Key Knowledge:
|
Epidemiology is the study of the distribution and determinants of diseases, as well as applications for controlling their spread
- Controlling disease spread requires the identification of the pathogen and an understanding of the mode of transmission
Pathogen Identification
Infectious pathogens can be identified in host organisms using a variety of biotechnological techniques
- Screening tests identify whether asymptomatic individuals (not showing symptoms) are at a high risk of being infectious
- Diagnostic tests provide a conclusive and definitive determination of the presence or absence of a disease in an individual
PCR Analysis
- A diagnostic technique that involves the use of fluorescent primers that are specific to viral sequences
- Amplification of the fluorescently tagged viral sequences (in a saliva sample) indicates infection in the host
Blood Cultures
- A diagnostic technique that involves growing and then identifying cultures of a pathogenic microbe from a blood sample
- Blood tests can also identify the presence of specific antibodies, but this is a screening technique (may be a past infection)
Western Blotting*
- A screening technique in which gel electrophoresis is used to isolate antigens that are specific to a given pathogen
- The isolated antigens are transferred to a membrane that is then stained with antibodies to allow for identification
ELISA*
- Enzyme-linked immunosorbent assays use ‘capture' antibodies that are fixed to a plate in order to trap specific antigens
- Following this, ‘detection' antibodies (linked to colour changing enzymes) are added to provide positive identification
* Tests that do not involve the amplification of the pathogen are typically used for screening as low pathogen levels will limit accuracy
Modes of Transmission
Transmission of infectious pathogens can occur via either direct or indirect mechanisms
- Direct transmission: Via physical contact (e.g. exchange of body fluids via kissing, transfusions, etc.) or droplet spread
- Indirect transmission: Via airborne transfer, non-living vehicles (e.g. food, faeces, fomites) or vectors (living organisms)
Animals can be important agents in the transmission of infectious pathogens either to, or between, human hosts
- Pathogens can be transferred to humans from infectious animals or shuttled between humans by unaffected animal carriers
Zoonotic Diseases
Diseases which are transferred from an animal host (usually a vertebrate) to a human host are called zoonotic diseases
- In this context, animals can act as a reservoir for the pathogen, allowing it to survive even if it is eradicated in humans
- This may facilitate the re-emergence of an infectious disease in a population where it had previously been eliminated
A specific example of a zoonotic disease is rabies, which is a viral disease that is transferred from dogs and bats (through bites)
- Many strains of bird flu or swine flu are also zoonoses that recombine with human strains (antigenic shift) to cause pandemics
Vector-Borne Diseases
Diseases which are transferred between human hosts by animal carriers (usually invertebrates) are called vector-borne diseases
- Mechanical vectors are animals that transmit pathogens externally and so remain uninfected (e.g. flies spreading germs)
- Biological vectors are infected animals in which the pathogen undergoes a part of its life cycle (e.g. ticks and Lyme disease)
A specific example of a vector-borne disease is malaria, an acute febrile disease that can be fatal in humans
- The parasite (Plasmodium falciparum) undergoes part of its reproductive cycle (juvenile form) inside the Anopheles mosquito
- The parasite is then transferred through bites to a human host, where it develops into an adult form that causes disease
Controlling Disease Spread
Scientific and social strategies may be employed to control the spread and distribution of a pathogenic infection
- Scientific strategies are interventions that act to reduce or stop the transmission and infection of a particular pathogen
- Social strategies are the policies put in place to support the scientific strategies and ensure their effective implementation
Scientific Strategies
Scientific strategies may vary according to the infectivity of the particular pathogen and its mode of transmission
- Vaccinations may be used to immunise the population and establish herd immunity
- Specific medications (e.g. antibiotics) may be developed via a process of rational drug design
- Contact tracing and health related databases may be used for monitoring and predicting disease spread
- Infectious individuals may be isolated through targeted protocols (such as quarantine or lockdowns)
- Pathogens may be eliminated via sterilisation procedures (e.g. water chlorination, fumigation)
- Disease-resistant vectors may be introduced via genetic engineering (e.g. sterile mosquitoes)
Social Strategies
Social strategies may commonly involve government legislations designed to support scientific interventions
- Advertisement campaigns designed to inform the public of associated risks (to help minimise exposure)
- Education programmes on appropriate hygiene practices (e.g. social distancing, correct use of insecticides)
- Providing adequate funding for scientific interventions (such as vaccination schedules, prophylactic equipment)
- Passing laws and mandates to ensure public compliance (e.g. customs act or vaccination mandate)
