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Togaviruses and Flaviviruses
Unique features of Togaviruses & Flaviviruses Enveloped Positive sense ssRNA Togaviruses replicate in the cytoplasm and
bud at the plasma membranes Flaviviruses replicate in the cytoplasm and
bud at internal membranes
Togaviruses and Flaviviruses
Virus groupVirus group Human pathogensHuman pathogens
Togaviruses
Alphavirus
Rubivirus
Arterivirus
Flaviviruses
Flaviviridae
Hepaciviridae
Pestivirus
Arboviruses
Rubella virus
None
Arboviruses
Hepatitis C virus
None
Togaviruses and Flaviviruses
Alphavirus and Flavivirus are discussed together because of similarities in the diseases that they cause, as well as in epidemiology.
Most are transmitted by arthropods and are therefore arboviruses (arthropod-borne viruses).
They differ in size, morphology, gene sequence, and replication.
Togaviruses and Flaviviruses
The alphaviruses and flaviviruses: These viruses have a very broad host
range, including vertebrates (e.g., mammals, birds, amphibians, reptiles) and invertebrates (e.g., mosquitoes, ticks). Diseases spread by animals or with an animal reservoir are called zoonoses.
Togaviruses
Alphavirus, Rubivirus, and Arterivirus. No known arteriviruses cause disease in
humans, so this genus is not discussed further.
Rubella virus is the only member of the Rubivirus group; it is discussed separately, because its disease manifestation (German measles) and its means of spread differ from those of the alphaviruses.
Togaviruses and Flaviviruses
The Flaviviridae include the flaviviruses, pestiviruses, and hepaciviruses (hepatitis C and G viruses).
Hepatitis C and G are discussed inhepatitis viruses.
ArbovirusesDiseaseDisease VectorVector HostHost DistributionDistribution diseasedisease
AlphavirusesAlphavirusesSindbis
Semliki forest
Venezuelan equine enceph.
Eastern equine encep.
Western equine encep.
Aedes & other mosquitos
Aedes & other mosquitos
Aedes, Culex
Aedes, Culiseta
Culex, Culiseta
Birds
Birds
Rodents,horses
Birds
Birds
Africa,Australia,India
East and west Africa
North,South,&Central America
North&South America, Caribbean
North & South America
Subclinical
Subclinical
Mild systemic,severe encephal.
Mild systemic,encephal.
Mild systemic,encephal.
ArbovirusesDiseaseDisease VectorVector HostHost DistributionDistribution diseasedisease
AlphavirusesAlphavirusesChikungunya Aedes Humans,
MonkeysAfrica, Asia Fever,
Arthralgia, arthritis
ArbovirusesDiseaseDisease VectorVector HostHost DistributionDistribution diseasedisease
FlavivirusesFlavivirusesDengue
Yellow fever
Aedes
Aedes
Humans, Monkeys
Humans, monkeys
Worldwide,esp. Tropics
Africa, South America
Mild systemic; break-bone fever, dengue hemorrhagic fever, and dengue shock syndrome
Hepatitis, hemorrhagic fever
ArbovirusesDiseaseDisease VectorVector HostHost DistributionDistribution diseasedisease
FlavivirusesFlavivirusesJapanese encephalitis
West Nile encephalitis
St. Louis encephallitis
Russian spring-summer encephalitis
Powassan encephalitis
Culex
Culex
Culex
Ixodes & dermocentor ticks
Ixodes ticks
Pigs, birds
Birds
Birds
Birds
Small mammals
Asia
Afr.,Eur.,CentralAsia,N.Amer
N. America
Russia
N. America
Encephalitis
Fever, encep., hepatitis
Encephalitis
Encephalitis
Encephalitis
Togaviruses & Flaviviruses/Clinical syndromes Alphavirus disease is usually characterized
as low-grade disease Can progress to encephalitis in humans
Flavivirus infections are relatively benign Serious aseptic meningitis, encephalitis,
hemorrhagic disesasehemorrhagic disesase can occur
Togaviruses & Flaviviruses/Clinical syndromes Hemorrhagic disesasesHemorrhagic disesases
Dengue Yellow fever viruses
Togaviruses & Flaviviruses/Clinical syndromes Hemorrhagic disesasesHemorrhagic disesases
Dengue virus Major worldwide problem 100 million cases of dengue fever/year 250 000 cases of dengue hemorrhagic fever/y Dengue shock syndrome
Togaviruses & Flaviviruses/Clinical syndromes Hemorrhagic disesasesHemorrhagic disesases
Yellow fever viruses Severe systemic disease Degeneration of the liver, kidney, heart +
hemorrhages “JAUNDICEJAUNDICE” Mortality rate: ~ 50% during epidemics
Togaviruses & Flaviviruses/Laboratory diagnosis Cell culture vertebrate and
mosquito cell lines difficult IF, hemadsorbtion
RT-PCR Serology
ELISA HI LA
Togaviruses & Flaviviruses/Treatment, Prevention, and Control No treatment! “The easiest means of preventing the spread
of any arbovirus is elimination of its vector and breeding grounds”
Vaccines Yellow fever live vaccine (17D strain) EEE, WEE, Japanese, Russian SSE killed
vaccines
Rubella virus
Same structural properties and mode of replication as the other toga’s
Rubella is a respiratory virus Does not cause readily detectable
cytopathologic effects
Rubella
One of the 5 classic childhood exantems Measles Roseola Fifth disease Chickenpox
Rubella
Rubella: “little red” in Latin “German measles” Infects URT local lymphe nodes
viremia Shedding respiratory droplets Only one serotype Natural infection lifelong
protective immunity
Congenital Rubella
“Serious congenital abnormalities in the child”
If the mother does not have antibody The virus can replicate in most tissues
of the fetus The normal growth, mitosis, and
chromosomal structure of the fetus’s cells can be altered by the infection
Congenital Rubella
The normal growth, mitosis, and chromosomal structure of the fetus’s cells can be altered by the infection
Improper development of the fetus, small size of the infected baby, and the teratogenic effects
Congenital Rubella
The nature of the disorder is determined by
1. The tissue affected
2. The stage of development disrupted
Congenital Rubella
~20% of women of childbearing age escape infection during childhood and are susceptible to infection unless vaccinated
Rubella/Clinical syndromes
Rubella disease Normally benign 3 day of maculopapular or macular rash
and swollen glands More severe in adults
Rubella/Clinical syndromes
Congenital disease The fetus is at major risk until the 20th
week of pregnancy Most common manifestations:
Cataracts Mental retardation deafness
Rubella/Laboratory diagnosis
Anti-Rubella IgM by ELISA Avidity test: Low avidity 4x increase in IgG Antibodies to rubella are assayed early
in pregnancy to determine the immune status of the woman
Rubella/Treatment, Prevention, and Control No treatment Vaccination (live) MMR vaccine