dr. sadia anjum lecture 7,8. dengue viruses the dengue viruses are members of the genus flavivirus...

Dr. Sadia Anjum

Lecture 7,8

Dengue Viruses

• The 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.

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.

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

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.

Den V proteins and their functions

Den V proteins and their functions

• NS1, 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 lifecycle

• It act as chapron for folding of E protein

Virus Replication and release

Viral binding and infection• Dengue 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

• 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 cell

• RNA 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. 

Viral Pathogenesis

Viral Pathogenesis

Viral Pathogenesis

Role of ADE in Dengue

Immune response to DENV

Bunyaviridae infections

Arthropod vector associated viral infections;

Exception – Hantaviruses – RVF – Aedes mosquito– CCHF – Ixodid tick– Hantavirus – Rodents– Less common– Aerosol– Exposure to infected animal tissue

Bunyaviridae in Animals

• RVF• Abortion – 100%• Mortality rate

– >90% in young– •5-60% in older animals

• • CCHF• • Unapparent infection in livestock• • Hantaviruses• • Unapparent infection in rodents

Transmission of CCHF Disease

o CCHFV usually circulates between asymptomatic animals and ticks in an enzootic cycle.

o This virus has been found in at least 31 species of ticks, including seven genera of the family Ixodidae (hard ticks).

o Members of the genus Hyalomma seem to be the principal vectors. Transovarial, transstadial and venereal transmission occur in this genus.

o Hyalomma marginatum is particularly important as a vector in Europe, but CCHFV is also found in Hyalomma anatolicum anatolicum andother Hyalomma spp.

Bunyaviridae

CCHFV 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 human

hemagglutination 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

CCHF is a Tick borne Disease

• CCHF 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.

CCHFV

CCHFV

• Crimean–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

Virus life cycle

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

Symptoms and Diagnostics

• The liver becomes swollen and painful.

• Disseminated intravascular coagulation may occur as well as acute kidney failure and shock, and sometimes acute respiratory distress syndrome

• Laboratory 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 RNA– sequence in blood or tissue, in a patient with a clinical history

compatible with CCH• At autopsy, the virus is most likely to be found in the lung, liver, spleen,

bone marrow, kidney and brain.

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