Pre Lecture on Ebola (4)

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Introduction to Communicable Diseases

The main lecture is pre-recorded by Professor Camelini Lacugay, an expert in public health and epidemiology from the ANU.
This lecture introduces the terminology and background needed to understand communicable diseases.
Camellini is a leading expert on Ebola and has worked with organizations like WHO and Médecins Sans Frontières.

Lecture Overview

The session will cover key definitions related to communicable diseases, approaches to studying them, and relevant language.
Topics include epidemiology, the epidemiological triad, dynamics of disease transmission, and understanding human reservoirs, sources, modes of transmission, and susceptible hosts.
The lecture aims to provide a scientific understanding of communicable diseases, not just common-sense interpretations.

Key Definitions and Concepts

Focus is on theories of etiology, places of transmission, modes of transmission, and diagnosis.
Therapies will not be discussed in this specific lecture.

Definition of Epidemiology

Epidemiology is the study of the distribution and determinants of health-related states and events in populations, and the application of this study to control health problems.
It involves looking at health patterns and trends, considering biological, social, economic, and environmental determinants, and aiming to control health problems.

Epidemiological Triad

The epidemiological triad involves the relationship between the agent of the communicable disease, the host, and the environment.
Demographic, biological, and socioeconomic characteristics are considered, along with biological, physical, chemical, nutrient, mechanical, and social agents of transmission.
The environment includes physical, biological, and social factors.

Biological Model

The biological model includes the pathogen, the host, and the environment.
The overlap of these three elements results in disease.

Communicable vs. Infectious Disease

A communicable or infectious disease is an illness due to a specific infectious agent capable of being directly or indirectly transmitted.
Transmission can occur from human to human, animal to human, animal to animal, or from the environment to humans.

Importance of Studying Infectious Diseases

It is important to study changes in the patterns of infectious diseases over time and across different regions.
Understanding the baseline is crucial for identifying new infections or unusual clusters of symptoms.
Epidemiology helps in the discovery of new infections and in determining if chronic diseases have an infective origin.

Terminology

Infection

Infection involves the entry, development, and multiplication of an infectious agent in the body.
Infection doesn't always cause illness.
Types of infection include:
- Colonization (e.g., Staphylococcus aureus on skin)
- Subclinical or inapparent infection (e.g., polio)
- Latent infection (e.g., herpes simplex)
- Manifest or clinical infection (e.g., cellulitis, abscesses, systemic infections)

Contamination

Contamination involves infectious agents on body surfaces, clothes, or substances like water and food.

Infestation

Infestation is the lodgment, development, and reproduction of arthropods or insects on the body or in clothing.
It can also describe the invasion of the gut by parasitic worms.

Contagious Disease

Contagious diseases are transmitted through contact (e.g., scabies, trachoma, STDs, leprosy).

Vector of Infection

A vector is an insect or living carrier that transports an infectious agent (e.g., malaria transmitted by the Anopheles mosquito).

Reservoir of Infection

A reservoir is any person, animal, plant, soil, or substance in which an infectious agent normally lives and multiplies.
It is the natural habitat of the infectious agent.

Incidence vs. Prevalence

Incidence is the number of new cases in a given time period.
Prevalence is the total number of cases at a given time.
Prevalence $=$ Incidence $*$ Duration of Disease

Epidemic, Endemic, and Pandemic

Epidemic: The unusual occurrence of a disease in a community clearly in excess of expected occurrence.
Endemic: The constant presence of a disease in a particular geographic area.
Hyperendemic: Disease constantly present at high incidence and/or prevalence rate, affecting all age groups equally.
Holoendemic: High level of infection beginning early in life and affecting most of the child population.
Pandemic: An epidemic affecting a large proportion of the population over a wide geographic area.

Exotic Diseases

Exotic diseases are those imported into a country where they do not otherwise occur (e.g., rabies in the UK).

Sporadic Diseases

Sporadic diseases occur irregularly and haphazardly from time to time and generally infrequently.

Attack Rates and Case Definitions

Attack rate: The proportion of non-immune exposed individuals who become clinically ill.
Index or primary case: The initial person who introduces an infection into a population.
Secondary cases: Those who subsequently contract the infection from the primary case.

Zoonosis and One Health

Zoonosis: An infection transmissible under natural conditions from vertebrate animals to humans.
Epizootic: An outbreak or epidemic of disease in an animal population.
One Health: The concept of interconnectedness between human health, animal health, and the environment.

Nosocomial and Opportunistic Infections

Nosocomial: Hospital-acquired infection unrelated to the patient's primary condition.
Opportunistic: Infections caused by organisms that take the opportunity provided by a defect in host defense (e.g., AIDS patients).

Controlling Infectious Diseases

Termination of transmission involves extermination of the infectious agent through surveillance and containment.
Elimination is the eradication of a disease from a large geographic area.

Reproductive Rate of Infection

R factor: Potential for an infectious disease to spread.
Influential factors: probability of transmission, frequency of population contact, duration of infection, virulence of the organism, and population immune proportion.

Dynamics of Disease Transmission

Chain of infection: source or reservoir, modes of transmission, and susceptible host.

Types of Reservoirs

Human: Primary, index, secondary cases, clinical, subclinical, latent, temporary, chronic.
Animal: Transmissible (rabies, plague, bovine TB).
Non-living: Soil (tetanus, anthrax).

Modes of Transmission

Direct: Touching, droplet infection, contact with soil, inoculation, transplacental.
Indirect: Vehicle-borne, airborne, fomite-borne, unclean hands.

Susceptible Host

Portal of entry (cuts, wounds).
Virulence: Degree of pathogenicity expressed as the ratio of overt infection cases to total infections.
If death is the only criterion, fatality rate is used.

Key Metrics

Serial Interval: Gap between onset of primary and secondary cases.
Infectious Period: Length of time a person can transmit disease.
Incubation Period: Time from exposure to development of disease.
Latent Period: Period between exposure and actual onset.

Transmission Probability Ratio (TPR)

P00: Transmission probability from unvaccinated infective to unvaccinated susceptible.
P01: Transmission probability from vaccinated infective to unvaccinated susceptible.
P10: Transmission probability from unvaccinated infected to vaccinated susceptible.
P11: Transmission probability from vaccinated infective to vaccinated susceptible.
When you compare ratios of P10/P00, P01/P00, and P11/P00, you can estimate vaccine effects.

Instructions

Listen to Professor Camillini's lecture. Complete assigned readings. Submit the template this week.