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Introduction

Routes of Infection - B. Catchpole, RVC 2008

Infection is a primary cause of disease. Four pathways of infection are identified: bacterial, viral, fungal, and parasitic, comprising protozoa and worms (helminths). In all cases, microorganisms can display similar characteristics. They can manifest as primary (obligate) pathogens or secondary (opportunistic) pathogens, the former of which always corresponds to disease, and the latter of which depends on prerequisites being fulfilled before causing disease. As such, secondary pathogens can be found in healthy animals, but they lie in wait until a primary cause compromises the immune system and then manifest symptoms. Secondary pathogens can even masquerade as commensal organisms, or those which operate in harmony with the animal, until they become pathogenic.

Microorganisms can enter the body by three routes: contact (typically dermatological infection), aerosol (typically respiratory infection), or orofecal (typically enteric infection). Infection can be either exogenous (via entry to epithelial surfaces) or endogenous (intracellular, extracellular, or vesicular).

Viruses

Virus Structure - B. Catchpole, RVC 2008

Structure

Viruses are very simple structures that do not carry any cellular machinery of their own. They depend on host cells to replicate and flourish. As such, their structure is streamlined, containing only:

  • Genetic material, which consists solely of viral RNA
  • Capsid proteins surrounding the RNA
  • A lipid envelope
  • Envelope glycoproteins (or spike proteins)

All of these elements can be recognized by the host as foreign material, and will provoke an immune response. Viruses have an advantage if they are able to penetrate host cell walls in that they are then capable of masquerading as host cells. The immune system must then counter by killing off host cells it recognizes as infected.


Replication

Viral Replication - B. Catchpole, RVC 2008

Viral lifespans have three distinct phases:

  • Infectious (extracellular): as they seek out a host target
  • Replicative (intracellular): as they overtake the host cell's utilities to multiply
  • Dissemination (extracellular): as they leave the host cell and migrate to those surrounding


Prevention and control

B Cell Immunity to Viruses - B. Catchpole, RVC 2008
T Cell Immunity to Viruses - B. Catchpole, RVC 2008
  • Viral evasion of immunity
    • Latency: Viruses can "hide" in host cells until the immune system is suppressed
    • Transformation: the virus incorporates into the host genome, activating an oncogene, which causes cells to become neoplastic
  • Adaptive Immunity to Viruses
    • The adaptive immune system has two main ways of dealing with viral infection: the first targets viruses in its initial extracellular phase, the second targets infected host cells:
      • B-lymphocytes are capable of producing Antibody to neutralize the spike proteins of the viral lipid envelope, preventing the virus from attaching to host cells
      • Cytotoxic (CD8+) T cells target infected cells, which present any number of danger signals


Antibiotics cannot treat viral infection. While anti-viral therapies do exist, they are costly and often ineffective. The alternatives, therefore, include vaccination (priming the immune response to create memory lymphocytes for a particular virus) or eradication.

Bacteria

Structure

File:Bacterial structure.svg
Bacteria Structure - Wikimedia Commons 2008

Bacteria offer the immune system a greater range of foreign components with which to be recognized, including:

  • Flagellum
  • Pili
  • Capsule
  • Cellular components: membrane, cell sap containing enzymes, genetic material, cell wall

Replication

The lifespan of bacteria differs from that of viruses in that it includes a colonization phase

Prevention and control

Parasites

References

  • Janeway, C.A., Travers, P., Walport, M. and Shlomchik, M.J. (2005) Immunobiology: The immune system in health and disease. 6th ed. New York: Garland Science Publishing.