This refers to a logical sequence of factors or links of a chain that are essential to the development of the infectious agent and propagation of disease. The six factors involved in the chain of disease transmission are:
- Infectious agent (etiology or causative agent)
- Portal of exit
- Mode of transmission
- Portal of entry
- Susceptible host
a. Infectious agent: An organism that is capable of producing infection or infectious disease. On the basis of their size, etiological agents are generally classified into:
- Metazoa (multicellular organisms). (e.g. Helminths).
- Protozoa (Unicellular organisms) (e.g. Ameobae)
- Bacteria (e.g. Treponema pallidum, Mycobacterium tuberculosis, etc.)
- Fungus (e.g. Candida albicans)
- Virus (e.g. Chickenpox, polio, etc.)
b. Reservoir of infection: Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.
Types of reservoirs
1. Man: There are a number of important pathogens that are specifically adapted to man, such as: measles, smallpox, typhoid, meningococcal meningitis, gonorrhea and syphilis. The cycle of transmission is from human to human.
2. Animals: Some infective agents that affect man have their reservoir in animals. The term “zoonosis” is applied to disease transmission from animals to man under natural conditions. For example:
- Bovine tuberculosis - cow to man
- Brucellosis - Cows, pigs and goats to man
- Anthrax - Cattle, sheep, goats, horses to man
- Rabies - Dogs, foxes and other wild animals to man Man is not an essential part (usual reservoir) of the life cycle of the agent.
3. Non-living things as reservoir: Many of the agents are basically saprophytes living in soil and fully adapted to live freely in nature. Biologically, they are usually equipped to withstand marked environmental changes in temperature and humidity. E.g. Clostridium botulinum etiologic agent of Botulism Clostridium tetani etiologic agent of Tetanus Clostridium welchi etiologic agent of gas gangrene
c. Portal of exit (mode of escape from the reservoir): This is the site through which the agent escapes from the reservoir. Examples include:
- GIT: typhoid fever, bacillary dysentery, amoebic dysentery, cholera, ascariasis, etc.
- Respiratory: tuberculosis, common cold, etc.
- Skin and mucus membranes: Syphilis
d. Mode of transmission (mechanism of transmission of infection): Refers to the mechanisms by which an infectious agent is transferred from one person to another or from a reservoir to a new host. Transmission may be direct or indirect.
1. Direct transmission: Consists of essentially immediate transfer of infectious agents from an infected host or reservoir to an appropriate portal of entry. This could be:
a. Direct Vertical Such as: transplacental transmission of syphilis, HIV, etc.
b. Direct horizontal Direct touching, biting, kissing, sexual intercourse, droplet spread onto the conjunctiva or onto mucus membrane of eye, nose or mouth during sneezing coughing, spitting or talking; Usually limited to a distance of about one meter or less.
2. Indirect transmission
a. Vehicle-borne transmission: Indirect contact through contaminated inanimate objects (fomites) like:
- Bedding, toys, handkerchiefs, soiled clothes, cooking or eating utensils, surgical instruments.
- Contaminated food and water
- Biological products like blood, serum, plasma or IV-fluids or any substance serving as intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not multiply or develop in the vehicle before it is introduced into man.
b. Vector-borne transmission: Occurs when the infectious agent is conveyed by an arthropod (insect) to a susceptible host.
1. Mechanical transmission: The arthropod transports the agent by soiling its feet or proboscis, in which case multiplication of the agent in the vector does not occur. (e.g. common house fly.)
2. Biological transmission: This is when the agent multiplies in the arthropod before it is transmitted, such as the transmission of malaria by mosquito.
C. Air-borne transmission: Dissemination of microbial agent by air to a suitable portal of entry, usually the respiratory tract. Two types of particles are implicated in this kind of spread: dusts and droplet nuclei.
Dust: small infectious particles of widely varying size that may arise from soil, clothes, bedding or contaminated floors and be resuspended by air currents.
Droplet nuclei : Small residues resulting from evaporation of fluid (droplets emitted by an infected host). They usually remain suspended in the air for long periods of time.
e. Portal of entry: The site in which the infectious agent enters to the susceptible host. For example:
- Mucus membrane
- Respiratory tract
f. Susceptible host (host factors): A person or animal lacking sufficient resistance to a particular pathogenic agent to prevent disease if or when exposed. Occurrence of infection and its outcome are in part determined by host factors. The term “immunity” is used to describe the ability of the host to resist infection.
Resistance to infection is determined by non-specific and specific factors:
- Skin and mucus membrane
- Mucus, tears, gastric secretion
- Reflex responses such as coughing and sneezing.
Genetic-hemoglobin resistant to Plasmodium falciparum Naturally acquired or artificially induced immunity. Acquired immunity may be active or passive.
Active immunity- acquired following actual infection or immunization.
Passive immunity- pre-formed antibodies given to the host.