Virus Characteristics

Virus General Characteristics

• Infectious agents are viruses the term originally referred to non-filterable poisons or venom, able to pass through small filters causing disease.
• Viruses are non-living, they do not possess the characteristics of living organisms like cellular order and energy production through respiration.
• They can't move, grow or reproduce on their own
• Viruses also cannot maintain homeostasis.
• These characteristics are only present when a host cell is present.
• The genetic material can be either DNA or RNA, but never both simultaneously.
• Outside a host cell, viruses are inert requiring a host to carry out replication
• Viruses lack any internal processing or regulation.
• Viruses are obligate intracellular parasites.
• Require electron microscopy to be seen.
• Size ranges from 20nm (parvo virus) to 14,000nm (pox virus)
• Bacteria are larger, between 1-10 microns (10,000nm)
• Viruses are host-specific, targeting certain species and cell types.
• Most organisms are susceptible to viral infections.
• While viruses target animals, plants and fungi they work differently in each organism.
• Bacteriophages are viruses that exclusively infect bacteria.
• Specificity of a given virus can change over time.

Virion Structure

• A virion is a completely developed infectious particle.
• The nucleic acid core of a virus is found outside of a host cell
• Viruses may have a DNA or RNA core, Retroviruses have a DNA core
• The core may be single or double stranded.
• They use the hosts materials to transcribe and translate
• A capsid is a protein coat which gives the virion its shape.
• Capsids are made of protein subunits called capsomeres
• The viral envelope is a lipid, protein, and carbohydrate coating external to the capsid, some viruses have this some do not.
• Spikes are protein or carbohydrate projections extending from the envelope surface of enveloped viruses.
• Spikes allow the virus to attach to host cells.
• Spikes have specific structures that are named.
• Antigenic drift is the mutation of spikes to help the virus avoid the host immune system.
• Capsid morphology is visible using electron microscopy and can be helical, polyhedral, or enveloped.
• Helical capsids are long and rod-like, comprised of tightly wound coils.
• Polyhedral capsids such as an icosahedron have 20 faces and 12 corners.
• Enveloped viruses have a spherical shape
• Complex viruses like bacteriophages are neither helical nor polyhedral

Virus Cultivation and Identification

• Viruses have an obligate intracellular life cycle, meaning they cannot reproduce without a host cell, thus requiring them to be cultured inside living cells.
• Living animal research involves taking over the cells makes them difficult to treat.
• Mice, rats, rabbits, guinea pigs, and non-human primates are often used.
• Host responses and immune reactions can be studied in animal models.
• Test animals are observed, euthanized, and examined post-infection.
• Results are theoretically applied to humans.
• Inoculating embryonated eggs involves injecting a chicken egg from outside the shell into the chick tissue.
• Injection can go to the chorioallantoic membrane, allantoic or amniotic fluid, or yolk sac, yielding varying results based on injection location.
• Viral growth can cause embryonic death, cell damage, or membrane lesions.
• Egg allergies can be triggered by vaccinations using eggs to grow viruses.
• In vitro growth is the literal growth of viruses in glass.
• Cell lines need to grow as a single layer in fluid media
• Antibiotics help maintain healthy cells by keeping them on a single flat surface.
• Viruses are added to cells to observe cytopathic effect to see the effects of the viruses
• Viral identification can be done visually but requires an electron microscope due to their small size.
• Serology involves using serum and antibodies to identify and indicate viral presence.
• Fluorescence, agglutination, and ELISA tests are conducted.
• The PCR (polymerase chain reaction) technique uses molecular probes to rapidly produce many copies of antigenic DNA or RNA segments.

Viral Multiplication

• They have very few genes, so use the host cells cellular components to build capsids and their proteins to reproduce.
• Stages of infection are similar for all types of viruses, including attachment, penetration, uncoating, biosynthesis, maturation, and release.
• To make a a protein, RNA is needed.
• The central dogma of life is DNA -> RNA -> protein through transcription and translation
• Retroviruses use reverse transcriptase.
• RNA need to be able to turn into DNA

Bacteriophages

• Attachment occurs by chance with a host cell receptor.
• Penetration involves an enzyme called lysozyme.
• Lysozyme from the bacteriophage is used to pierce the host cell membrane and inject viral DNA into the cytoplasm.
• Uncoating is not needed following penetration
• Biosynthesis involves using parts made by the bacterial nucleoid.
• Maturation involves bacteriophages that are assembled in the cytoplasm.
• Release happens following the lytic and lysogenic cycles

Bacteriophage Viral Cycles

• The lytic cycle lyses releases infectious virions and kills the cell immediately.
• Lysogenic cycle maintains itself in the viral genome, hides while the cell replicates and divides and spreads until triggered at a later date.
• Bacterium undergoes binary fission where viral DNA incorporated to multple cells.

DNA Viruses

• Attachment happens when virions develop specific surface proteins/spikes to allow attachment to host receptors, these receptors are heritable traits.
• Penetration occurs when the host cell brings the virus via endocytosis.
• If there is an envelope, there will be fusion with the host cell membrane
• Uncoating is when the protein coat is removed, and viral DNA released.
• Biosynthesis involves viral DNA entering the nucleus, where mRNA is transcribed and proteins are produced.
• Maturation involves the virions that are repackaged in the host nucleus.
• Release happens through budding, lysis, or rarely reverse penetration.
• Budding coats a non-enveloped virus in the host cell membrane, and is not recognized as foreign

RNA Viruses

• Biosynthesis involves RNA entering the cytoplasm and making its own mRNA to produce viral proteins in the host ribosome.
• Maturation reassembles viral components in the cytoplasm which allows for its release
• Release

Latent Vs Persistent Viral Infections

• A latent virus happens when the host is at equilibrium until the virus suddenly causes disease, and remains undetected.
• During the latent phases no symptoms occur and the virus is hiding.
• Relapsing and remitting viruses, such as herpes simplex 1 in the trigeminal nerve, can lead to cold sore outbreaks and reactivation.
• Reactivation often happens with 10-15% of people
• With Varicella, the virus in the ganglia lead to shingles with 10–20% of adults getting the virus
• FeLV can be progressive or regressive and potentially fatal.
• Persistent viruses slowly build up over time and cause disease with viral levels detected.
• HIV and FIV are examples of persistent viruses, with a chronic slow infection

Types of Viruses

• Common DNA and RNA viruses exist.
• DNA viruses include:
  • Adenoviridae which causes the common cold.
  • Poxviridae, such as smallpox, cause pus-filled skin lesions.
  • Herpesviridae has 100 known types, some can cause cancer.
    • Includes diseases such as Herpes Simplex I (cold sores), HSV II (genital herpes)
    • Varicellavirus -(Chickenpox)
    • Epstein‐Barr virus (Mononucleosis)
  • Equine Herpes virus can be a Respiratory, neurologic and venereal disease.
  • Feline Herpes virus is in the feline URI which is in the RCP vaccine
  • Papovaviridae transforms normal cells into cancer cells, such as warts
• Common RNA viruses include
  • Picornaviridae, such as Poliovirus.
  • Togaviridae, EEE, WEE and VEE include symptoms such as head pressing, and neurological signs.
  • Arboviruses causes the virus to be transmitted by arthropods.
  • Rhabdoviridae can trigger the Rabies virus
  • Reoviridae. Colorado tick fever can cause respiratory infection, and enteric infection.
  • Retroviruses such as the diarrhea

Viral Replication and Process

• Replication differs compared to other virus types.
• Process: typically is DNA->RNA->protein, but retroviruses use reverse transcriptase to double strand the DNA
• Viral DNA incorporates into the host genome.
• It is protected from immune response and antiviral drugs.
• This can lead to different outcomes/types of infections
  • Latent infection only replicates when the host cell divides.
  • Olysis kills the cell and allows it to become extracellular.
  • Often, budding is used
  • Sometimes, oncogenesis leads to cancer.
• Common retroviruses include:
  • HIV/FIV acquired from an immunodeficiency virus
  • FeLV the helper T-cells - (HIV-AIDS) causes immunodeficiency and cancer where the Immune System is suppressed.
  • EIA
  • Oncoviruses
• Cancer is caused from a viral genome insertion that activates a space in the host genome, also known as an oncogene. "Transformation" causes growth of the cell.
• Cancer cells often lack contact inhibition.
• Main virus types are DNA, RNA, and Retro

Plant and Fungal Viruses

• Tobacco mosaic virus was the first plant virus discovered.
• Economic impact equals $60B lost in crop yields worldwide annually
• Plant viral diseases are caused by viroids, specifically naked pieces of RNA without a protein coat
• Transmission of plant viruses differs from animal viruses
• Often spread through plasmodesmata of plants and requires a vector
• Mycoviruses focuses on viruses in fungi pathogenic to plants.
• If a virus infects a fungus that kills plants, it may decrease the virulence of the fungus and protect the plant.
• This could have therapeutic uses in humans with more study and research.
• They rely on intracellular transmission through the hyphae and so transmission is limited.

Prions

• Prions are infectious, but non-living proteinaceous particles that have no DNA or RNA.
• They can also cause abnormally folding the proteins
• Can be passed to offspring and transmitted by ingestion or chronic exposure.
• Prion diseases are caused by the crowding and crushing of normal structures resulting in an encephalopathy.
• Scrapie in sheep causes the protein to crowd or squish in the brain
• Bovine Spongiform Encephalopathy in cows (Creutzfeldt-Jacob Disease) causes squishing.
• Chronic Wasting Disease (CWD) in cervids causes holes.