Epidemiologic Mind : Infectious Diseases Epidemiology and Component Of The Infectious Process
Definition
Communicable disease (infectious disease) – is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment.
4.2 Components of the infectious process
The infectious process of a specific disease can be described by the following components, which constitute of the chain of disease transmission.
1. The Agent
2. Its reservoirs
3. Its portal of exits
4. Its mode of transmission
5. Its portals of entry
6. The human host
I. The Agents
The agents in the infectious process range from viral particles to complex multi-cellular organisms
II. Reservoirs
A reservoir is an organism or habitat, in which an infectious agent normally lives, transforms, develops and/or multiplies. Reservoirs for infectious agents may be humans, animals, plants or other inanimate objects.
Some diseases with human reservoirs are:
-Most bacterial and viral respiratory diseases HIV/AIDS/Sexually Transmitted Infections (STIs), measles, typhoid etc.
All infected humans, whether showing signs and symptoms of the disease or not, are potential sources of infection to others. A person who does not have apparent clinical disease, but is a potential source of infection to other people is called a Carrier. An example of carrier is a person infected with HIV. A person infected with HIV might not have the signs and symptoms but he/she is capable of transmitting the infection to others Some diseases are transmitted to human beings from animals.
These diseases are called zoonoses. Examples: Rabies, anthrax, etc.
III. Portal of Exit
Portal of exit is the way the infectious agent leaves the reservoir. Possible portals of exit include all body secretions and discharges: Mucus, saliva, tears, breast milk, vaginal and cervical discharges, excretions (feces and urine), blood, and tissues. For example feces is the portal of exit for the eggs of hook worm.
IV. Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts. Transmission may be direct or indirect.
1. Direct Transmission
1.1 Direct contact : Occurs when there is contact of skin, mucosa, or conjunctiva with infectious agents directly from person or vertebrate animal, via touching, kissing, biting, passage through
the birth canal, or during sexual intercourse. Example: HIV/AIDS/STIs, rabies
1.2 Direct Projection : is transmission by projection of saliva droplets during coughing, sneezing, singing, spitting or talking. Example: common cold
1.3 Transplacental: is t ransmission from mother to fetus through the placenta. Example: syphilis, HIV/AIDS
2. Indirect transmission
The following are the different types of indirect transmission.
2.1 Vehicle-borne : Transmission occurs through indirect contact with inanimate objects fomites: bed sheets, towels toys, or surgical instruments; as well as through contaminated food, water, IV fluids etc.
2.2 Vector-borne : The infectious agent is conveyed by an arthropod to a host. Vectors may be biological or mechanical.
Biological vector : A vector is called biological vector if the agent multiplies in the vector before transmission.
• Example: anopheles mosquito is a biological vector for malaria.
Mechanical vector : A vector is called mechanical vector if the agent is directly infective to other hosts, without having to go through a period of multiplication or development in the vector. The vector simply carries the agent by its body parts( leg, proboscis etc) to convey it to susceptible hosts.
Example: Flies are mechanical vectors for the transmission of trachoma.
2.3 Airborne: which may occur by dust or droplet nuclei (dried residue of aerosols)
Example: Tuberculosis. When pulmonary tuberculosis patients cough, they emit many aerosols which consists the agents of tuberculosis. When these aerosols dry droplet nuclei will be formed. These droplet nuclei will remain suspended in the air for some time. When another healthy susceptible individual breaths he/she will inhale the droplet nuclei and become infected with tuberculosis.
V. Portal of entry - is the site where an infectious agent enters a susceptible host.
Examples:
-Nasal mucosa is portal of entry for common cold
-Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI. Susceptible human host: The susceptible human host is the final link in the infectious process. Host susceptibility or resistance can be seen at the individual and at the community level.
Host resistance at the community (population) level is called herd immunity. Herd immunity can be defined as theresistance of a population to the introduction and spread of aninfectious agent, based on the immunity of a high proportion ofindividual members of the population, thereby lessening thelikelihood of a person with a disease coming into contact with susceptibles.
Example - If 90 % of the children are vaccinated for measles, the remaining 10 % of the children who are not vaccinated might not become infected with measles because most of the children (90 %) are vaccinated. That means transmission from infected person to other susceptible children will not be easier.
Thanks For Visiting ! Keep Your Healthy !
0 comments:
Post a Comment