Virology: Structure and Function of Viruses

Viral Structure and Composition

• A virion is an intact infectious virus particle, consisting of viral protein, nucleic acid, and lipid envelopes.
• Hemagglutinin is a naturally occurring glycoprotein that causes red blood cells to agglutinate or clump together.
• Viral protein facilitates the transfer of viral nucleic acid from one host cell to another, protects the viral genome against inactivation by nucleases, and participates in the attachment of the virus particle to a susceptible cell.
• There are two types of viral proteins: structural proteins present in the coat of the virus, and non-structural proteins, including hemagglutinin and enzymes.

Viral Nucleic Acid

• Viruses contain a single kind of nucleic acid, either DNA or RNA, which encodes the genetic information necessary for replication.
• The genome may be single or double-stranded, circular or linear, and segmented or non-segmented.
• The type of nucleic acid, its polarity, and its size are major characteristics used for classifying viruses into families.

Viral Lipid Envelopes

• Lipid is acquired when the viral nucleocapsid buds through a cellular membrane during maturation.
• Lipid-containing viruses are sensitive to treatment with ether and other organic solvents, indicating that disruption or loss of lipid results in loss of infectivity.
• Nonlipid-containing viruses are generally resistant to ether and detergents.

Symmetry of Viruses

• Icosahedral viruses have a rigid, polygon-shaped structure with 12 corners/vertices and 20 surfaces/sides.
• Examples of icosahedral viruses include herpesvirus, parvovirus, papovavirus, adenovirus, picornavirus, and reovirus.
• Enveloped/helical viruses have protein subunits (capsomeres) packed in a helical array around the viral nucleic acid, forming a spiral tube or helix.
• Complex viruses neither have icosahedral nor enveloped/helical structures.

Virulence Factors

• Some viral particles can replicate during the defensive inflammation responses of the host, such as during virus-induced fever.
• Extremely virulent strains can eventually evolve by mutation and natural selection within the virus population inside a host.

DNA Viruses: Herpesviruses

• Herpesviruses are enveloped, large viruses with a double-stranded DNA genome, belonging to the family Herpesviridae.
• Sources of herpesviruses include saliva, skin lesions, and respiratory secretions from infected, asymptomatic carriers.
• Herpesviruses produce an initial acute infection followed by a period of latent infection, with reactivation resulting in recurrent infection.
• Herpesviruses exhibit a wide spectrum of disease, including gingivostomatitis, pharyngitis or tonsillitis, lesion on the face, herpes encephalitis, and eczema.

RNA Viruses

• RNA viruses replicate in the cytoplasm and have a simpler mechanism of replication, with early and late mRNA not clearly separated.
• There is no host cell RNA polymerase that can use the viral parental RNA as a template for synthesis of complementary RNA strands.
• Examples of RNA viruses include enteroviruses, polioviruses, coronaviruses, and orthomyxoviruses.

Enteroviruses

• Enteroviruses are transient inhabitants of the human alimentary tract and may be isolated from the throat or lower intestine.

Polioviruses

• Polioviruses are shed by infected individuals into the environment, spreading rapidly in areas of poor sanitation.
• Poliovirus causes polio or poliomyelitis, entering through the mouth and multiplying in the intestine.
• The virus mainly affects nerves in the spinal cord or brain stem, resulting in flaccid paralysis.
• Prevention and control are achieved through a series of vaccinations.

Arboviruses in the Philippines

• Examples of prevalent arboviruses in the Philippines include dengue (DEN), Japanese encephalitis (JE), chikungunya (CHIKV), and Zika virus (ZKV).

Coronaviruses

• Coronaviruses are large, enveloped RNA viruses that cause common colds and may cause lower respiratory tract infections and gastroenteritis in infants.
• Novel coronaviruses have been identified as the cause of severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

Fungal Infections

• Yeast infections, such as candidemia, can spread through eyes, endocardium, meninges, kidney, liver, spleen, and bone, and may cause cross-infection (mother to baby).
• Diagnosis is through blood culture test.
• Cryptococcosis is a fungal infection caused by Cryptococcus neoformans, which is found worldwide and is an opportunistic invasive fungus.

Clinical Classification of Mycoses

• Superficial mycoses affect the mucus membrane or keratinized tissue.
• Subcutaneous mycoses are due to the implantation of environmental fungi.
• Systemic mycoses cause widespread disease and are serious and fatal, often affecting healthy individuals and opportunistic infections in immunocompromised individuals.

Filamentous Fungi Infections

• Examples of filamentous fungi infections include dermatophytosis (ringworm infection), which appears as red patches on the affected area of the skin and later spreads to other parts of the body.