Medical Virology: Viral Structure & Classification

• Medical virology is the study of viruses that cause disease in humans.
• It encompasses the structure, classification, replication, pathogenesis, diagnosis, prevention, and treatment of human viruses.

Viral Structure

• Viruses consist of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid.
• Some viruses possess an outer envelope derived from the host cell membrane.
• The capsid safeguards the viral genome and facilitates attachment to host cells.
• Viral genomes are small, encoding only essential replication proteins.

Viral Classification

• Viruses are classified based on nucleic acid type (DNA or RNA).
• Classification also considers capsid structure (icosahedral, helical, or complex).
• The presence or absence of an envelope influences viral classification.
• Virion size affects classification.
• Host range determines viral classification.
• The International Committee on Taxonomy of Viruses (ICTV) maintains a universal virus taxonomy.

Viral Replication

• Viral replication involves attachment where the virus binds to host cell receptors.
• Penetration occurs through endocytosis or membrane fusion.
• Uncoating releases the viral genome from the capsid.
• Replication of the viral genome utilizes host cell or virally encoded enzymes.
• Assembly involves the creation of new viral particles.
• Release occurs as the virus exits the host cell via lysis or budding.
• Viruses exploit host cell machinery for replication.
• This exploitation often leads to cell damage or death.

Viral Pathogenesis

• Viral pathogenesis is the process by which viruses cause disease.
• Entry into the host can occur via the respiratory tract, gastrointestinal tract, skin, or mucous membranes.
• Spread within the host can be local or systemic through the bloodstream or lymphatic system.
• Cell tropism describes how viruses infect specific cell types based on viral receptors on the cell surface.
• Viruses can directly damage cells through lysis, apoptosis, or syncytia formation, known as cytopathic effects.
• The host immune response contributes to viral clearance.
• The immune response can also cause immunopathology.
• Some viruses establish persistent infections, either latent or chronic, known as viral persistence.

Viral Infections

• Viral infections can be acute, with rapid onset and resolution.
• Chronic infections persist for extended periods.
• Chronic infections may be asymptomatic or cause chronic disease.
• Examples of common viral infections include influenza, common cold, and RSV (respiratory infections).
• Norovirus and rotavirus are examples of gastrointestinal infections.
• Measles, chickenpox, and rubella are exanthems.
• Encephalitis, meningitis, and polio are neurological infections.
• HIV, herpes simplex virus, and human papillomavirus are sexually transmitted infections.
• Hepatitis A, B, C, D, and E are types of hepatitis.

Laboratory Diagnosis of Viral Infections

• Viral culture involves growing viruses in cell culture.
• Direct detection identifies viral antigens or nucleic acids in clinical samples.
• Serology detects antibodies against viruses in serum.
• PCR is widely used for rapid and sensitive detection of viral nucleic acids.

Prevention of Viral Infections

• Vaccination prevents viral infections by stimulating antibody production.
• Handwashing and avoiding close contact with infected individuals are important hygiene practices.
• Vector control manages insect vectors that transmit viruses.
• Safe sex practices prevent sexually transmitted viral infections.
• Screening of blood and organ donations prevents transmission of bloodborne viruses.

Treatment of Viral Infections

• Antiviral drugs treat some viral infections.
• Antiviral drugs target specific steps in viral replication.
• Neuraminidase inhibitors are used for influenza.
• Reverse transcriptase inhibitors and protease inhibitors are used for HIV.
• Nucleoside analogs are used for herpes simplex virus and varicella-zoster virus.
• Interferon is used for hepatitis B and C.
• The development of new antiviral drugs is ongoing.
• Immunotherapy, such as monoclonal antibodies, can treat viral infections.