Module 7: IQ 1-2 (HSC:3)

Infectious disease

A disease that is caused by a pathogen or infectious agent and can be transmitted from one organism to another

non-infectious disease

A disease that is NOT caused by a pathogen and is unable to be passed from one organism to another

Pathogen

An infectious biological agent that is capable of causing a disease or illness in a host organism. They live and reproduce at the expense of the host and release toxins, damage tissue or outcompete other microflora for nutrients.

3 groups of pathogens

microorganisms, macroorganisms, non-cellular

Non cellular pathogens

viruses and prions

Prions description

Prions do not contain any genetic material (DNA or RNA). They are misfolded proteins measuring in the nanometre range. Normal host proteins responsible for producing neurone synapses are specified by the Prnp gene. If an organism inherits a mutated Prnp gene, an abnormal protein is produced

Prions: Structure

Composed of amino acids and polypeptides. The 3D structure is different to a normal protein.

Prions: mechanism of infection

Abnormal prions induce misfolding in normal proteins. They activate enzymes that convert into long fibre structures (prions). These are resistant to high temperatures, high pressure, digestion by lysosomes, UV radiation and toxic chemicals

Prions: impact on host

Mostly attacks brain or nerve cells. The degradation of neurone synapses leads to cell death in the brain. As naturally occurring cells remove these dead nerve cells, holes are left in the brain, resulting in Transmissible Spongiform Encephalopathy (TSE).

Prions: example

Creutzfeldt-Jakob disease (CJD)

Prions: transmission

Direct: Eating beef containing the infection In-direct: Contaminated surgical instruments or transplanted organs Inheritance: Mutated Prnp gene

Prions: treatment

none

Virus: description and reproduction

They are non-living pathogens but possess both living and nonliving features. 30-300nm. They cannot reproduce independently. They must use the host's organelles and reproductive mechanisms. This often results in cell lysis (bursting), which spreads the virus to more cells.

Virus: Structure

Consists of a protein capsule (capsid) containing genetic information in the form of DNA or RNA. Surface antigens (proteins) allow the virus to bind to the complementary receptor proteins on a host cell.

Virus: mechanism of infection

The virus enters the cell and inserts its genetic material into the host's genome. It redirects the host's protein synthesis to produce more capsids and replicate its genetic material.

Virus: examples

Influenza A, AIDS, Covid 19, Hepatitis B, Measles and Rabies

Virus: transmission

Airborne droplets, contact with infected surfaces, contaminated food/water or vectors.

Virus: Treatment

Prevention: vaccines Treatment: antivirals

Cellular pathogens

bacteria, protozoa, fungi, macroparasites

Bacteria: description

They are unicellular, microscopic prokaryotic organisms that lack membrane-bound organelles and a nucleus. They measure in the micrometre range.

Bacteria: strucure

Cell wall and cell membrane, sometimes have a capsule or flagellum. Genetic material in plasmids.

Bacteria: classification / reproduction

Often classified by shape: spherical, rod, spiral, comma or oval. Multiply rapidly by binary fission.

Bacteria: mechanism of infection

They steal nutrients and secrete toxins that harm host cells and tissues

Bacteria: examples

Tuberculosis, Cholera, Salmonella, Tetanus, leprosy

Bacteria: transmission

Direct contact, airborne water droplets, contaminated food or water

Bacteria: treatment

Antibiotics (destroys cell wall and disrupts metabolism)

Protozoa: description

They are unicellular, eukaryotic organisms that have cell membranes and organelles but no cell wall.

Protozoa: structure

Measured in micrometres. Heterotrophic (cannot photosynthesize). Mobile in water.

Protozoa: reproduction

Binary fission and sexual reproduction (higher diversity)

Protozoa: mechanism of infection

They may attach to the intestine to prevent nutrient absorption or invade blood cells

Protozoa: examples

Giardiasis, malaria, cryptosporidiosis

Protozoa: transmission

Frequently via vector transmission or contaminated water

Fungi: description

They are eukaryotic organisms that can be unicellular or multicellular. micrometre to millimeter in size. They are saprophytes or parasites

Fungi: structure

Possess membrane bound organelles, a nucleus and a rigid cell wall made of chitin. They are not mobile.

Fungi: reproduction

Spores, relies on wind for transportation and germinate in cool, moist environments. Can also sexually reproduce.

Fungi: mechanism of infection

They survive on skin surfaces and release mycotoxins to cause disease, often forming colonies.

Fungi: examples

athletes foot, thrush, plant rust

Fungi: transmission

Direct contact with an infected person or animal or indirect contact with moist shared surfaces

Macroparasites: description

They are multicellular eukaryotic pathogens visible to the naked eye

Macroparasites: structure

Possess specialised tissue but no cell wall.

Macroparasites: types and reproduction

Endoparasites: Live inside the host's body to obtain food. Ectoparasites: Live outside the host's body, consuming blood and injecting neurotoxins. Reproduce sexually

Macroparasites: mechanism of infection

Cause direct tissue damage or act as vectors that carry other microscopic pathogens to healthy hosts, potentially causing secondary infections.

Macroparasites: examples

tapeworm infestation, ticks, fleas, lice

Macroparasites: transmission

Faeces-oral transmission, direct contact or environmental exposure

What conditions do parasites require to survive?

1. Optimal temperature (usually ~37° or slightly colder)
2. High moisture and humidity
3. Optimal pH (usually neutral, ~7)
4. Available nutrients (from the host)
5. Optimal oxygen concentration (aerobic or anaerobic)
6. Access to a susceptible host (entry/exit, weakened immunity)

Direct contact transmission

This occurs when there is physical contact between an infected person and a susceptible host. This involves the physical transfer of pathogens through an immediate connection between infected and host. Eg. physical touch, sexual contact, droplet spread, bodily fluid exchange

indirect contact transmission

Refers to contactless transmission where a pathogen is transferred via an intermediary. It utilises an intermediate mode of transmission. Eg. airborne, fomites, contaminated food/water, animal to person

Vector transmission

Requires a third living organism to carry the pathogen from one host to another. This organism does not typically have the disease but is able to pass it on. Usually spread by blood sucking insects, including mosquitoes, ticks and fleas. Eg. biological or mechanical

What was the predominant theory of disease transmission before germ theory?

Miasma theory

What did miasma theory propose about disease transmission?

Diseases were transmitted by a poisonous vapour containing suspended particles of miasma.

In what decade was germ theory developed?

In the late 1860s.

Who focused on the causes of contamination in germ theory?

Louis Pasteur

Who focused on the transmission and identification of specific pathogens?

Robert Koch

What are Koch's postulates?

A 4-step procedure to verify if a specific pathogen causes a specific infectious disease.

What is the first step of Koch's postulates?

Association: The suspected pathogen must be in all diseased organisms but absent in healthy ones.

What is the second step of Koch's postulates?

Isolation: The pathogen must be isolated from the affected host and grown in a pure culture.

What is the third step of Koch's postulates?

Inoculation: The isolated pathogen must cause the same disease when introduced into a healthy organism.

What is the fourth step of Koch's postulates?

Re-isolation: The pathogen must be re-isolated from the newly infected host and shown to be the same as the original.

What was Pasteur's primary role in microbiology?

Disproving spontaneous generation and demonstrating that microorganisms arise from pre-existing organisms.

What was the purpose of Pasteur's Swan-Necked Flask Experiment?

To prove that air carries microbes and that they do not arise spontaneously.

What was the observation in Pasteur's Swan-Necked Flask Experiment?

The broth remained sterile for weeks until the swan-neck was removed.

What did Pasteur discover about fermentation?

Microbes were responsible for the fermentation of wine and beer.

What term did Pasteur coin for bacteria that operate without oxygen?

Anaerobic bacteria.

What is pasteurization?

Heating liquids to 60-80°C for 1-2 hours to kill most microbes without destroying the product's flavor.

How did Pasteur contribute to the silk industry?

He identified that the disease pebrine was hereditary and spread through spores in diseased moth eggs.

What principle did Pasteur develop after investigating anthrax?

The Principle of Immunity, proving that injecting a weakened or dead strand of a pathogen could provide resistance.

What was the outcome of Pasteur's public trial with vaccinated sheep?

25 vaccinated sheep survived a lethal dose of anthrax, while 25 unvaccinated sheep died.

Define 'endemic'

The usual number of people affected by a particular disease within a population in a given area, which remains constant.

Define 'outbreak'.

A sudden increase in the occurrence of a particular disease above endemic levels, within a population at a certain time in a given area.

What is an epidemic?

An outbreak that occurs within a population in a localized area.

What is a pandemic?

An outbreak that occurs on a global scale (multiple countries/continents).

What conditions must be met for an epidemic to occur?

High pathogen numbers in a population and a high number of susceptible hosts with easy transmission.

What is virulence

The ability of a pathogen to infect and cause harm to its host.

What are some factors that can affect the transmission of a virus?

Virulence of the virus strain, population density, population mobility, host exposure and susceptibility, cultural beliefs, and public health infrastructure.

When did the first appearance of EVD occur?

In 1976 with two simultaneous outbreaks in South Sudan and the Congo.

What species caused the major 2014-2016 Ebola epidemic in West Africa?

Zaire ebolavirus species.

What is the natural host of the Ebola virus?

Fruit bats.

How is the Ebola virus initially transmitted to humans?

From an infected animal.

What role do cytokines play in the immune response to Ebola?

They initiate an immune response, but excessive release can lead to blood vessel leakage and shock.

What are the initial symptoms of Ebola Virus Disease?

Fever, fatigue, muscle pain, headache, and sore throat.

What are severe symptoms of Ebola Virus Disease?

Vomiting, diarrhea, rash, impaired kidney and liver function, internal and external bleeding.

How long can the Ebola virus persist in the body after recovery?

More than 9 months in the testicles, eyes, CNS, placenta, and milk of pregnant women.

What is the current treatment for Ebola Virus Disease?

There is no cure; patients receive supportive care with fluids and treatment of specific symptoms.

What was significant about the Ebola vaccine developed in 2016?

It was the first successful vaccine for Ebola Virus Disease.

What are some prevention methods for Ebola Virus Disease?

Cook meat thoroughly, wear personal protective equipment, maintain good hygiene, and refrain from unprotected sex.

What is the incubation period for Ebola Virus Disease?

2 to 21 days.

What are the key components of Ebola prevention and control?

Surveillance of hot spots, contact tracing, laboratory testing, safe burials, social mobilization, and quarantine.

What species does root rot effect?

fruit and nut trees, shrubs, and vegetables

What causes root rot?

fungi such as Pythium, Phytophthora, Rhizoctonia, and Fusarium.

What are the symptoms of root rot in plants?

gradual or quick decline, stunted growth, small pale leaves, wilted or yellowed leaves, branch dieback, and thinning of the canopy.

How is root rot transmitted?

waterlogged soil, contaminated water, and infected root contact with susceptible roots.

What is the impact of root rot on agricultural production?

decreases yield, reduces the quality of fruits and seeds, increases management costs, and lowers farmer profits.

Which plants does Cucumber Mosaic Virus effect?

plants in the Cucurbitaceae family, such as cucumbers and melons

What causes Cucumber Mosaic Virus?

Tobamovirus genus

What are the symptoms of Cucumber Mosaic Virus in infected plants?

light and dark mottling, mosaic patterns, blistering, curling, stunted growth, and misshapen or bumpy fruit.

How is Cucumber Mosaic Virus transmitted?

indirect physical contact, sap from infected plants, pollinating bees, seeds, soil, and water.

What is the effect of cucumber mosaic virus on agricultural production?

decreases yield, causes premature plant death, and necessitates the removal of diseased plants, leading to financial losses.

What species does fire blight effect?

apples, pears, and quinces

What causes fire blight?

The bacterium Erwinia amylovora

What are the symptoms of fire blight?

dark brown or black infected tissues, wilting, bending of young shoots, and production of a thick yellow liquid in warm, humid weather.