Pathogenesis of Viral Diseases

Pathogenesis of Viral Diseases

The fundamental process of viral infection is the expression of the viral replicative cycle in a host cell.
 

The steps for the infectious process involving viruses are:

1. Enter a host
2. Contact and enter susceptible cells
3. Replicate within the cells
4. Spread to adjacent cells
5. Cause cellular injury
6. Engender a host immune response
7. Be either cleared from the body of the host, establish a persistent infection, or kill the host
8. Be shed back into the environment

1. Entry, Contact, and Primary Replication

  1. The first step in the infectious process is the attachment and entrance of the virus into a susceptible host and the host’s cells.
  2. Entrance may be accomplished through one of the body surfaces (skin, respiratory system, gastrointestinal system, urogenital system, or the conjunctiva of the eye).
  3. Other viruses enter the host by needle sticks, blood transfusions and organ transplants, or by insect vectors (organisms that transmit the pathogen from one host to another).
  4. Some viruses replicate at the site of entry, cause disease at the same site (e.g., respiratory and gastrointestinal infections), and do not spread throughout the body.
  5. Others spread to sites distant from the point of entry and replicate at these sites.
  6. For example, the enteroviruses enter through the gastrointestinal tract but produce disease in the central nervous system.

2. Viral Spread and Cell Tropism

  1. Mechanisms of viral spread vary, but the most common routes are the bloodstream and lymphatic system.
  2. The presence of viruses in the blood is called viremia.
  3. In some instances, spread is by way of nerves (e.g., rabies virus, herpes simplex, and varicellazoster viruses.
  4. Viruses exhibit cell, tissue, and organ specificities. These specificities are called tropisms (Greek trope, turning).
  5. A tropism by a specific virus usually reflects the presence of specific cell surface receptors on the eucaryotic host cell for that virus. 

3. Cell Injury and Clinical Illness

  1. Destruction of the virus-infected cells in the target tissues and alterations in host physiology are responsible for the development of viral disease and clinical illness.
  2. Some tissue (e.g., intestinal epithelium) can rapidly regenerate after a viral attack and withstand extensive damage.
  3. Other tissues, such as nervous system tissues, are not able to regenerate and may never resume normal functioning after damage has occurred.
There are four generally accepted patterns of a viral infection:
  1. In lytic infections the virus multiplies and kills the host cell immediately and new virions are released.
  2. In persistent viral infections the virus lives in the host cell and releases small numbers of virions over a long period of time. This causes little damage to the host cell.
  3. In latent infections, the virus resides in the cell but produces no virionsSome viruses can transform the host cell into a cancer cell that becomes the focal point for a tumor.

4. Host Immune Response

  1. Both humoral and cellular components of the immune response are involved in the control of viral infections.

5. Recovery from Infection

  1. The host will either succumb or recover from a viral infection. 
  2. Recovery mechanisms involve nonspecific defense mechanisms and specific humoral and cellular immunity.
  3. The relative importance of each of these factors varies with the virus and the disease.

6. Virus Shedding

  1. The last step in the infectious process is shedding of the infectious virus back into the environment.
  2. This is necessary to maintain a source of viruses in a population of hosts.
  3. Shedding often occurs from the same body surface used for entry.
  4. During this period, an infected host is infectious and can spread the virus.
  5. In some viral infections, such as a rabies infection, humans are dead-end hosts because virus shedding does not occur.
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Pathogenesis of Viral Diseases

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