RNA Viruses and Prions

Composed of 8 genera where orthoreovirus, orbivirus, coltivurs, and rotavirus infect humans

ds RNA

Icosahedral

No envelope is present

Reoviruses are unusually stable to heat, to a pH range of 3.0-9.0, and to lipid solvents, but they are inactivated by 95% ethanol, phenol, and chlorine.

Rotavirus

  • Transmission is believed to be fecal-oral
  • Major agents of severe diarrhea in infants and young children
  • appears to cause a substantial portion of cases of gastroenteritis in children aged 6 months to 2 years.

Picornaviruses (ssRNA)

Wider range of illnesses than most other, if not all virus families

Smallest naked icosahedral

Two genera- enterovirus and rhinovirus(approx. 105 serotypes)

  • Major enteroviral pathogens that affect humans- poliovirus, enterovirus, coxsackevirus
  • Infection may be asymptomatic or show clinical syndromes such as aseptic meningitis, encephalitis, common cold, conjunctivitis, hepatitis (hepatitis A)

Togaviruses (ssRNA)

Icosahedral, Enveloped

Divided into Alphaviruses and Rubiviruses

  • Rubella virus causes Rubella (“German Measles”)
  • Transmitted by respiratory route from human to human
  • The live, attenuated Rubella virus vaccine is given universally to young children in the US and many other developed countries. It is given in two shots (at 15 months and 5 years) and produces lasting immunity in more that 90% of recipients.

Flaviviruses (ssRna)

Icosahedral

Divided into 7 subgroups of families

Viruses of this family are arthropod-borne viruses in both human and veterinary medicine (mainly by mosquitos and ticks)

  • St. Louis Encephalitis, West Nile fever, yellow fever,

Orthomyxoviruses (ssRNA)

Spherical, enveloped

3 family members: influenza A, B, C

Humans can be infected by A (subtypes H1N1, H1N2, H3N2), B, & C

Antigenic variability of influenza A viruses are on the hamagglutinin & neuraminidase proteins

Influenza viruses can change

  • One is called “antigenic drift.” Small changes in the virus that happen continually over time. Antigenic drift produces new virus strains that may not be recognized by the body's immune system.
  • Mutation is responsible for the annual epidemics (the antigenic drift, occur every 2-3 years)
  • The other type of change is called “antigenic shift.”

Paramyxoviruses (ssRNA)

Enveloped, helical nucleocaspid

Paramyxoviruses are an important class of viruses which are associated with respiratory ailments, and common childhood diseases such as measles and mumps.

  • Measles-main route of transmission is via inhalation
  • Complications- ear infections, pneumonia, encephalitis, Sub-SclerosingPanencephalitis
  • Mumps- normally affects children under 10 years of age. The virus is transmitted by respiratory secretions
  • Complications include- Meningitis (15% of cases), which may include neck stiffness and Deafness (rare), Pancreatitis, Myocarditis

Rhabdoviruses

There are more than 200 rhabdoviruses known

Bullet shaped virus, enveloped

Rabies

  • Entry occurs by wound or abrasion of skin directly into bloodstream (animal bite - e.g. vampire bats in S. America)
  • Man is a dead-end infection for the virus, normal hosts are foxes, dogs, cats, bats, skunks, etc
  • Although in veterinary medicine rabies vaccines are used as a preventive measure, vaccination of humans takes place mainly after exposure to a rabid animal
  • no effective drug treatment against rabies

Bunyavirus (ssRNA)

Large number of viruses that infect animals, insects and plants

Small enveloped spherical virus with a helical capsid

Spreads by mosquitoes, ticks, sandflies

The exceptions are the Hantaviruses, which are spread via aerosolized rodent urine and feces

Most bunyaviruses cause severe hemorrhagic fevers and encephalitides

Hantaviruses cause severe Adult Respiratory Distress Syndrome

Retroviruses

Non-icosahedral, enveloped viruses

Possess 2 copies of ss RNA

Divided into 2 subfamilies: Spumaretrovirinae and Orthoretrovirinae

Ecode for reverse transriptase

Enzyme that allows for transcription of DNA using RNA as template

DNA copy provirus in host cells

Activation: irradiation, chemicals, immunological stimulation

Oncogenic retroviruses- cancer causing retroviruses act rapidly

Lesser Known RNA viruses

Filovirus- can cause severe hemorrhagic fever in humans

  • 2 members of this family- Marburg virus and Ebola virus
  • May appear in different shapes

Arenavirus- lassa fever

  • generally associated with rodent-transmitted disease in humans
  • Spherical
  • Calcivirus- GI diseases

Prions

Prions (PREE-ons) are proteins that are unique in their ability to reproduce on their own and become infectious.

Prion diseases or transmissible spongiform encephalopathies (TSEs) are a family of rare progressive neurodegenerative disorders that affect both humans and animals