Chain of Infection and Disease Transmission

Study Notes

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

Description

Test your knowledge on the chain of infection and disease transmission, including the infectious disease process, types of reservoirs, carriers, zoonoses, and modes of transmission. This quiz is based on Lecture 4 of Medicine Year 2 Level 6 by Dr. Alaa Hassan.