Pathogens

What is a pathogen? Examples?

Anything that can cause a disease

• Bacteria

• Viruses

• Fungi

• Protozoa

• Prions

• Parasitic invertebrates

What is an disease? Infectious vs Non Infectious?

A disease is any condition that impairs the normal functioning of the body

Infectious disease-

• Caused by a pathogen which can move between hosts

Non-infectious disease-

• Not caused by pathogens therefore cannot be transferred between people E.g. genetic

Infectious disease main components?

Host (animal/plant) in or on which another organism parasitically lives

Parasite (Organism) that lives on or in another organism and causes it harm

• Interference with normal body functions

• Deprive host of vital nutrients

Virulence — the ability of a pathogen to cause disease

• A measure of severity of disease

• Virulent = extremely noxious, damaging, etc. Rapid and severe

Infectious diseases— how are they spread?

Infection must occur (the invasion of a host by a pathogen)

Can occur via:

• Water droplets/spores in air

• Water & food

• Bodily fluids

• Other animals

Types of diseases?

Inert diseases — can survive for years without infecting hose (tetanus and anthrax)

Viruses — which need a host to survive & reproduce

Vector — moves from one host to another using another organism

What is an incubation period? & Why do pathogens need one

Time between infection & onset of symptoms

• Pathogen needs time to divide many times to reach sufficient numbers to cause disease

• Pathogen may need time to reach a particular organ/place in host

• Pathogen may produce toxins (which can take time)

What are symptoms? How do symptoms help with diagnosis

Symptoms are effects the pathogen has on the body of the hose.

• high temperature and rash develop in the measles

Diseases usually have characteristic symptoms that help doctors to diagnose the cause of the disease without isolating the pathogen

Disease recovery = when symptoms disappear

Terminology recap. Disease, Infectious disease, Infection, invasiveness, virulence & communicability

What is Koch’s Postulates? (1884-1890)

Koch developed four criteria to demonstrate that a specific disease is caused by a particular agent.

1. The specific agent must be associated with every case of the disease.

2. The agent must be isolated from a diseased host and grown in culture.

3. When the culture-grown agent is introduced into a healthy susceptible host, the agent must cause the same disease.

4. The same agent must again be isolated from the infected experimental host.

Phases of infectious disease?

How to classify infectious diseases?

+ By geography

• Endemic/Enzootic - The constant presence of a disease or infectious agent within a given geographic area.

• Epidemic/Epizootic - The occurrence in an area of a disease or illness in excess of what may be expected on the basis of past experience for a given population (in the case of a new disease, such as AIDS, any occurrence may be considered "epidemic").

• Pandemic/Panzootic - A worldwide epidemic affecting an exceptionally high proportion of the global population.

Give an example of an infectious disease (Influenza)

How to transmit infectious diseases?

Direct physical contact

• skin contact

• bodily fluids

Indirect transmission

• Formites (clothing & other infected objects)

• Dust

• Air

• Water droplets

• Food/water

• Fecal matter

What is a reproductive rate?

Formula for spread of infectious disease? & Assumptions?

Assumptions

• Population is fixed (no entries/births or departures/deaths)

• Latent period is zero

• Infectious period = disease duration

• After recovery, individuals are immune

People can be in one of three states

• Susceptible to the infection (S)

• Infected and infectious (I)

• Recovered/immune (R*)

What is the chain of infection?

In order for disease to occur and spread from one individual to another, certain conditions must be met.

If any one condition is not met, the transmission of the disease will not happen.

Explanation of chain of infection elements?

Infectious Agent – pathogen such as a bacteria or virus.

Reservoir – a place the pathogen can live.

Examples: human body, animals, the environment, fomites.

Fomites are objects contaminated with infectious material that contains pathogens.

Ex: doorknobs, bedpans, linens, instruments.

Portal of Exit – way to escape from the reservoir in which it has been growing.

• Urine, Feces & Semen

• Saliva, Mucous discharge (snot) & Tears

• Respiratory & Gastrointestinal tract

• Skin & Blood

Mode of Transmission – way in which it can be transmitted to another reservoir or host where it can live.

• Can be through direct contact or airborne droplet.

• Contaminated hands

Portal of Entry – way to enter the new reservoir or host.

• Respiratory tract, mucous membranes, and gastrointestinal tract are common.

• Damaged skin.

Susceptible Host – one that is capable of being infected.

• Microorganisms must be present in large enough quantity to be virulent.

• The host must be susceptible.

• Individuals with an immunity to certain pathogens will not be susceptible.

What is a infectious disease case?

Case = person diagnosed as having a disease disorder injury or condition

Primary case = The first disease case in the population

Index case = the first disease case brought to attention of the epidemiologist

Secondary case = persons infected once a disease has been introduced to a population

Levels of diagnosis for cases

Suspect → Probable → Confirmed

Types of carriers & short definition?

Active carrier: Exposed/Harboring pathogen

Convalescent carrier: Harbor pathogen & In Recovery (Infectious)

Healthy (or Passive) carrier: Exposed to and harboring pathogen (Not ill/Not showing any symptoms)

Incubatory carrier: Exposed to and harbor pathogen, initial symptoms, ability to transmit a disease.

Intermittent carrier: Exposed to and habor pathogen, can spread disease.

Reasons why a disease can emerge/re-emerge

• Appearance of a previously unknown agent

• Evolution of a new infectious agent

• Spread of an infectious agent to a new host

• Spread of an infectious agent to new locations

• Acquisition of resistance to anti-microbial drugs

• Deliberate introduction to population

How to reduce spread of infectious disease?

• Vaccines (immunisation)

• Antimicrobial drugs

• Good personal hygiene/sanitation (education)

• Protection against mosquitoes

• Quarantine

Herd immunity

Based on notion that if a population or group is mostly protected from a disease by immunisations (> 85%), then chance of a major epidemic occurring is limited.

Herd immunity provides barrier to direct transmission of infections through population.

Lack of susceptible persons stops spread of a disease through throughout group.