Introduction to Viruses

Introduction to Viruses

• Louis Pasteur first proposed the term "virus" in the 1890s
• Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus
• Loeffler and Frosch discovered an animal virus that causes Foot-and-Mouth Disease (FMD) in cattle

Definition of a Virus

• A virus is a non-cellular particle made up of genetic material and protein that can invade living cells and reproduce
• Viruses are submicroscopic, obligate intracellular parasites
• They are inert outside of a host cell

Terminology

• Virology: the study of viruses
• Virus particle (virion): the extracellular form of a virus
• Viral Latency: some viruses have the ability to become dormant inside a host cell, e.g. HIV and Herpes viruses

Properties of Viruses

• Non-cellular organisms enclosed in a protective envelope
• Presence of spikes helps in attaching the virus to the host cell
• Do not grow, respire, or metabolize, but reproduce
• Surrounded by a protein coat (capsid) and have a nucleic acid core comprising DNA or RNA
• Considered both living and non-living things

Types of RNA Viruses

• Double-stranded (ds) RNA virus: e.g. Reovirus
• Single-stranded (ss) RNA virus: further classified into two types:
  • Positive sense RNA (+RNA): e.g. Poliovirus, Hepatitis A, Rabies virus, Influenza virus
  • Negative sense RNA (-RNA)

Diversity of Animal Viruses

• Nonenveloped ssDNA: Parvovirus
• Enveloped ssRNA: partially dsDNA, e.g. Hepadnavirus
• Enveloped dsDNA: e.g. Papovavirus
• ssRNA: e.g. Picornavirus, Rhabdovirus, Togavirus, Orthomyxovirus
• dsDNA: e.g. Poxvirus, Adenovirus, dsRNA, Bunyavirus, Coronavirus, Reovirus
• RNA viruses: e.g. Retrovirus

Structure of the Virus

• A virus is either DNA or RNA, protected by a protein coat called a capsid
• Protein coat (capsid) made of individual protein subunits called capsomeres
• Enveloped or non-enveloped (naked)
• Additional layers: carbohydrates, lipids, other proteins

Viral Glycoproteins

• Reside on the surface of virions and are often the sole component of the virus that interacts with the external environment
• H spikes help a virus enter a cell, and N spikes help it leave

Prions

• Causative agents of Kreutz Feld Jakob disease, scrapie, bovine spongiform encephalopathy (BSE), and kuru
• Incorrectly folded proteins cause normal proteins to fold in such a way as to be non-functional

Techniques in Cultivating and Identifying Animal Viruses

• Primary purposes of viral cultivation: to isolate and identify viruses in clinical specimens, to prepare viruses for vaccines, and to do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells
• Using live animal inoculation: specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits
• Using bird embryos: enclosed in an egg, nearly perfect conditions for viral propagation
• Using cell (tissue) culture techniques: most viruses are propagated in some sort of cell culture

Treatment of Animal Viral Infections

• Antiviral drugs block virus replication by targeting one of the steps in the viral life cycle
• Interferon medication shows potential for treating and preventing viral infections
• Vaccines stimulate immunity
• Antivirals can only be used to treat certain viral infections and do not "kill" or disarm the virus permanently
• Vaccines can only be used to prevent infections (both viral and bacterial) from leading to disease

The Viral Envelope

• Enveloped viruses take a bit of the host cell membrane in the form of an envelope
• In the envelope, some or all of the regular membrane proteins are replaced with viral proteins
• Some proteins form a binding layer between the envelope and the capsid
• Glycoproteins remain exposed as spikes (peplomers) essential for attachment
• Naked: transmitted through feces or orally due to ability to live in the environment
• Enveloped: transmitted only by direct contact (blood, sexually, respiratory, insect/animal bites)

Replication Properties and Site of Replication

• Replication within the cytoplasm of the host cell: e.g. all RNA viruses except Influenza virus
• Replication within the nucleus and the cytoplasm of the host cell: e.g. Influenza virus, Poxvirus
• Replication within the nucleus of the host cell: all DNA viruses except Pox virus
• Replication of the virus through the double-stranded DNA intermediate: e.g. all DNA viruses, Retrovirus, and some tumor-causing RNA viruses
• Replication of the virus through a single-stranded RNA intermediate: e.g. all RNA viruses except Reovirus and tumor-causing RNA viruses