Dr. Sadia Anjum Lecture - PowerPoint Presentation

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Dr. Sadia Anjum Lecture 7,8

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Dengue VirusesThe dengue viruses are members of the genus 

Flavivirus in the family 

Flaviviridae

.

Along with the dengue virus, this genus also includes a number of other viruses transmitted by mosquitoes and ticks that are responsible for human diseases. Flavivirus includes the yellow fever, West Nile, Japanese encephalitis, and tick-borne encephalitis viruses.

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Dengue Virus

There are four different strains of dengue virus. DEN-1, DEN-2, DEN-3, and DEN-4. These four viruses are called serotypes because each has different interactions with the antibodies in human blood serum. 

These serotypes are very similar (65%) so similar in fact that the immune system recognizes all of them after seeing only one. 

 In the first infection virus particles will be captured and processed by so-called antigen presenting cells. 

These viruses will be presented to T-cells causing them to become activated. And likewise B-cells will encounter their antigen free floating and become activated. B-cells produce antibodies. 

Human T-cells each are programmed to recognize a specific pattern (or antigen). Antibodies

are used (among other things) to tag the viruses to encourage their uptake by macrophages (called

opsonization

) and inactivate them.

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The change in distribution of dengue serotypes

The distribution of dengue serotypes in 1970 (a) and 2004 (b).

Guzman, M. G. 

et al.

 Dengue: A continuing global threat. Nature Reviews Microbiology 8, S7–S16 (2010). All rights reserved

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Genome and Structure

The dengue virus genome is a single strand of RNA. It is referred to as positive-sense RNA because it can be directly translated into proteins.

The viral genome encodes ten genes. The genome is translated as a single, long polypeptide and then cut into ten proteins.

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Den V proteins and their functions

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Den V proteins and their functionsNS1, associates with the membrane on the cell surface and in the RNA replication complex. NS1

lacks a transmembrane domain, and it associates with the membrane  via lipid rafts.Smaller M protein Apart from

fprming

the prM–E complex, little is known about the cellular or biochemical properties of prM/M.M is capable of inducing apoptosis in a sequence and localisation-dependent manner , and  and interacts with host proteins during the entry and assembly stage of the virus lifecycleIt act as chapron

for folding of E protein

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Virus Replication and release

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Viral binding and infectionDengue virus binds to its receptor, and this process is mediated by envelop protein (E).

In mammalian cell, DEN 1–4 serotypes bind with

Heparan

sulfate, nLc4Cer, DC-SIGN/L-SIGN and Mannose receptors.

DEN-2 serotype also binds with HSP70/HSP90, GRP78, CD14- associated protein and two unknown proteins having trypsin resistance and trypsin sensitive properties.

DEN 1–3 serotypes as well bind with

Laminin

receptor.

DEN 2–4 serotypes also bind with an unknown protein having the property of serotype specific binding. After initial attachment of the virus with particular receptors on

th

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The viral particle is fused into acidic lysosomes through receptor-mediated endocytosis.

After that, viral particle is uncoated and the RNA is released in host cellRNA is released in host cell where it directs the synthesis of viral proteins.

Once

all the essential proteins are synthesized, viral RNA starts copying to generate a minus strand, which is then transcribed to new plus stranded molecules.

In only few hours after infection, tens of thousands copies of viral molecules are produced from a single viral molecule leading to cell damage and in severe cases to death. 

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Viral Pathogenesis

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Viral Pathogenesis

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Viral Pathogenesis

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Role of ADE in Dengue

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Immune response to DENV

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Bunyaviridae infections Arthropod vector associated viral infections; Exception – Hantaviruses

RVF – Aedes mosquitoCCHF – Ixodid tick Hantavirus – Rodents

Less common

Aerosol

Exposure to infected animal tissue

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Bunyaviridae in AnimalsRVF Abortion – 100%

Mortality rate >90% in young•5-60% in older animals• CCHF• Unapparent infection in livestock

•

Hantaviruses

• Unapparent infection in rodents

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Transmission of CCHF DiseaseCCHFV usually circulates between asymptomatic animals and ticks in an enzootic cycle.

This virus has been found in at least 31 species of ticks, including seven genera of the family Ixodidae

(hard ticks).

Members of the genus

Hyalomma seem to be the principal vectors. Transovarial, transstadial and venereal transmission occur in this genus. Hyalomma marginatum is particularly important as a vector in Europe, but CCHFV is also found in Hyalomma anatolicum

anatolicum

andother

Hyalomma

spp.

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BunyaviridaeCCHFV is a member of the

Nairovirus genus of the family Bunyaviridae. Other genera within the family include Orthobunyavirus, Hantavirus,

Phlebovirus

, and

Tospovirus.there are seven recognized species in the genus Nairovirus containing 34 viral strains. The most important Serogroups are the CCHF group, which includes CCHFV, and Hazara virus, which has not been demonstrated to be pathogenic to humanhemagglutination inhibition (HI), complement fixation (CF) and agar gel diffusion and precipitation (AGDP) tests have shown the virus to be

antigenically

related to no other viruses except: to

Hazara

with which it constitutes the CCHF group

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CCHF is a Tick borne DiseaseCCHF spreads to humans either by tick-bites, or through contact with viraemic

animal tissues during and immediately post-slaughter.CCHF outbreaks constitute a threat to public health services because of its epidemic potential, its high case fatality ratio (10-40%), its potential for nosocomial outbreaks and the difficulties in treatment and prevention

Clinical disease is rare in infected mammals, but commonly severe in infected humans, with a 30% mortality rate.

Outbreaks of illness are usually attributable to handling infected animals or people.

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CCHFV

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CCHFVCrimean–Congo hemorrhagic fever virus has a tripartite genome consisting of large (L), medium (M), and small (S) negative-sense RNA segments that encode the viral RNA polymerase, the glycoproteins, and the nucleocapsid protein (NP), respectively (

Clerx et al., 1981). Viral infection is detected by host cells via Toll-like receptor (TLR)-dependent and -independent mechanisms which results in ubiquitination

and

ISGylation

of target proteins to induce the innate immune system. Ubiquitination involves a regulatory protein called Ubiquitin (Ub) which conjugates with target (viral) proteins and forms an Ub-tag which in turn, recognized by proteosome for destruction

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Virus life cycle

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Symptoms Fever, fatigue, dizziness, myalgia's, and prostration

Signs of bleeding range from only conjunctival hemorrhage, mild hypotension, flushing, and petechiae to shock and generalized mucous membrane hemorrhage and evidence of pulmonary, hematopoietic, and neurologic dysfunction

Renal insufficiency is proportional to cardiovascular compromise except in Hemorrhagic Fever and Renal Syndrome in which it is an integral part of the disease

severe bruising, severe nosebleeds, and uncontrolled bleeding at injection sites can b seen, beginning on about the fourth day of illness and lasting for about two weeks

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Symptoms and DiagnosticsThe liver becomes swollen and painful.

Disseminated intravascular coagulation may occur as well as acute kidney failure and shock, and sometimes acute respiratory distress syndromeLaboratory diagnosis of CCHF can be made by finding a positive serological test result, evidence of viral antigen in tissue by

immunohistochemical

staining and microscopic examination,

identification of viral RNAsequence in blood or tissue, in a patient with a clinical history compatible with CCHAt autopsy, the virus is most likely to be found in the lung, liver, spleen, bone marrow, kidney and brain.

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serology. Crimean-Congo hemorrhagic fever can be diagnosed by serology.

Tests detect CCHFV-specific IgM, or a rise in IgG

titers in paired acute and convalescent sera.

IgG

and IgM can usually be found with mindirect immunofluorescence or ELISA after 7-9 days ofillness