togaviruses & bunyaviruses

ARBOVIRUSES:TOGAVIRUSES & BUNYAVIRUSES

EDWARD-BENGIE L. MAGSOMBOL, MDFPCP, FPCC, DASNC

Associate Professor, Department of MicrobiologyFatima College of Medicine

I. INTRODUCTION New pathogenic viruses emerging due

to human practices 500 known arboviruses, 100

pathogenic to humans and 50 to animals

includes unrelated viruses that are arthropod-borne but now also includes non-arthropod-borne but morphologically similar viruses (Hantaan)

arthropods: mosquitoes, ticks, flies. mites

clinical syndromes: mostly mild, acute, undifferentiated febrile illness

complications: rash, hemorrhage, encephalitis, hepatitis or renal failure

found in almost all parts of the world including the Arctic

Ex. of outbreaks: Rift Valley Fever, Egypt

(1970s) Yellow fever, West Africa (1986-

1991)

II. VIRUS STRUCTURE AND GENETICS

Families: Togaviridae, Flaviviridae, Bunyaviridae Genera: 1. Togaviridae: 4 members but Alphavirus is the only arthropod borne a. Alphavirus : ssRNA (+) stranded

Spherical, 60-70 nm, enveloped 3 structural CHONs: capsid,

glycosylated E2 and E1 WEE, EEE, VEE

b. Rubivirus : Rubella c. Pestivirus and Arterivirus (non-pathogenic

to humans)

2. Flaviviridae: ssRNA, (+) stranded, 40-50 nm,

enveloped 3 structural CHONs: capsid, glycosylated

CHON small, membrane

associated non-

glycosylated CHON Ex. dengue, yellow fever, Jap B

Encephalitis, St. Louis Encephalitis, Russian

Spring Summer Encephalitis, Prowasan

3. Bunyaviridae: Bunyamwera, Phlebovirus,

Nairovirus, Hantavirus

80-100 nm, 3-segment genome of (-) stranded RNA

3 glycosylated CHONs enveloping virions

nucleocapsid (M or middle RNA segment)

(S or small RNA segment)

(L or large RNA segment)

no matrix proteins polymerase, other CHONs

alpha, flavi, and bunya viruses are all sensitive to lipid solvents and heat

important survival mechanism: generating genetic diversity

escape from immune responses, adaptation to new hosts, alter replication patterns

1. genetic drift = deletions, mutations and inversions (alpha, flavi, rhabdo)

2. genetic shift = genome segment reassortment (bunya, orbivirus)

recombination= not known for arboviruses

III. DIAGNOSIS due to prior exposure, absolute Ab titers

may be misleading virus-specific peptides virus-specific IgM titers helpful only in

some cases most accurate method: isolation of virus from

serum culture via: intracerebral

inoculation of suckling

mice continuous cell

lines (mosquito

cells)

TOGAVIRUSES

• Enveloped, positive, single-stranded RNA viruses with icosahedral nucleocapsid

• Genome encodes early (non-structural) and late (structural) proteins

• Replicate in the cytoplasm and bud at the plasma membrane

TOGAVIRUSES

Virus Group Human Pathogens

AlphavirusRubivirusPestivirusArterivirus

ArbovirusesRubella virusNoneNone

TOGAVIRUSES

Alphavirus Vector Host Distribution Disease

Sindbis Aedes & other mosquitoes

Birds Africa, Australia, India

Subclinical

Semliki Forest

Aedes & other mosquitoes

Birds East and West Africa

Subclinical

VEE Aedes, Culex Rodents, horses

North, South & Central America

Mild systemic, severe enceph.

EEE Aedes, Culiseta

Birds North & South America, Caribbean

Mild systemic; enceph.

WEE Culex, Culiseta

Birds North & South America

Mild systemic; enceph.

Chikungunya Aedes Humans, monkeys

Africa, Asia Fever, arthralgia, arthritis

TOGAVIRUSES: Mosquito Vectors

Culex

Aedes triseriatus Culiseta melanura

TOGAVIRUSES: Alphaviruses

• Classified as arboviruses (arthropod-borne viruses). To be an arbovirus, the virus must be able to:1. Infect both vertebrates & invertebrates2. Initiate sufficient viremia in a vertebrate

host for a sufficient time to allow acquisition of the virus by the invertebrate vector

3. Initiate a persistent productive infection of the salivary gland of the invertebrate to provide virus for the infection of other host animals


• With broad host range including vertebrates and invertebrates

• Diseases spread by animals or with an animal reservoir zoonotic

• Structurally similar to Picornaviruses but larger


• Capsid proteins of all Alphaviruses are similar in structure and are antigenically cross-reactive

• Individual viruses with different tissue tropisms

• Enters the cell by receptor-mediated endocytosis bind to ribosomes as mRNA


• Major differences between Alphaviruses and Flaviviruses are in the organization of their genome and their mechanism of protein synthesiso Entire flavivirus genome translated into a

single polyproteino Replicate in the cytoplasm and bud at the

plasma membraneso Flavivirus structural genes are at the 5’ end

of the genome structural proteins synthesized first and with the greatest efficiency


• MOT: bite of an arthropod

• Humans are usually “dead-end” hosts due to absence of persistent viremia

• Female mosquito acquire the viruses by taking a blood meal from a viremic vertebrate host

• Most common vector is the mosquito. Birds and mammals are the usual reservoirs.


• Diseases occur during the summer months and rainy seasons

• Urban outbreaks occur when the reservoirs for the virus are humans or urban animals.

• Humans can be reservoirs for dengue, yellow fever, and chikungunya.


• Viruses are cytolytic

• Viruses establish systemic infection and viremia.

• Viruses are good inducers of interferon, accounting for the flu-like symptoms of infection.


Initial viremia Systemic symptoms within 3 to 7 days of infection

Replication in cells of monocyte-macrophage

system

Secondary viremia

Brain via endothelial cells lining small vessels of the brain or the choroid plexus

Liver, skin, vasculature (depending on tissue tropism of the virus)


• Disease characterized as low-grade disease

• Flu-like symptoms correlate with systemic infection during the initial viremia

• EEE, WEE, and VEE progress to encephalitis in humans (fever, headache & dec. consciousness 3-10 days after infection) generally resolve without sequelae but with possibility of paralysis, mental disability, seizures and death


Chikungunya

• Swahili for “that which bends up”

• Vector: Aedes aegypti

• Crippling arthritis with serious disease due to infection with the virus


Disease Mechanisms of Togaviruses & Flaviviruses

DiseaseFlu-like

symptoms

Ence-phalitis Hepatitis

Hemor- rhage Shock

VEE + +

EEE + +

WEE + +

Dengue + + + +

Yellow fever + + + +

St. Louis enceph + +

Japanese enceph + +

West Nile enceph + +


Laboratory Diagnosis:

1. Culture• can be grown in both vertebrate and

mosquito cell lines but difficult to isolate

2. Cytopathologic studies3. Immunofluorescence4. Reverse transcriptase-PCR5. Serology – hemagglutination inhibition,

ELISA, latex agglutination• Presence of specific IgM or 4-fold rise in

titer between acute and convalescent sera indicate a recent infection

TOGAVIRUSES: Rubella Virus

• Same structural properties and mode of replication as other togaviruses.

• Unlike other togaviruses:1. It is a respiratory virus2. It does not cause readily detectable

cytopathologic effects

• Rubella means “little red”

• Maternal infection correlated with several other severe congenital defects


• Replication of the virus prevents replication of superinfecting picornaviruses by means of heterologous interference

• Infects upper respiratory tract local LN (+ LAD) viremia infection of other tissues

• Prodromal period lasts approx. 2 weeks

• Viral shedding: prodromal period and for as long as two weeks after onset of rash


• Antibody generated after the viremia correlates with appearance of rash limit viremic spread

• Immune complexes most likely cause the rash and arthralgia associated with the infection.

• CMI important in resolving the infection

• Natural infection produces lifelong protective immunity


Congenital Infection

• Occurs if mother does not have antibody virus replicates in the placenta spread to fetal blood supply & throughout the fetus

• Alter normal growth, mitosis, and chromosomal structure of the fetus’s cells

• Nature of the disorder determined by:1. Tissue affected2. Stage of development disrupted


Congenital Rubella Syndrome

• Most serious outcome of infection

• Fetus at risk until 20th week AOG

• Most common manifestations: cataracts, mental retardation, deafness


Rubella Disease (German Measles)

• Normally benign in childreno I.P. = 14 – 21 dayso 3-day maculopapular or macular

rash + swollen glands

• Infection in adults may be more severeo Arthralgia & arthritiso Thrombocytopenia or post-

infectious encephalopathy – rare


Laboratory Diagnosis

• Isolation difficult & rarely attempted

• Diagnosis confirmed by the presence of anti-rubella-specific IgM

• 4-fold increase in specific IgG antibody titer between acute and convalescent sera recent infection

BUNYAVIRIDAE

• “Supergroup” of at least 200 enveloped, segmented, negative-strand RNA viruseso Broken down into four genera on the

basis of structural and biochemical features

• Unlike other negative-strand RNA viruses, they do not have a matrix protein

• Replicates in cytoplasm

BUNYAVIRIDAE

Genus Members Insect vector

Pathologic Conditions

Vertebrate Hosts

Bunyavirus Bunyamwera, California enceph virus, La Crosse virus

Mosquito Febrile illness, encephalitis, febrile rash

Rodents, small mammals, primates, marsupials, birds

Phlebovirus Rift Valley fever virus, sandfly fever virus

Fly Sandfly fever, hemorrhagic fever, encephalitis, conjunctivitis, myositis

Sheep, cattle, domestic animals

Nairovirus Crimean-Congo hemorrhagic fever virus

Tick Hemorrhagic fever

Hares, cattle, goats, sea- birds

BUNYAVIRIDAE

Genus Members Insect vector

Pathologic Conditions

Vertebrate Hosts

Uukuvirus Uukuniemi virus

Tick --- Birds

Hantavirus Hantaan virus

Sin Nombre

None

None

Hemorrhagic fever with renal syndrome, ARDS synd.

Hantavirus pulmonary syndrome, shock, pulmonary edema

Rodents

Deer mouse

BUNYAVIRIDAE

• Virus can infect humans and arthropods

• Virus in arthropod can be transmitted to its eggs allow virus to survive during winter

• Effects:1. Neuronal & glial damage + cerebral

edema encephalitis2. Hepatic necrosis (e.g. Rift Valley Fever)3. Leakage of plasma & erythrocytes through

vascular endothelium hemorrhagic fever

BUNYAVIRIDAE

• MOT: vectors mosquitoes, ticks or Phlebotomus flies

• Virus transmitted to rodents, birds and larger animals become reservoirs

• Transmission occurs during summer but unlike other arboviruses, many of the Bunyaviridae can survive a winter in the ova of the mosquito

BUNYAVIRIDAE

• California encephalitis group: Aedes triseriatus & Culiseta

• Hantaviruses without arthropod vector maintained in a rodent species specific for each viruso Humans are infected by close contact with

rodents or through the inhalation of aerosolized rodent urine

BUNYAVIRIDAE

BUNYAVIRIDAE

Clinical Syndromes:

• Incubation period: approx. 48 hours

• Non-specific febrile flu-like illness

• Fever lasts approximately 3 days

BUNYAVIRIDAE

Clinical Syndromes:

1. Encephalitis illnesses (e.g. La Crosse)• Sudden onset after I.P. of ~ 1 week• Fever, headache, lethargy, vomiting• Seizures in 50% of patients

2. Hemorrhagic fevers (e.g. Rift Valley)• Petechial hemorrhages, ecchymoses,

epistaxis, hematemesis, melena, gum bleeding

BUNYAVIRIDAE

Clinical Syndromes:

Hantaan virus• Korean hemorrhagic fever• Headache, petechial rash, shock, and

renal failure• Rodent-borne

Sin Nombre virus• Hantavirus pulmonary syndrome• Influenza-like symptoms followed by

acute respiratory failure• Endemic in deer mice (Peromyscus)

BUNYAVIRIDAE

Laboratory Diagnosis:

• Serologic tests to confirm diagnosis

• ELISA – detect antigen to clinical specimen from patients with intense viremia

• RT-PCR – Sin Nombre virus

THANK YOU!

togaviruses & bunyaviruses

Health & Medicine

singlestranded rna viruses

bunya viruses

small rna segmentl

enveloped3 structural

louis encephalitis

glycosylated e2

segment genome

middle rna segments