Bunyaviridae

Bunyaviridae

Clayton M. Johnston

Bunyaviridae

• Largest family of mammal affecting viruses (250 viruses)

• Arthropod- or rodent-borne vectors• Most are amplified in vertebrate hosts

BunyaviridaeGenus Disease(s)

Bunyavirus LaCrosse encephalitisPhlebovirus Rift Valley FeverNairovirus Crimean-Congo Hemorrhagic FeverTospovirus Plant VirusHantavirus Hemorrhagic Fever with Renal

SyndromeHantavirus Pulmonary Syndrome

Structure

• Virion Structure• Genomic Structure• Structural Proteins

Virion Structure

• Spherical• 80-120 nm diameter• Enveloped• Helical nucleocapsid• NO matrix protein

Genomic Structure

• (-) sense• Linear ssRNA• Three segments:

– Large (L) codes for viral polymerase– Medium (M) codes for G1 and G2

glycoproteins– Small (S) codes for nucleocapsid

Structural Proteins

• Membrane Glycoproteins• Nucleocapsid Protein• Viral Polymerase

Structural Proteins

Membrane glycoproteins (G1 and G2)

Nucleocapsid proteins (N)

Polymerase (L)

Membrane Glycoproteins

• G1 and G2• Integral membrane proteins• Important in cell entry and pathogenesis

Nucleocapsid Protein

• Complexes with genomic vRNA in virus, as well as with cRNA after infection, but not with mRNA

• Necessary for virus replication and packaging

Viral Polymerase

• RNA-dependent RNA polymerase• Complexed with ribonucleocapsid in virion• Endonuclease activity to cleave host mRNA• Transcriptase activity for making cRNA

and mRNA from vRNA• Helicase activity to unwind vRNA during

transcription

Viral Replication

• Receptor mediated endocytosis• Occurs in cytoplasm• Budding at Golgi apparatus or cell

membrane

Hantavirus Replication Cycle

• Attachment• Entry and Uncoating• Primary Transcription• Translation• Genome Replication• Secondary Transcription• Virion Assembly• Virion Release

Attachment

• Viral G1 and G2 glycoproteins interact with cell surface receptors

• Pathogenic hantavirus bind β3 integrins• Non-pathogenic hantaviruses bind β1

receptors

Entry and Uncoating

• Virus particles bound to integrin receptors are taken in by receptor mediated endocytosis

• Newly formed vesicles are acidified• Acidic environment changes confirmation of G1

and G2 glycoproteins• Viral and cell membranes fuse• Genomic material and polymerase are released

into cytoplasm

Attachment and Entry

Primary Transcription

• Transcription of negative sense vRNA to mRNA

• Viral polymerase transcribes nucleoprotein-coated vRNA

• Capped oligonucleotides from cell’s own mRNA are used to prime transcription (similar to Influenza virus)

Translation

• L and S segments of mRNA are translated on free ribosomes in cytoplasm

• M segment mRNA is translated on ER-bound ribosomes

Translation

Genome Replication

• vRNA is used as a template by viral polymerase to make cRNA

• cRNA is used as a template to make more negative sense strands of vRNA

Secondary Transcription

• Extra vRNA synthesized during replication is used as template to make mRNA

• Since more template is present after vRNA is replicated, more mRNA can be transcribed, and more viral proteins can be made

• Persistent infection

Virion Assembly

• Membrane-bound G1 and G2 peptides are transported to Golgi apparatus and carbohydrates are attached by N-linked glycosylation

• vRNA complexes with N nucleocapsid protein, forms looped panhandle structure, and complexes with polymerase

Virion Assembly

Virion ReleaseTwo Mechanisms

• Nucleocapsid complexes bud into the Golgi membrane with G1 and G2 embedded

• Virion particle is formed inside Golgi apparatus

• Virions are transported to cell membrane by vesicles and released by exocytosis

• G1 and G2 embed into cell membrane through Golgi vesicles

• Virions bud from cell membrane, not through Golgi apparatus

Attachment

Entry

Uncoating

Transcription

Translation

Replication

Assembly

Release

LaCrosse EncephalatisBunyavirus

• Mostly infects children younger than 16• Ades mosquitoes are the common vector• Squirrels and chipmunks are the amplifying

host• Most common bunyavirus infection in the

United States

LaCrosse EncephalatisBunyavirus

• Targets the brain• Symptoms may include:

– Fever– Convulsions– Drowsiness– Focal neurological signs

Rift Valley FeverPhlebovirus

• Most spread by sandfly or Ades species of mosquitoes

• Causes abortion in livestock• Highly infectious by aerosolized blood• Distribution follows that of the host vectors• Immunization of livestock is the most

effective way to control and prevent the disease

Rift Valley FeverPhlebovirus

• Febrile disease in humans• Targets the liver• Symptoms often include:

– Fever– Encephalitis– Retinal vasculitis (which may lead to blindness)

Rift Valley FeverDistribution Map

Crimean-Congo Hemorrhagic FeverNairovirus

• Transmitted by ticks• Appears in the Middle East and Africa• Targets the liver and vascular endothelium• Symptoms include:

– Headache– Pain in limbs– Often bleeding from many orifices

Crimean-Congo Hemorrhagic FeverNairovirus

Hantavirus

• Enveloped• ssRNA• Virions 98 nm in

diameter• Genome consists of

three RNA segments

TransmissionVectors

• Transmitted via aerosolized rodent urine, feces, and saliva– Deer mouse (Peromyscus maniculatus) – Cotton rat (Sigmodon hispidus) – White-footed mouse (Peromyscus leucopus) – Striped field mours (Apodemus agrarius) – Bank vole (Clethrionomys glareolus) – Rat (Rattus)

Hemorrhagic Fever with Renal SyndromeHantavirus

• Liver and vascular enothelium are targeted• Symptoms include:

– Hemorrhage– Acute renal failure– Fever

• Over 15% mortality rate

Hantavirus Pulmonary SyndromeHantavirus

• Lungs are targeted• Symptoms include

– Fever– Acute respiratory distress

• Over 50% mortality rate• Shock and cardiac complications often

contribute to death

Prevention and Control

• Vaccines• Hygiene• Vector Control

Vaccines

• E. coli expressed truncated nucleocapsid as an immunogen

• Naked DNA• Recombinant non-pathogenic virus• Rodent brain-derived• Cell culture derived• Inactivated virus – being tried out in China

Hygiene

• Prevent aerosolization of virus from roden excrement

• Dampen surfaces with bleach before cleaning

• Control rodents and human contact with rodents

Host Defenses and Immune Response

• Interferon is produced• Humor antibody has been shown to be

related to the disappearance of virus from blood

• Cytotoxic T-cells attack infected host cells• Inflammatory response

Treatment

• Early aggressive intensive care• Early use of inotropic agents (Dobutamine)• Early ventilation• Careful monitoring:

– Oxygenation– Fluid balance– Blood pressure

Treatment

• General care, alleviation of symptoms• Ribavirin (Hemorrhagic Fever with Renal

Syndrome)• ECMO (Hantavirus Pulmonary Syndrome)

Ribavirin• Administered

intravenously• Shown to be effective

against Hemorrhagic Fever with Renal Syndrome

• Not shown to be effective against Hantavirus Pulmonary Syndrome causing strains

Extra Corporeal Membrane Oxygenation(ECMO)

• Removes blood from the body and artificially removes CO2 and adds O2

• Costly• Difficult

ECMO

Laboratory Diagnosis

• Serology (ELISE for IgM)• Immunohistochemistry• Reverse transcription and polymerase chain

reaction (RT-PCR)• Virus isolation• Direct detection of antigen in blood and

urine• Immunofluorescent test for antibodies

Problems Diagnosing Hantavirus

• Symptoms often confused with influenza• Common signs of upper respiratory disease

such as sore throat, sinusitis, and ear pain not usually present

• Abdominal pain often misinterpreted as appendicitis

• Many doctors outside endemic regions fail to recognize or have sufficient testing

Friday, March 26, 2004PARKS AND PEOPLE Glacier National ParkDeath of Jerry O'NealThe employees of Glacier National Park are deeply saddened to learn of

the untimely passing of Deputy Superintendent Jerry O'Neal. O’Neal died early yesterday morning at a Kalispell, Mont., hospital following a brief illness.

O'Neal, 61, came down with an unknown illness last week and had undergone blood work and other diagnostic tests over the past few days. He was admitted to the Kalispell Regional Medical Center on Wednesday and died at 5:30 a.m. Thursday.

Friday, April 2, 2004OPERATIONS NOTE Public HealthHantavirus UpdateIn view of the untimely death of Glacier National Park Deputy Superintendent Jerry

O'Neal on March 25th from hantavirus pulmonary syndrome, we are issuing the following precautions and annual reminders.

Hantavirus pulmonary syndrome (HPS) is a viral disease transmitted to humans primarily through the inhalation of airborne dusts laden with the virus from infected rodent droppings (urine and saliva may also be sources of infection). Although hantaviruses have been a threat to human health worldwide for at least 50 years, HPS was first recognized in the United States in 1993 around the Four Corners area of the Southwest. Since then it has since been identified throughout the United States. Although rare, HPS is potentially deadly – mortality rates between 40 and 50% are common.