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

Test your understanding of viruses, including the concept of a virus, how they infect cells, and the history of virology. Learn about the pioneering work of Louis Pasteur, Ivanovski, Beijerinck, Loeffler, and Frosch, and assess your knowledge of viruses and their effects on living organisms.