Lesson 1c: Chain of viral infection

The chain of viral infection describes the process by which a virus moves from one host to another, ultimately causing disease. It includes several key stages, from the initial contact with the host to the transmission of the virus to new individuals. This chain typically follows a series of steps, each of which is crucial for viral survival, replication, and spread.

Chain of Viral Infection:

1. Reservoir of Infection (Source)
   • The reservoir is the natural habitat where the virus lives and multiplies. It can be a human, an animal, or the environment.
   • Example:
     • Influenza virus has both humans and birds as reservoirs.
     • Rabies virus reservoirs include wild animals like bats, raccoons, and foxes.
2. Portal of Exit
   • The virus must leave the reservoir or infected host to spread to another individual. The portal of exit is the site through which the virus is shed from the body.
   • Examples:
     • Respiratory tract: Coughing or sneezing expels viruses like SARS-CoV-2 and influenza.
     • Gastrointestinal tract: Viruses like norovirus and rotavirus are shed in feces.
     • Blood or bodily fluids: Viruses like HIV and hepatitis B are spread through blood, sexual fluids, or from mother to child.
3. Mode of Transmission
   • The virus moves from the reservoir to a new host through various transmission modes. This can happen directly or indirectly.
   • Examples:
     • Direct contact: Physical contact with an infected person (e.g., touching lesions caused by herpes simplex virus or exposure to infected blood in HIV).
     • Droplet transmission: Inhalation of respiratory droplets (e.g., SARS-CoV-2, influenza).
     • Fecal-oral transmission: Ingesting food or water contaminated with the virus (e.g., norovirus).
     • Vector-borne transmission: Insects like mosquitoes or ticks transmit the virus (e.g., Zika virus via mosquitoes, Tick-borne encephalitis virus).
4. Portal of Entry
   • The virus enters the new host through a specific entry point, usually determined by the virus’s route of transmission.
   • Examples:
     • Respiratory tract: Viruses enter through inhalation (e.g., influenza virus, SARS-CoV-2).
     • Gastrointestinal tract: Viruses are ingested via contaminated food or water (e.g., norovirus, hepatitis A).
     • Skin or mucosa: Viruses enter through breaks in the skin or mucosal surfaces (e.g., HIV, herpes simplex virus).
     • Bloodstream: Viruses directly enter the bloodstream via insect bites (e.g., Zika virus) or through contaminated needles (hepatitis B).
5. Susceptible Host
   • The virus infects a host that is susceptible, meaning they lack immunity to the virus. Factors like age, immune status, and underlying health conditions influence susceptibility.
   • Examples:
     • Immunocompromised individuals: People with weakened immune systems (e.g., HIV patients) are more susceptible to infections like cytomegalovirus.
     • Young children or the elderly: These groups may have weaker immune defenses and be more prone to viral infections like RSV (respiratory syncytial virus) or influenza.
6. Replication and Disease in the Host
   • After the virus enters the new host, it begins the process of replication within host cells, leading to the spread of the virus throughout the body (dissemination). This can cause symptoms and disease.
   • Examples:
     • HIV replicates in immune cells (CD4+ T cells), weakening the immune system over time.
     • Influenza virus replicates in the respiratory tract, causing symptoms like fever, cough, and body aches.
     • Hepatitis B replicates in liver cells, causing liver inflammation and, over time, potential liver damage.
7. Transmission to New Hosts
   • Once the virus has replicated in the host and spread within their body, it is eventually shed and transmitted to new hosts, continuing the infection cycle.
   • Examples:
     • Infected individuals with influenza cough, releasing viral particles that can infect others nearby.
     • A person with hepatitis B can transmit the virus to others via blood or sexual contact.
     • A mosquito that bites an individual infected with Zika virus can become infected and transmit the virus to the next person it bites.

Diagram of the Chain of Viral Infection:

1. Reservoir of Infection → 2. Portal of Exit → 3. Mode of Transmission → 4. Portal of Entry → 5. Susceptible Host → 6. Replication and Disease → 7. Transmission to New Hosts

Breaking the Chain of Infection:

To prevent the spread of viral infections, the chain can be broken at various points:

• Vaccination: Boosts immunity, reducing the number of susceptible hosts (e.g., measles vaccine).
• Quarantine and isolation: Prevents transmission from infected individuals by blocking portals of exit (e.g., isolating COVID-19 patients).
• Hygiene and sanitation: Proper handwashing and clean water reduce the risk of fecal-oral transmission (e.g., norovirus).
• Vector control: Reducing mosquito populations can prevent vector-borne transmission (e.g., for Zika virus or malaria).

By understanding this chain, public health measures can be better targeted to prevent and control viral outbreaks.