

|Arenaviruses |Spherical, eveloped, and ssRNA two segments (ambisense +/-), 50-300 nm in |Maturation by budding from host cell, contain host |

| |diameter. |ribosomes |

|Filioviruses |Envelope, ss(-)RNA, filamentous and highly pleomorphic, 80 nm in diameter and |Similar to other (-) RNA viruses, rapid packaging of |

| |300-1400 nm long |genome, maturation by budding from cell membrane. |

Pathogenesis: three manifestations of arbovirus disease exist: 1) viral tropisms for human organs, if CNS, aseptic meningitis or encephalitis, 2) particular damage to major organ systems (e.g. liver and yellow fever), 3) hemorrhagic fever with damage to small blood vessels resulting in hemorrhage (e.g. intestines)

3) To recognize the diseases caused by arboviruses.

|Classification |Virus |1° Vector |Disease |

|Togaviruses |Western Equine |Mosquito |1/1000 cause symptoms, mild, nonspecific febrile illness, aseptic meningitis, encephalitis (5% |

|(Alphaviruses) |Encephalitis | |mortality). |

| | | |1/25 cause severe illness in children 40-years |

| |Yellow Fever |Mosquito |Abrupt onset of fever, chills, headache, and hemorrhage. May progress to severe vomiting |

| | |(urban) |(sometimes with gastric hemorrhage), bradycardia, jaundice, and shock. Caribbean, Central and |

| | | |South America (maybe SE US) |

| |Dengue |Mosquito (urban|Fever, an erythematous rash, severe backing joint pain. Severe form in the Far East - shock, |

| | |or sylvatic) |pleural effusion, and hemorrhage often followed by death. Worldwide distribution, all cases in |

| | | |the US imported. |

| |Japanese B. |Mosquito |High proportion of human infections are subclinical: especially in children. Development of |

| |Encephalitis | |encephalitis is severe and often fatal. Transmission cycle is similar to St. Louis encephalitis|

| | | |and Western equine encephalitis viruses. |

| |West Nile Fever |Mosquito (tick)|Incubation 1-6 days, typically mild, fever, headache, backache, generalized myalgia, generalized|

| |Virus | |lymphadenopathy (common), pharyngitis in gastrointestinal symptoms (nausea, vomiting, abdominal |

| | |2° birds |pain). |

| | | |May also result in aseptic meningitis or meningoencephalitis - death in a small number of |

| | | |patients (elderly). |

| | | |Lab findings include leukopenia, CSF pleocytosis and elevated protein. |

| | | |Linked to several deaths in the US (1999) and Arizona (2003) |

| |Powassan Virus |Tick |Isolated in Ontario and fetal human case of encephalitis, not significantly related to humans. |

|Bunyaviruses |Bunyavirus |Mosquito |Abrupt onset of encephalitis frequently with seizures (5-18 years of age), widely distributed in|

| |(California) | |the Midwest CA, WI, MN, OH, IN. |

| |Hantaviruses |Rodents |Hemorrhagic fever (Korean War), US outbreak related to deer mice (1993), fulminant respiratory |

| | | |disease with high mortality (67%), transmission by inhalation of infected rodent excreta |

| | | |(conjunctival route or direct contact with skin breaks). No human to human transmission in US. |

| | | |Ribavirin. |

|Reoviruses |Colorado Tick Fever |Tick |Incubation 3-6 days, sudden onset of headache, muscle pain, fever, and occasionally |

| | | |encephalitis, leukopenia (consistent feature). 1% symptomatic. |

4) To discuss the diagnosis, treatment and prevention of encephalitis and hemorrhagic fever.

|Disease |Diagnosis |Treatment and prevention |

|Encephalitis |Virus culture, |Treatment: supportive care |

|Hemorrhagic fever |Viremia in blood, |Prevention: primarily of avoidance of contact with potentially |

| |Antibody test, |infected arthropods. |

| |Nucleic Acid genome - PCR, |Vaccination: Western, Eastern equine encephalitis virus infection for |

| |Detection of IgM - specific antibody |horses, but can be used for laboratory personnel, yellow fever vaccine|

| |within few days of infection |(live attenuated) single subcutaneous dose (lasts 16-19 years) 10-year|

| | |boosters |

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