MICROBIO 2.6 - ENVELOPED RNA VIRUSES

Enveloped RNA Viruses

• Enveloped RNA viruses are viruses that use RNA as their genetic material and have a lipid envelope surrounding the capsid

Viral Classifications

• RNA viruses can be enveloped or non-enveloped
• Enveloped RNA viruses can be single-stranded positive sense, single-stranded negative sense, or retrovirus
• Examples of single-stranded positive sense enveloped RNA viruses: Togaviruses, and Coronaviruses
• Examples of single-stranded negative sense enveloped RNA viruses: Rhabdoviruses, Paramyxoviruses, Orthomyxoviruses, Arenaviruses, and Bunyaviruses
• Example of retrovirus: Lentiviruses

Class Objectives

• Be able to differentiate naked & enveloped RNA viruses and how they differ from DNA viruses in structure, replication, and morphology
• Able to define "negative sense" (& positive sense, & how they differ) and importance of segmentation
• Be able to compare & contrast naked & enveloped RNA viruses
• Distinguish enveloped RNA viruses and their diseases
• Diagnose enveloped RNA viruses based on signs, symptoms, diagnostic tests
• Prescribe appropriate prevention and treatments including vaccinations and therapeutics
• Describe the steps of infection and stages of rhabdoviridae presentation
• Explain what the filoviridae are

Negative Sense Enveloped RNA Viruses

Key characteristics of negative sense RNA viruses:

• Rhabdoviridae: linear RNA shape, negative RNA sense, helical capsid shape, example is Rabies
• Paramyxoviridae: linear RNA shape, negative RNA sense, helical capsid shape, examples are Measles, Mumps, RSV, and HMPV
• Arenaviridae: segmented RNA shape, negative (ambisense) RNA sense, helical capsid shape, example is Lassa Fever
• Bunyaviridae: segmented RNA shape, negative (Ambisense) RNA sense, helical capsid shape, examples are Hantavirus, and Rift Valley Fever
• Orthomyxoviridae: segmented RNA shame, negative RNA sense, helical capsid shape, example is influenza

Rhabdoviridae- Rabies

Key concepts

• Rabies virus is a zoonotic pathogen classified in the family Rhabdoviridae, genus Lyssavirus
• Rabies is mainly found in dogs in developing countries; in developed countries, it is perpetuated by bats and mammalian carnivores
• Rabies virus is transmitted to humans through the bite of an infected animal
• The virus spreads from the bite site via nerves in the peripheral nervous system to the central nervous system
• The disease manifests after a variable incubation period, approximately 1 to 3 months, but ranging from a few weeks to over 1 year

Damage from Rabies

• Signs and symptoms include acute, progressive fever, headache, difficulty swallowing, paresthesia, increased muscle tone, hypersalivation, paralysis, and hydrophobia
• Lesions are minimal but microscopic pathology may include intracytoplasmic inclusions in neurons, known as Negri bodies

Diagnosis of Rabies

• Rabies can be diagnosed postmortem by fluorescent antibody demonstration of viral antigen in the brain
• Or antemortem by detection of antibody in patient serum or cerebrospinal fluid
• Or viral antigen or nucleic acid/virus in saliva, skin biopsy, or brain biopsy

Treatment and Prevention of Rabies

• Rabies can be controlled by vaccinating domesticated and wild animals
• Rabies has been prevented in humans by wound treatment, rabies immune globulin, and rabies vaccination

Rhabdoviridae- Rabies Disease Mechanisms

• Rabies is usually transmitted in saliva from a bite wound by a rapid rabid animal
• Rabies virus is not very cytolytic and remains cell associated except in the salivary gland
• The virus replicates in the muscle at the bite site with little/no symptoms (incubation phase)
• After weeks/months, the virus infects the peripheral nerves and travels to the brain (prodrome phase)
• Brain infection causes coma and death (neurologic phase)
• During the neurologic phase, the virus spreads to the glands, skin, and other body parts
• Rabies infection does not elicit an antibody response until the central nervous system spread
• Administration of antibody can block progression of the virus and disease if given early enough

Rhabdovirus - Rabies

• Incubation: 1-3 months, up to 12 months
• Moves passively within axoplasm of peripheral nerves
• Signs & symptoms include fever, headache, increased muscle tone, difficulty swallowing, hydrophobia
• 20%-50% triggered by pain associated with swallowing water
• Focal & generalized seizures, disorientation, & hallucinations are common during neurologic phase
• Paralysis (15%-60%) may be the only manifestation, leading to respiratory failure

Diagnosing Rabies

• Negri bodies present in tissue
• Consider if the bite resulted in a skin break
• Has rabies been previously reported in the area and if it is rabid or escaped
• Is species known to commonly being get infected and is the biting animal observed

Rabies Progression

• Acute nervous system infection
• Prevention involves dog vaccination
• Fatal (if w/ out vaccination prior to onset of symptoms)
• Five stages of infection:
  • Incubation- 1-3 months (highly variable)
  • Prodrome- 2-10 days; nonspecific symptoms include tingling, pain at wound site, respiratory, GI, and CNS
  • Neurologic- hyperactivity/hydrophobia and paralysis, rapid & irregular breathing
  • Coma
  • Death- Cardio-respiratory arrest

Rhabdovirus - Rabies

• Diagnostic tests - Fluorescent antibody technique (stain the brain to detect rabies virus)
• Prevention includes:
  • Vaccinating pets (& wild animals)
  • Postexposure prophylaxis: Local wound treatment (soap + water), administer antibody (HRIG or human rabies immunoglobulin), and vaccination (4 doses in deltoid over 2 weeks; immunosuppressed patients get 5 doses)

Rhabdo/Viral life cycle basics

• Recognize & attach, binding to specific receptors
  • Enveloped: Use surface VAPs
• Penetration- Virion moves along host cell surface where it encounters entry receptors (co-receptors)
  • Fusion- Viral envelope fuses w/ host cell membrane causing virus to enter
• Uncoating: released from capsid, genome accessible to translation & transcription machinery
• Viral lifecycle:
  • Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell promoting viral entry into the cell
  • The viral genome functions as a template to synthesize complementary RNA strands produced by viral enzymes
  • New copies of the viral genome RNA are made using the complementary RNA strands as templates, and capsid proteins (cytosol) and glycoproteins (ER)
  • A capsid assembles around each genome molecule, then released as a new virus

Assembly and Release

• Self-assembly of the genome and structured proteins produces the nucleocapsid
• Assembly: Progeny viral RNA, early & late proteins, & capsid
• Release: is usually budding since they are enveloped, this requiring M protein

Positive Sense Viruses

• ssRNA is classified according to polarity of RNA
• act as mRNA & are directly translated and are infectious by themself

Negative Sense Viruses

• Requires viral rna to replicate
• Negative-sense strand is used as a template to produce mRNA transcripts before translation

Filoviridae

• Are enveloped, single-stranded, negative sense linear RNA viruses with helical nucleocapsids (like Rhabdovir)
• Ebola is considered more lethal than Marburg (~50% mortality)
• Both Ebola and Marburg have human-human transmission
  • Marburg- usually bat infested caves
  • Ebola- primate & fruit bats
• Incubation of Filoviridae is 2-21 days (~8-10 on avg.)
• Signs & Symptoms are hemorrhagi fever

Ebola and supportive therapy

• Supportive therapy, pain management, and dialysis are treatments as well as 2 monoclonal antibody treatments (Inmazeb & Ebanga)
• FDA has approved a live-attenuated rVSV-ZEBOV vaccine for Zaire ebolavirus-recombinant vesicular stomatitis virus (VSV; harmless in humans)
• There are currently no vaccines or mAbs for Marburg or Sudan Virus, but some are in the pipeline.
• Ebola progresses to disseminated intravascular coagulation, shock, & death

Scientist

Jean-Jeacques Muyembe, PhD

• Is a field epidemiologist & microbiologist
• Experienced mysterious outbreak in 197, like yellow fever or typhoid
• Saw hemorrhaging when he went to take blood
• Sent samples to hospital in Belgium & named Ebola (white scientists) after river in village but never received credit

Scientists who died battling Ebola:

• Sheik Humarr Khan, MD
• Mbalu Fonnie, nurse
• Alice Kovoma, nurse
• Alex Moignoi, nurse
• Mohamed Fullah, lab technician

Paramyxoviridae

• Have single-stranded, nonsegmented, negative-sense RNA genomes
• Have a lipid envelope derived from the host cell membrane
• Contain membrane glycoproteins that determines virulence
• Includes causes of some of the oldest known viral diseases and several respiratory diseases
• Measles virus is spread by aerosolized droplets.
• Measles virus replicates in the respiratory tract and then spreads in the bloodstream.
• Damage is associated with formation of giant cells in the respiratory epithelium and is enhanced by the host immune response.
• Diagnosis usually clinical
• Live, attenuated virus vaccine is protective.

Important Receptors of the family

• G: glycoprotein (HN, H, G) - Transmembranous envelope Viral attachment protein
• H is Hemagglutinin, HN is a fusion Hemagluttinin-Neuraminidase

Paramyxoviridae- Measles

• Pathognomonic (characteristic of a particular disease) symptoms
• Mucosal eruption (enanthem) & Koplik spots (pic on right)
• Leukopenia (decreased white blood cells)
• Three C's- cough, coryza (swelling of mucus membrane in nose) & conjunctivitis
• Serious- Acute disseminated encephalomyelitis (ADEM; demyelinating autoimmune)
• Serious- Subacute sclerosing panencephalitis (SSPE) (7-10 y after, mental impairment, personality changes, and myoclonus)
• Immunosuppression known as “immune amnesia", so watch for bacterial, viral, and protozoan infections
• Vitamin A deficiency & HIV/AIDS leads to poorer outcome
• Treatment is Measles immunoglobulin, SSPE is interferon

Measles Mumps and Rubella

• Measles is AKA rubeola
• Prevention include using a live, attenuated, MMR vaccine

Mumps

• Very communicable disease with one serotype, only infects humans
• Found worldwide & endemic in late winter & early spring
• Spread via inhalation of large respiratory droplets & person-person contact
• Contagion period precedes symptoms (7 days prior to symptoms)
• Virus may cause asymptomatic shedding
• Often asymptomatic, or sudden onset parotitis that is almost always bilateral & with fever; redness & swelling of parotid duct & other glands
• CNS involvement ~50% of patients; 10% may exhibit mild meningitis, 5/1000 cases of encephalitis
• Unvaccinated people, (especially infants <1 y.o., & immunocompromised) at greatest risk
• Prevention- Live attenuated vaccine (Jeryl Lynn strain) of MMR vaccine
• Immunity- life-long

Paramyxoviridae- Respiratory Syncitial Virus

• Most common cause of fatal acute respiratory tract infection in infants & young children
• 75% of children seropositive for RSV by 1 y.o.
• RSV is cytotoxic to respiratory epithelial cells, with resultant damage to ciliary function(barking cough/croup)
• Bronchiolitis- cough, wheezing, dyspnea, laryngotracheobronchitis (aka croup), & look the same in chest radiograph
• Treatment: passive immunoprophylaxis/Palivizumab (a monoclonal antibody/Reduces hospitalizations (high risk ~ 50%)
• Entry: Respiratory syncytial virus (RSV) is spread to the respiratory tract by airborne droplets and fomites.
• Spread and Replication: RSV infection is confined to the respiratory tract.
• Damage: Damage results from viral cytotoxicity and immune-mediated pathology
• Treatment and Prevention: (no vaccine; monoclonal antibody) Palivizumab, protects at-risk and immunocompromised children through the seasonal occurrence of RSV.

Paramyxoviridae- Human Metapneumovirus (HMPV)

• HMPV was discovered by Dutch virus hunters in 2001
• Genetically related to avian metapneumovirus, believed to came over humans.
• 7% seasonal upper respiratory infections, peaked 2023 (11% samples)
• One study found 2nd most common cause of respiratory infections
• 14 million infections in 2018, more than 600,000 hospitalizations, & > 16,000 deaths
• Common symptoms include: Runny or Stuffy nose, Cough, shortness of breath, Store throat, Wheezing Fever and Rash
• Doctors test testing often done in hospitals: after Covid & Influenza test are performed
• No antiviral Treatments, treatment: URI supportive

Segmented Genome

• Capacity to exchange genome segments during co-infection by a process called reassortment
• With multiple selection pressures can promote or prevent the emergence of reassortant viruses in the population.
• When ≥2 viruses infect a single host, each package genome segments from the other virus into a hybrid virion

Reassortment:

• Reassortment can increase viral fitness by enabling reassortants to escape immune recognition, but it can also decrease viral fitness by uncoupling essential protein sets that interact optimally when kept together.

Positive Sense Replication process:

• Positive-sense strand act as mRNA and are directly translated by cellular ribosome to produce viral proteins, hence it is infectious

Ambisense RNA

• In this process each strand has regions of both + and - polarity