Chain of Infection and Disease Transmission

Chain of Infection

• The chain of infection is a perpetual cycle of 6 links that begins when an agent leaves its reservoir or host through a portal of exit and is conveyed by some mode of transmission to infect a susceptible host.
• The 6 links of the chain of infection are:
  • Presence of microbiological agent
  • Presence of reservoir
  • Portal of exit
  • Mode of transmission
  • Portal of entry (inlet)
  • Presence of susceptible host

Agent

• Microorganisms are responsible for disease production (viruses, bacteria, protozoa, parasites, fungi, etc.)
• Agent factors that affect disease transmission:
  • Infectivity
  • Pathogenicity
  • Virulence
  • Antigenicity

Infectivity

• The ability of an agent to invade and multiply (produce infection) in a susceptible host
• Measured by the ease and spread of infection
• Secondary Attack Rate (SAR) is used to measure infectivity: SAR = (Number of secondary cases / Number of susceptible individuals) x 100

Pathogenicity

• The ability of the organisms to produce specific clinical reaction after infection
• Refers to the proportion of infected persons who develop clinical disease
• Examples: High pathogenicity (Measles, Chickenpox), Low pathogenicity (Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS)
• Measured by: Ratio of clinical to sub-clinical cases

Virulence

• The ability of organisms to produce severe pathological reaction
• Refers to the proportion of persons with clinical disease who become severely ill or die
• Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses
• Measured by: Case fatality rate = (Total number of deaths from a disease / Total number of cases of that disease) x 100

Antigenicity (Immunogenicity)

• The ability of the organism to produce specific immunity (antibodies or antitoxin)
• Measured by: Second attack frequency

Reservoir of Infection

• The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies
• Types of reservoirs: Humans, animals, and the environment
• Two types of human reservoir exist: Cases (persons with symptomatic illness) and Carriers

Carrier

• A person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others
• Carriers are dangerous because they:
  • Do not show any clinical manifestation
  • Take no precautions
  • Are difficult to discover
  • Can remain infectious for a long time, leading to repeated introduction of the disease