L5 Epidemiology (from individual patients -> patterns in population)

The Language of Epidemiology

• Epidemiology

  • Study of the occurrence, distribution, and determinants of health and disease in a population

• Public health

  • The health of the population as a whole

• Identifying the nature of a disease and its transmission - major goal of epidemiology

  • A single case may be clinically important; a cluster of related cases can signal a population - level problem!

One Health Connection in Epidemiology

Developed vs Developing Countries

• Why is there a difference?

  • lack of infrastructure

• Is there influence of national income or not?

  • economically feasible countries could make vaccines, others could not

  • country might be doing great, lack attention to parts of population that is struggling (overcrowding)

The Language of Epidemiology - 1

• Surveillance: the observation, recognition, and reporting of diseases as they occur

• Trace the spread of disease to identify its origin and mode of transmission

• The incidence of a disease is the number of new cases of the disease in a given period of time

• The prevalence of a disease is the total number of new and existing cases in a population in a given time

Pattern and geographic extent of disease in populations

• Endemic- disease is constantly present in a population, at low incidences. Individuals that are infected with a pathogen that causes endemic disease are called reservoirs (human or non-human animals)

• Epidemic- disease occurs in a large number of people in a population at the same time; “outbreak”

• Pandemic- is widespread, global occurrence

The Language of Epidemiology - 2

• Disease outbreak = occurs when a number of cases of a disease are reported in a short period of time

• To cause a disease, a pathogen must replicate and grow inside a host

• A well-adapted pathogen lives in balance with its host

  • Chronic infections: Host and pathogen survive

• New pathogens sometimes emerge for which the host has no resistance

  • Acute infections: Pathogen can be lethal

• Carriers - Asymptomatic but infected individuals (a type of reservoir)

The Language of Epidemiology - 3

• Stages of disease

  • Infection: The organism invades and colonizes the host

  • Incubation period: the time between infection and onset of symptoms

  • Acute period: The disease is at its height

  • Decline period: Disease symptoms are subsiding

  • Covale

The Language of Epidemiology - 4

Coevolution of Host and Pathogen

• Virulence of the parasite in host-to-host transmission diminishes, and resistance of the host increases (e.g., myxoma virus introduced to control rabbits in Australia)

  • A host-to-host pathogen that kills its host before it can infect another host may become extinct

• If a pathogen does not rely on host-to-host transmission (i.e., vectors or environmental reservoirs), it may remain extremely virulent (e.g., E.coli in hospitals)

The Host Community

• Herd immunity- resistance of a group to infection due to immunity of a high proportion of the group

• If a high proportion of individuals are immune to an infection, the

Infectious Disease Transmission and Reservoirs

• Direct (host-to-host) transmission

  • Infected individual transmits a disease directly to a susceptible host without the assistance of an intermediary (e.g., fl

Infectious Disease Transmission and Reservoirs - 1

• Zoonosis is any disease that primarily infects animals but is occasionally transmitted to humans

  • Control of a zoonotic disease in the human population will not eliminate the disease. Ex. Anthrax

  • Some infectious diseases have complex life cycles involving an obligate transfer from a nonhuman host to humans followed by transfer back to the nonhuman host

    • E.g., Malarial parasite shuttling between humans and anopheles mosquitoes

Characteristics of Disease Epidemics

• Common-source epidemic- Onset of a given epidemic is indicated by a sharp rise in the number of cases reported daily over a brief interval… may arise from contamination of water or food

  • Ex. cholera

• Host-to-host epidemic- slow, progressive rise and a gradual decline

  • Takes time to be transmitted from one host to the next

  • Ex. COVID, influenza and chicken pox

Common vehicles: food, water and air

• Vehicle: A nonliving source of an infectious agent that is responsible for the infection of a group of people

  • Food safety laws and regulations lowered incidence of foodborne pathogens

  • Water purification reduced the incidence of waterborne illnesses

    • Ex. typhoid, cholera

  • Airborne pathogens are difficult to control

    • Ex. COVID pandemic

Public Health Approach to Control

• Controls directed against the reservoir (‘break the chain of infection”)

  • Animal reservoirs- immunization or culling

  • Human reservoirs- quarantine, immunization, and treatment

  • Vaccine: Used by the WHO to successfully eradicate smallpox in humans (only important reservoir)

Public Health Approach to Control

• Isolation

  • Separation of persons having active disease

• Quarantine

  • Restricts the movement of an individual who may have been exposed

• Surveillance

  • The observation, recognition, and reporting of diseases

  • Reportable diseases are the diseases under surveillance

• Pathogen eradication- removal of a pathogen from any reservoir

  • Smallpox: eradicated globally

  • Polio: nearly eradicated. In 2014, only 359 cases of polio were reported worldwide

  • Both diseases were eradicated by use of vaccines

• Question: What disease was successfully eradicated from animals globally? Rinderpest (cattle plague)

Emerging and Reemerging Diseases

Epidemiology assists in surveillance and eradication of human pathogens, but even after eradication, epidemiology is also important for preparedness planning if diseases emerge or re-emerge

• Emerging diseases- sudden increase in prevalence

  • Ex. COVID, Dengue, Zika, Lyme disease, E.coli

• Reemerging diseases- increase in prevalent after having been under control

  • Ex. Tuberculosis, gonorrhea, pertussis

• What are some “emergence/reemergence factors”?

  • Human behavior, such as failure of vaccine use and breakdown of public health measures

  • Microbial adaptation and change: RNA viruses mutate rapidly

    • Difficult to control with immunization

  • Catastrophic events: poor drinking water quality after hurricanes

  • Others

Examples of Pandemics: HIV/AIDS

• Human Immunodeficiency Virus (HIV)/Acquired immunodeficiency syndrome (AIDS)

  • HIV kills CD4 T cells

  • Detected in the US in 1981

  • >70 million infected worldwide with HIV

  • >25 million mortality from AIDS

  • Transmitted through sexual contact or blood

Examples of Pandemics: Cholera

• Cholera

  • Causes severe, water-loss diarrhea

  • Typically transmitted though ingestion of contaminated water containing bacteria, Vibrio cholerae

  • Largely restricted to developing countries

  • Endemic in Africa, Southeast Asia, and the Indian subcontinent, and Central and South America

Public Health Threats from Microbial Weapons

• Illegal under national/international law; US has no first use policy [Pres. Ford signed Biological Weapons Convention (BWC) in 1975]

• Biological warfare is the use of biological agents to induce damage

• Biological weapons are organisms or toxins that are:

  • Easy to produce and deliver

  • Safe for use by the offensive party

  • Able to incapacitate or kill individuals under attack in a consistent and reproducible manner

  • Spreads rapidly in the introduced population with devastating consequences; typically, unusual clusters of disease, based on unusual locations, disease severity, route of exposure, etc.

    • → Prompts epidemiologic investigation, even before intentional release is known!

Public Health Threats from Microbial Weapons - 1

• Many pathogenic bacteria or viruses have potential for weaponization

• Bacterial toxins, such as botulinum toxin from Clostridium botulinum, are also potential biological weapons