7.1.1 - Transmission during epidemic Flashcards
(12 cards)
Epidemic
When there is a sudden increase the occurrence of a particular disease in a given population
Endemic level
The usual level of a disease found within a population. This level is steady, neither decreasing nor increasing in a population.
Pandemic
When epidemics of a particular disease are occurring across multiple countries or continents
Factors affecting disease transmission in a pandemic
Transmissibility of the pathogen
- The pathogen’s mode of transmission
- Timing of the contagious period
- Pathogen’s characteristics
Susceptibility of the host population
- Level of immunity
- Population density
- Level of preparedness for the disease
Pathogen’s mode of transmission
Factors affecting transmission
Transmissibility of the pathogen
Air
- E.g. COVID virus, measles virus, and influenza.
- Airborne pathogens are the most likely to cause epidemics, as they are difficult to intercept and can easily travel through the air via droplets or aerosols
Water
- E.g. cholera, giardiasis
- Can engender rampant epidemics through contamination of a shared water supply
- However, transmission can be curbed through sanitary measures such as rigorous water purification processes
Body fluids and blood
- E.g. ebola, HIV/AIDS
- Can cause epidemics, but less potently transmissible as they require close contact between an infected person and a non-infected person to spread
- Transmission can be slowed with precautionary measures e.g. isolation of infected individuals, use of PPE by medical staff, and use of condoms for sexually transmitted diseases
Vector-borne
- E.g. malaria, dengue fever
- Diseases transmitted by a vector tend to be limited to the vector’s geographical range
- Unlikely to develop into pandemics, although they may cause epidemics in regions where they are endemic
Timing of the contagious period
Factors affecting transmission
Transmissibility of the pathogen
- Diseases that can become contagious before the affected person realises they are infected can spready very rapidly (if it is infectious before symptoms appear/become significant), as the patient may be asymptomatic initially. E.g. COVID, chicken pox, measles
- Some diseases only become infectious later, after symptoms have appeared. This makes their spread easier to control as affected individuals can be effectively isolated. E.g. smallpox is infectious only after the appearance of symptoms, and hence was the first disease to be eliminated by an immunisation program
Characteristics of the pathogen
Factors affecting transmission
Transmissibility of the pathogen
- High transmissibility + enough virulence to seriously infect hosts = can spread quickly and widely, very dangerous. E.g. measles, COVID, influenza
- Low transmissibility or virulence = spread more slowly, less threatening
- Extremely high virulence = slower spread, because hosts are likley to die before spreading the infection.
- However, some pathogens can continue to spread after the death of the host, for example Ebola and cholera
DNA vs RNA viruses
- Viruses can replicae up to a trillion per day
- RNA viruses, like COVID, are able to evolve rapidly into new variants as they can bypass the host cell’s editing process, which correct many mutations and reduces the number of new variants
- RNA viruses can begin replicating immediately after they enter the cell’s cytoplasm, as they do not need to be first transcribed before their genetic code can be translated by ribosomes (unlike DNA viruses)
- High replication and mutation rates of viruese gives them significant ‘plasticity’, boosting their chances of crossing the species barrier and causing infections
Host population level of immunity
Factors affecting transmission
Host population susceptibility
- Historically 80% of the world’s worst pandemics have occurred in populations with minimal immunity to the disease, e.g. measles, plague, TB, and Ebola, smallpox
- Lack of immunity to the disease allows it to spread rapidly and uncontrollably
- Vaccines, exposure, and other factors can help build up immunity in a population over time
Population density
Factors affecting transmission
Host population susceptibility
- High density + high mobility → increased susceptibility, as there is increased contact between members of a population and hence a higher chance of contacting infected hosts
Lack of preparation
Factors affecting transmission
Host population susceptibility
- Disorganised response by authorities, such as lack of timely quarantine implementation, travel restrictions, shortage of essential equipment like masks and oxygen, effective antimicrobials, and delayed rollout of effective vaccines can make the host population more susceptible
Transmission case study: cholera pandemic
- Cholera is caused by the bacteria vibrio chloerae in contaminated food or water
- Symptoms include vomiting, muscle cramps, and diarrhoae
- Causes severe dehydration
Scope of the pandemic
- 1826-37
- Reached from India across western Asia, Europe, Britain, the Americas, and China and Japan
John Snow
- British physician
- Mapped areas that had been affected by cholera and interviewed families on their water sources
- Found that subjects exposed to unclean water sources were significantly more likely to contract cholera
- This theory caused political controversy, with some government officials rejecting his findings
- Findings finally accepted as effective when in 1866, households in cholera-affected areas of New York began following Snow’s theory and disinfected houses, which reduced infection
