Infectious Diseases Week 1

what is the chain of infection model

a step-by-step guide showing how germs spread from one person or place to another using six key links

what are the 6 links of the chain of infection model

1. Infectious agent,

2. Reservoir,

3. Portal of exit,

4. Means of transmission,

5. Portal of entry,

6. Susceptible host

How can the Chain of Infection be broken

by infection control measures implemented by health care workers

what is an infectious agent

Causative agent of infectious disease

what are examples of infectious agents

bacteria, viruses, fungi, parasites

How are infectious agents classified

By structure, size, metabolism, and whether they are single-cellular or multi-cellular

what is a reservoir

where a pathogen exists

Any person, animal, arthropod, plant, soil, or substance (or combination of these) in which an infectious agent normally lives and multiplies

What is the purpose of a reservoir

ensures survival of the pathogen and allows pathogens to reproduce before being transmitted to a susceptible host

What is the goal of pathogen eradication

To remove all of a pathogen from any reservoir

How many cases of polio were reported worldwide in 2014

Only 359 cases

What are animate reservoirs

People, insects, birds, and other animals

What are inanimate reservoirs

Soil, water, food, faeces, intravenous fluid, and equipment

what are 2 examples of coronaviruses

Middle East Respiratory Syndrome (MERS)

Severe Acute Respiratory Syndrome (SARS)

what is the main reservoir for MERS

bats, camels

what is the main reservoir for SARS

not 100% identified but probably civets, bats

What is the main reservoir for influenza viruses

Birds (domestic and wild)

Where do influenza viruses usually infect and replicate in birds

In the lung or gut of birds

What other susceptible hosts can influenza be transmitted to

Pigs, humans, horses, etc

what is A portal of exit

the area of the body through which a pathogen can be transmitted to others or to the environment

Can pathogens use the same portal for entry and exit

yes

what are the 4 main portals of exit in humans

Alimentary canal

Genitourinary system

Respiratory system

Skin

what are examples of the alimentary canal as a portal of exit

vomiting, diarrhea, saliva

what are examples of the Genitourinary system as a portal of exit

bodily fluids, sexual contact

what are examples of the Respiratory system as a portal of exit

secretions from coughing, sneezing, or talking

what are examples of the skin as a portal of exit

open wounds

What are zoonoses

Pathogens transmitted from vertebrate animals to humans

What obstacle do zoonotic pathogens face

Evading immune defences within the reservoir or intermediate host to survive and proliferate

What happens once a zoonotic pathogen crosses the species barrier by infecting humans

Its success in the human population will depend on whether it can acquire the ability of sustained human-to-human transmissibility

What are anthroponoses

Pathogens that have acquired the ability of sustained human-to-human transmission

what are 6 Factors influencing successful human-to-human transmission:

Quantity of organisms present in host ●Ie., viral load (for example, detectable levels of HIV)

Suitability of the environment to enable transmission ●e.g., densely vs. sparsely populated areas

Pre-existing immunity in a community ●e.g., vaccination levels, prior immunity from previous exposure

Immune evasion

Socioeconomic and educational factors ●e.g., hygiene, knowledge about spread of infections

Sexual behaviour

What are the 7 common ways germs are spread

Direct or indirect contact

Airborne

Fomites

Food

Water

Bodily fluids

Vectors

What is a portal of entry

the sites through which pathogens enter the body

What does infection refer to

The movement of microbes into hosts

What are the four types of portals of entry

Body openings/mucous membranes

Skin

Placenta or via breast feeding

Parenteral route

What are mucous membranes

Tissues that line the body cavities that are open to the environment

What type of environment do mucous membranes provide for pathogens

A moist, warm environment that is hospitable to pathogens

Where are mucous membranes found

In the respiratory, gastrointestinal, and genitourinary tracts, as well as the conjunctiva of the eye

How do respiratory pathogens enter the body

Through mucous membranes of the respiratory tract or via the nasolacrimal ducts when a person rubs their eyes

How are respiratory pathogens transmitted

By aerosol droplets released during coughing and sneezing

What are examples of respiratory pathogens

Influenza viruses

rhinoviruses

Ebola

S. pneumoniae

H. influenzae

L. pneumophila

Y. pestis

B. anthracis

How are gastrointestinal pathogens transmitted

Generally via faecal-contamination through vehicles such as food and water

What are examples of gastrointestinal pathogens

C. jejuni, S. typhimurium, E. coli, H. pylori, V. cholera, Shigella

What are examples of genitourinary pathogens

Treponema pallidum

Neisseria gonorrhoeae

Chlamydia trachomatis

papilloma viruses

Herpes simplex II, HIV

What are nosocomial pathogens

Pathogens often responsible for hospital-acquired infections, particularly urinary tract infections

What are examples of nosocomial pathogens

Proteus mirabilis

P. vulgaris

E. coli, and S. epidermidis (normally non-pathogenic)

What is the skin's usual function as a portal of entry

The outer layer of packed, dead skin cells usually acts as a barrier to pathogens

How can some pathogens enter through the skin

Through openings or cuts, or by burrowing into or digesting the outer layers of skin

What is the parenteral route

Not a true portal of entry but a means by which the portal of entry can be circumvented – involves a break in the skin such as cuts, scrapes, puncture wounds, bites, and burns

Where are pathogens deposited via the parenteral route

Directly into tissues beneath the skin or mucous membrane

What viral pathogen enters via the parenteral route

Rabies virus

What bacterial pathogens enter via the parenteral route

S. aureus, C. perfringens, P. aeruginosa

What insect vector-transmitted pathogens enter via the parenteral route

Plasmodium, Leishmania, Trypanosoma

what are some known pathogens that cross the placenta

toxoplasma gondii (toxoplasmosis)

cytomegalovirus

what are effects of pathogens that cross the placenta

abortion

mental retardation

microcephaly

What is a susceptible host

The person whose resistance to the causative agent is lacking or compromised

What are the factors that can make someone more susceptible to infection

HIV, AIDS, primary immune deficiencies

Chemotherapy, as a result of cancer

Surgery and medications e.g. after organ transplants

Age

Family medical history

Immunisation status

Lifestyle

Sex

Etc.

What are the methods for breaking the chain of infection

Elimination of sources of infection (reservoirs)

Appropriate handling and disposal of body secretions ● E.g., Vomitus, faeces, sputum, blood, etc.

Appropriate handling of contaminated items,

Segregation of waste categories and disposal

Hand washing

Wearing PPE (E.g., masks, goggles, etc.)

Discarding needles/sharps appropriately

Washing linen

Social distancing

Quarantine

what is incidence

Measures the number of new cases of a specific disease that develop in a population in a specified time period

what is prevalence

Measures the proportion of a population who have a specific disease in a given time period

Regardless of when the disease first developed

what is public health

refers to the health of the population as a whole

What is a carrier

A pathogen-infected individual showing no signs of clinical disease, who are potential sources of infections

What stage of disease might carriers be in

The incubation period of the disease

How can carriers be identified

Using diagnostic techniques, including culture and immunoassays

What is an example of a carrier

typhoid mary

what are examples of Public health controls directed against common vehicles

Food laws lowered incidence of foodborne pathogens

Water purification reduced incidence of typhoid fever

what pathogens are still difficult to control

Airborne pathogens are difficult to control

what are examples of Public health controls directed against animal reservoirs

Mad cow disease UK mid 1990s – Cattle culled

Bird flu outbreak 2024 – Over 1 million poultry and wild birds culled

Varroa destructor mite outbreak 2024 – Bees destroyed

What are public health controls directed against human reservoirs

Isolation

quarantine

surveillance

immunisation

What is isolation

Limiting contact of infected individuals (e.g., 1.5 m social distancing, working from home)

What is quarantine

Restricting movement of actively infected individuals

What is surveillance in public health

The observation, recognition, and reporting of diseases (e.g., contact tracing)

What is an example of immunisation as a public health control

Smallpox, measles, mumps, rubella, tetanus

What is surveillance in epidemiology

The observation, recognition, and reporting of diseases as they occur

What is the goal of epidemiologic research

To search for causal factors that influence one's risk of disease, to identify a cause so that appropriate action can be taken

Where does surveillance information flow from

From public and healthcare providers (clinicians, laboratories, hospitals, health departments)

Where does surveillance information flow to

To public and healthcare providers

How do infectious disease death rates compare between developed and developing countries

Infectious diseases cause fewer deaths than non-infectious diseases in developed countries

in developing countries, infectious diseases account for almost half of all deaths

What is an epidemic

A disease outbreak that occurs within a specific geographical area (e.g., HIV/AIDS)

What is a pandemic

A widespread occurrence of an infectious disease across the world at a particular time (e.g., COVID-19)

What is an endemic disease

A disease outbreak that is consistently present but limited to a particular region (e.g., Malaria)

What does epidemiology study

The occurrence, distribution, and determinants of health and disease in a population using a systematic approach

What questions does an epidemiologist attempt to determine

What caused the disease?

Who is affected and at what rates?

Where is the disease occurring?

When is the disease occurring?

Why/How is the disease occurring?

What are the two main approaches that epidemiologists employ

descriptive epidemiology

analytical epidemiology

what are the reasons for compiling and analysing data

Identifying information

Demographic information

Clinical information

Risk factor information

what does clinical info allow

Allows creation of an epidemic curve, description of illness spectrum

what does risk factor info do

Helps tailor investigation to the specific disease in question

What does compiling data help epidemiologists do

provides important clues to the causes of the disease, and these clues can be turned into testable hypotheses

Before counting cases, what must epidemiologists decide

What to count – i.e., what to call a case

What is a case definition

A set of standard criteria to classify whether a person has a particular disease, syndrome, or other health condition, including person, place, time, and clinical features

What are the three classifications of cases based on the case definition

1. Confirmed (usually has laboratory verification),

2. Probable (usually has clinical features without lab verification),

3. Suspected (usually has fewer typical clinical features)

Can case definitions change

Yes. Case definitions can change as more information is obtained

What was the first case definition for SARS based on

Clinical symptoms and either contact with a case or travel to an area with SARS transmission, published by CDC on March 21, 2003

What was the updated SARS case definition

After a novel SARS-CoV was determined to be the causative agent, an interim surveillance case definition included laboratory criteria for evidence of infection with the SARS-associated coronavirus

What does analytic epidemiology search for

Causes and effects (why and how) a specific public health problem exists

what does analytical epidemioligy test

hypotheses of causal relationships by quantifying the association between exposures and outcomes

What is the key feature of analytic epidemiology

A comparison group

What are the three main types of analytical epidemiology studies

Cohort study

Case control study

Cross sectional study

What is a cohort study

Longitudinal tracking of a group of people over time

What is a case control study

Retrospective study that compares individuals with and without a condition

What is a cross sectional study

Observational study that analyses data at a single time-point to assess relationships