virology

Definition: Viral-host interactions
- Viral and host processes that occur during viral infection enable both the virus and the host to respond to each other.

A virus relies on cell processes to successfully replicate.
Viral replication cycle involves significant biochemical and structural modifications in the host cell leading to cell damage, termed cytopathic effects.
- These effects may alter cell functions or completely destroy the host cell.

At the molecular level, viruses invade host cells:
- They manipulate cellular mechanisms to replicate, survive, and induce disease.
- Viruses depend on host cells due to their reliance on them for their life cycle.
Protein-Protein Interactions:
- Viruses utilize interactions with the host's cellular proteins to exploit cellular machinery.

Host Cell Destruction:
- New viruses emerge from the host cell through lysis, leading to cell death.
- Some viruses acquire part of the host cell's membrane during lysis, forming an envelope around their capsid.

Viruses target specific host cells using:
- Receptor sites on the host cell membrane for attachment, facilitated by attachment proteins located in the capsid or glycoproteins in the viral envelope.
This interaction specificity determines which host species and particular cells within the host can be infected.

Requirement for Host Cells:
- Viruses lack the ability to reproduce independently; they require a host cell to replicate and spread.
Stages of Viral Replication:
- Six basic stages are essential, varying significantly across virus types and families.

Structure:
- Core of genetic material (either DNA or RNA) encapsulated in a protective protein coat called a capsid.
- Sometimes an envelope made of glycoproteins surrounds the capsid.
Infection Process:
- Viruses are adept at latching onto and infiltrating host cells.

The host can combat viruses:
- By employing immune defenses that identify and destroy infected cells.

Pathogenesis:
- The process by which viral infections progress to disease.
Pathogenic Mechanisms:
1. Implantation of virus at the portal of entry.
2. Local replication of the virus.
3. Spread to target organs (disease sites).
4. Dispersal of the virus to external environments.

Key Factors:
1. Accessibility of virus to tissues.
2. Susceptibility of the host cells to viral multiplication.
3. Effectiveness of the host's immune defenses against the virus.

Direct Damage Mechanisms:
1. Diversion of host cell's energy resources.
2. Disruption of macromolecular synthesis in the host cells.
3. Competition of viral mRNA for cellular ribosomes.
4. Utilization of cellular transcriptional factors by viral promoters.
5. Inhibition of the host's interferon responses.
Indirect Damage Mechanisms:
1. Integration of the viral genome into the host genome.
2. Induction of mutations.
3. Initiation of inflammation due to immune response.

Definition: The affinity of specific viruses for particular body tissues.
Factors Determining Tropism:
1. Presence of specific cell receptors for the virus.
2. Availability of transcription factors capable of recognizing viral sequences.
3. The host cell’s ability to sustain viral reproduction.
4. Physical barriers and environmental conditions (temperature, pH).

Entry Mechanisms:
- Viable entry routes include respiratory, gastrointestinal, skin-penetrating, and genital exposures.
Local Spread:
- Viruses may spread extracellularly or intracellularly leading to localized infections and shedding of the virus.
Viremia and Neural Dissemination:
- Commonly through the lymphatic system or directly spreading via nerve pathways (as seen with rabies).

Poliomyelitis Overview:
- Caused by the poliovirus, responsible for causing muscle paralysis through infection of the spinal cord.
Poliovirus Characteristics:
- Belongs to the family Picornaviridae and has a positive-sense RNA genome approximately 7500 nucleotides long.

Transmission commonly occurs via fecal-oral route, particularly in areas with inadequate sanitation.
Polio can manifest with or without symptoms (95% asymptomatic).
Risk Factors:
- Pregnant women, individuals with weakened immune systems, and those without vaccinations.

Non-paralytic (Abortive) Polio:
- Presents flu-like symptoms lasting days or weeks:
  - Fever, sore throat, headache, vomiting, fatigue, stiff neck, and muscle pains.
Paralytic Polio:
- Affects a minority (1%) leading to:
  - Loss of muscle reflexes, severe muscle pain, limb weakness, respiratory complications.
- Subtypes:
  - Spinal Polio: Affects lower limbs and breathing.
  - Bulbar Polio: Impacts swallowing and respiratory functions.

Post-Polio Syndrome:
- Occurs in 64% of survivors decades post-infection, leading to:
  - Progressive muscle and joint pain, fatigue, and swallowing difficulties.

Definition: Life-threatening liver infection caused by the hepatitis B virus (HBV).
Transmission: Occurs through contact with infected blood and body fluids.

HBV is a partially double-stranded DNA virus belonging to the family Hepadnaviridae.
It can lead to chronic infection resulting in cirrhosis and liver cancer.

Acute Infection:
- Symptoms may not always appear, particularly in young children. When they do:
  - Jaundice, fever, fatigue, abdominal discomfort, joint pain.
Chronic Infection:
- Symptoms similar to acute but often asymptomatic until advanced stages.

Transmission Routes:
- Unprotected sexual contact, sharing needles, open wounds, and from mother to child during birth.
Diagnostic Methods:
- Blood tests to detect HBV antigens and antibodies.

Vaccination:
- Preventative measures include vaccinations for at-risk individuals.
Medications for Chronic Cases:
- Include antivirals and supportive care like lifestyle changes and liver monitoring.
- Common drugs include Entecavir, Tenofovir, and Lamivudine.

Vaccination:
- Recommended for all newborns and at-risk populations.
Hygiene Practices:
- Regular hand washing, avoiding direct contact with contaminated surfaces, and proper needle sterilization.
Health Monitoring:
- Regular blood tests for at-risk individuals to monitor liver function and potential viral activity.