HSC BIOLOGY
HSC Biology
Module 7: Infectious Disease
Terence Cheng
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Year 12
2021-2022
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HSC BIOLOGY
Course Content
Module 7: Infectious Disease
Causes of Infectious Disease
2
2
Types of Pathogens
2
Koch and Pasteur
5
Modes of Transmission
8
Case Study: Malaria
10
Case Study: Dengue Fever
13
Microbial Testing of Food and Water Samples
15
Plant Disease – Panama Disease
18
Animal Disease – Foot Rot
19
Adaptations of Pathogens
20
Responses to Pathogens
21
Case Study: Myrtle Rust (Fungal Infection)
21
Physical Defences Against Infection
22
Chemical Defences Against Infection
23
Immunity
25
The Innate Immune System
25
The Adaptive Immune System
27
Prevention, Treatment and Control
32
Factors that Limit the Spread of Infectious Diseases
32
Methods for Preventing Disease Spread
33
The Effectiveness of Pharmaceutical Treatments
35
Case Study: Management and Quarantine in COVID-19 in Australia
36
Historical and Current Strategies for Predicting and Controlling Disease
37
Aboriginal Medicine
38
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Module 7: Infectious Disease
Causes of Infectious Disease
Inquiry Question 1: How are diseases transmitted?
Types of Pathogens
What are Pathogens?
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Prions
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An infectious disease is a disease that can be spread from one organism to another and is
caused by a pathogen.
A pathogen is a causative agent for infectious disease.
The host is the living being that the pathogen, or other disease-causing microorganism
normally resides in.
A Prion is a type of protein found in all brains that is harmless in its normal form.
However, abnormal (misfolded) prions can cause prion diseases.
They are about 10nm in size.
Misfolded prions are able to pass the misfolding onto nearby, normal prions, causing
exponential infection of all prions in the brain. Together, they clump together in large fibres
which cause instant death to neurons, leaving holes in the brain. This eventually leads to
rapid memory loss, dementia and death.
EXAMPLE: Creutzfeldt-Jakob Disease (CJD)
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It is the human equivalent of BSE (Mad Cow Disease), which originated from a cow that had
eaten sheep meat that contained the prion disease Scrapie.
You get can get CJD by eating beef contaminated with BSE. The misfolded prions cannot be
destroyed by heat or Enzymes.
Symptoms include memory loss, dementia and coma.
Viruses
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A Virus is a tiny structure with a protein coat that encloses genetic material (either DNA or
RNA).
They can be from 30-300nm in size.
They can be crystallised.
They reproduce by being engulfed into a cell and essentially turning into a virus-producing
factory that then releases new viral cells into the surroundings.
EXAMPLE: Ross River Fever
Caused by a Virus belonging to the family Torgaviradae.
Transmitted to humans by mosquitoes.
Found in all Australian States and Territories
Symptoms include fever, chills, muscle aches, swollen lymph nodes and joint pain.
There is currently no cure, but treatments include headache tablets and bed rest.
Bacteria
- Bacteria are prokaryotic, single-celled organisms (no membrane bound organelles or
nucleus)
- Most respire anaerobically, allowing them to live in low oxygen habitats such as the
digestive tracks of animals.
- Bacteria are killed using disinfectant and bacterial diseases are treated with antibiotics.
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EXAMPLE: Salmonella
Caused by anaerobic bacteria belonging to the genus salmonella.
Transmitted by eating food contaminated with Salmonella bacteria.
Symptoms include nausea, vomiting, fever and diarrhea, which are caused by the toxins
released by the Salmonella bacteria
Salmonella bacteria can be killed by recooking contaminated food.
Protozoans
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Protozoans are unicellular eukaryotic organisms that are their own kingdom.
They are about 50 µm in size.
They are classified based on the way they move (e.g. with their flagella or cilia or other)
Most protozoans do not cause disease and thus, are not pathogens.
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EXAMPLE: African Sleeping Sickness
Caused by the Protozoan Trypanosoma.
It is transmitted by the bite of the Tsetse fly.
Symptoms include fever, headaches, joint pains, poor coordination and confusion.
Without treatment, the disease is fatal.
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Fungi
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Fungi are eukaryotic organisms that have cell walls but no chloroplasts.
They could be unicellular or multicellular.
Unicellular fungi are about 4 µm in size.
They are either parasitic (living on a host) or saprophytic (living on dead matter)
Diseases caused by fungi are called mycotic diseases.
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EXAMPLE: Tinea a.k.a Ringworm
Athlete’s Foot is common type of Tinea that affects the foot.
It is most commonly caused by the fungus Epidermophyton Floccosum T.
Spreads by sharing footwear or shower floors with an Athlete’s Foot sufferer.
The fungus grows in moist areas such as between toes or on the soles of the feet.
Symptoms: flaky and reddened skin in affected areas.
Macro-Parasites
- Macro-Parasites are large parasites that can be seen with the naked eye such as tapeworms.
- Therefore, most are greater than 1 mm in size.
- They could be ectoparasites (found externally) or endoparasites (found internally)
EXAMPLE: Taeniasis a.k.a Tapeworm Disease
- Caused by Pork Tapeworm.
- Pork Tapeworms spend the larvae part of their life cycle in pigs and the tapeworm part of
their life cycle in human intestines.
- It is transmitted to humans by eating undercooked pork that contain live larvae, which
develop into young tapeworms once in the intestine.
Koch and Pasteur
The Chain of Infection
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The Chain of Infection is how pathogens are spread, made up of 6 different links.
To prevent spread of disease, one or more of the links need to be broken.
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1) An Infectious Agent is the organism that has the ability to cause disease, so the pathogen.
Ways to break this link: seek prompt treatment if you are ill and use the correct
tools/cleaning agents to remove the pathogen.
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2) A Reservoir is an environment where pathogens can thrive or reproduce. This includes
tabletops, doorknobs and people.
Ways to break this link: clean the environment to remove the pathogens.
3) The Portal of Exit is the method the pathogen leaves the reservoir. This could be through
sneezing or coughing.
Ways to break this link: wear protective equipment such as masks.
4) The Mode of Transmission is how pathogens are carried from one place to another, such
as through the air or on the hands of a healthcare worker.
Ways to break this link: handle food properly, wash hands and control air flow.
5) The Portal of Entry is the way pathogens enter the host. This could include breaks in the
skin or other orifices in the body.
Ways to break this link: Taking proper care of wounds, washing hands and wearing PPE.
6) The Susceptible Host is someone who cannot fully defend against the pathogen. This
includes the elderly, burn patients or people with compromised immune systems.
Ways to break this link: Identifying those at higher risk of infection and vaccination.
Germ Theory
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Up until the 18th century, the spontaneous generation theory claimed rotting matter created
microbes. The theory also claimed the microbes were spread through Miasma, or clouds of
vapour.
Germ Theory stated that germs already exist in the environment and that they do not
spontaneously generate. It also stated that microbes caused decay, not the other way
around.
Both Pasteur and Robert Koch provided evidence for germ theory through their
experiments.
Pasteur’s Experiments on Microbial Contamination
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Pasteur designed an experiment that tested whether sterile nutrient broth could generate
microbial life.
He set up 2 flasks with sterilised nutrient broth in them. He bent the neck of one of the
flasks into a swan neck shape and broke off the neck of the other flask.
Over time, dust particles from the air fell into the broken flask while it collected in the neck
of the Swan flask.
As a result, the broth in the broken flask became cloudy, a sign that it teemed with microbial
life while the broth in the intact flask remained clear- without the introduction of dust on
which microbes could travel, no life arose.
Thus, through Pasteur’s experiments, he disproved the notion of spontaneous generation.
Koch’s Postulates
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Koch’s Postulates are steps taken to prove that a certain pathogen is the causative agent of a
certain disease.
1) The suspected causative agent must be identified as absent from all healthy organisms
but present in abundance in all diseased organisms.
2) The causative agent must then be isolated from the diseased organism and grown in pure
culture. (Culture in biology means the growing of microorganisms in a specially prepared
nutrient medium)
This step is necessary because if you just take a sample from a diseased organism and
inoculate it into another and it gets sick, all you know is one of the pathogens in the sample
is the causative agent, but you don’t know which one.
3) The cultured agent must cause the same disease symptoms when inoculated into a
healthy, susceptible organism.
4) The same causative agent must be reisolated from the inoculated, diseased organism.
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In other words, when the causative agent of the second organism’s disease is isolated, it
must be the same one isolated in step 2.
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Limitations of Koch’s Postulates
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It won’t work if you can’t isolate the suspected causative agent in pure culture, for
example if it is a virus, which needs a host to reproduce.
▪
It won’t work if multiple pathogens cause the same disease, so you can prove that one of
the pathogens is a causative agent of a disease, but you can’t say it is the causative
agent of the disease. An example is the common cold, which is caused by multiple
viruses.
▪
It also won’t work if one organism causes multiple diseases, for example streptococcus
pneumoniae, which can cause meningitis or pneumonia.
Modes of Transmission
Direct Contact
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Direct Contact is when an individual physically comes into contact with a person or animal
with a disease.
This could be through touching, kissing, coughing and sneezing, or in animals, bites and
scratches.
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EXAMPLE 1: Transmission of the Covid-19 virus when cough droplets from an infected
individual lands on their face.
EXAMPLE 2: Transmission of stomach flu through kissing, which facilitates mouth to mouth
contact.
Indirect Contact
- Indirect Contact is when an individual comes into contact with a surface or environment
where pathogens are spread into by an infected person.
- A surface could be a doorknob, a syringe, or contaminated food while an environment could
be a hospital ward.
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In these places, (reservoirs) pathogens may be able to live without hosts for a long period of
time. The abundance of nutrients in reservoirs could also allow these pathogens to grow.
Objects that become contaminated with pathogens are called fomites.
Infections spread by indirect contact could also be through vehicle transmission. This is the
spread of pathogens by contaminated air, food or water.
EXAMPLE 1: Transmission of the HIV virus through an unclean syringe used to collect an
infected individual’s blood, which is then used on another patient
EXAMPLE 2: Transmission of athlete’s foot by walking on the floor surrounding a public
swimming pool, which has become a reservoir for the EFT fungus after being walked on by
an infected individual.
Vector Transmission
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A Vector is an organism that helps transmit infection from one host to another, or between
a reservoir and a host. Thus, vector transmission is when a vector spreads an infection.
A Biological vector is an organism in which the pathogen undergoes part of its life cycle.
EXAMPLE: Mosquitoes that spread malaria through plasmodium parasites that are able to
develop while still inside the mosquito.
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A Mechanical vector is an organism that physically transfers the pathogen from one
host/reservoir to another host/reservoir without being infected themselves.
EXAMPLE: Flies which can spread cholera on their feet without being infected themselves,
from a contaminated water source, carrying it to food, which is then eaten by people.
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Note: because Vector Transmission is not a form of direct contact, it also falls under indirect
contact, just that it is a specific type of indirect contact, just like vehicle transmission.
Case Study: Malaria
Key Facts
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Malaria is a life threatening disease caused by the Plasmodium parasite, transmitted to
people by Anopheles Mosquitoes.
It is preventable and curable.
According to the WHO, in 2020 there were 241 million cases of Malaria and 627,000. Deaths
from Malaria in the world.
Africa was home to 95% of Malaria Cases and 96% of Malaria Deaths.
Children under 5 accounted for 80% of all Malaria deaths in the region.
More about the Protozoan that Causes Malaria
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Malaria is caused by plasmodium parasites, which are protozoans, and are thus eukaryotic
and unicellular.
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There are five species of parasites in the plasmodium genus that cause malaria in humans.
The two of greatest threat are called plasmodium falciparum (mainly found in sub-Saharan
Africa) and plasmodium vivax (mainly found outside of sub-Saharan Africa).
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The life cycle of plasmodium parasites involves alternation of generations.
Half of its life cycle is spent in a human, where sporozoites use red blood cells to undergo
multiple fission into gametocytes.
The other half of its life cycle is spent inside the Anopheles mosquito. This is where
gametocytes fertilise into sporozoite zygotes, which move to their salivary glands, from
which they are injected into the blood of another human.
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Transmission (Chain of Infection)
1) Infectious Agent: The plasmodium parasites.
2) Reservoir: These parasites are found in the red blood cells of infected people.
3) Portal of Exit: When a mosquito bites an infected person, a small amount of blood is taken in
which contains the microscopic plasmodium parasites.
4) Mode of Transmission: Thus, the mosquito becomes a biological vector for the plasmodium
parasites, which can still develop as it is still inside the blood meal taken in by the mosquito.
Side note: Malaria can also be spread through other forms of indirect contact such as blood
transfusions, organ transplants, or the share use of syringes contaminated with blood.
5) Portal of Entry: About 1 week later, when the mosquito takes its next blood meal, these
parasites mix with the mosquito’s saliva and are injected into the person being bitten.
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6) Susceptible Host: Anyone can get Malaria. Most cases occur in people who live in countries
with Malaria Transmission, however, people from countries with no Malaria can become
infected when they travel to countries with Malaria.
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Despite this, Malaria is not contagious because it cannot be spread by direct contact
between people. You cannot get it from casual contact with people infected with Malaria.
Symptoms
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Symptoms of Malaria are due to anaemia, the loss of functional red blood cells.
The first symptoms- fever, headache and chills, usually appear 10-15 days after the infective
mosquito bite and may be mild and difficult to recognise as Malaria.
If left untreated, Plasmodium F (S.S African) Malaria can progress to severe illness and death
within a period of 24 hours.
Prevention
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Over the last 2 decades, there has been an expanded access to WHO-recommended Malaria
prevention tools and strategies, which include:
Effective vector control such as insecticide-treated mosquito netting and indoor residual
spraying.
The use of preventative antimalarial drugs for both people living in Malaria-prone regions
and travellers to these areas.
Prevalence
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About half the world’s population is currently at risk of Malaria.
Some groups are particularly at risk of contracting Malaria and developing severe Disease
including:
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Infants and children under 5 years of age.
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Pregnant women.
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Patients with HIV/AIDS.
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People with low immunity moving to areas of intense malaria transmission such as
migrant workers and travellers.
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Malaria mostly occurs in poor, subtropical areas of the world. Africa is most affected
because:
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Anopheles mosquitoes live there, which can transmit very efficiently.
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Warm weather conditions all year round allow mosquitoes to thrive, and
transmission to occur at higher rates.
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Scarce resources and socio-economic instability have hindered efficient Malaria
control activities.
Case Study: Dengue Fever
Key Facts
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Dengue Fever is caused by the Dengue Virus.
Its main mode of transmission is vector transmission. It is most commonly transmitted
through the Aedes Aegypti mosquito.
Dengue fever is highly prevalent in South Asia, South-east Asia and South America.
Symptoms include serious flu-like symptoms, however, in some cases, a more severe form of
the disease occurs, which is deadly.
Transmission (Chain of Infection)
1) Infectious Agent: The Dengue Virus.
2) Reservoir: Humans infected with Dengue Fever.
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3) Portal of Exit: When a mosquito bites a person infected with Dengue Fever, a small amount
of blood is taken in which contains the Dengue virus particles.
4) Mode of Transmission: Thus, the Aedes mosquito becomes a biological vector for the
Dengue Virus.
5) Portal of Entry: About 1 week later, when the mosquito takes its next blood meal, the virus
particles are injected into the person being bitten.
6) Susceptible Host: Anyone can get Dengue Fever. Most cases occur in people who live in
countries where the Aedes Aegypti Mosquito thrives, however, people from countries where
these mosquitoes don’t live can become infected when they travel to these countries.
Incidence of Dengue Fever
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In recent decades, the number of cases of Dengue Fever have increased dramatically
worldwide.
Currently, it is estimated there are around 400 million cases per annum.
The relationship between climate and dengue fever is as follows: Tropical climate causes an
increase in the incidence of dengue fever, whereas cooler, drier regions have lower dengue
fever incidence.
This is because the mosquitoes that cause Dengue fever have an optimal habitat of warmer
climates.
Thus, climate changes that make Australia warmer will provide suitable habitat for these
mosquitoes and so Dengue Fever could reach Australia.
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Prevalence
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Dengue Fever typically affects children 2-15 years old at a higher rate than adults.
Epicentres of outbreaks are typically located in major cities and mostly affect urban and semi
urban areas.
Mortality Rate
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Dengue Fever causes around 22 million global deaths a year.
Current Preventative Measures
- Avoiding mosquito bites by using insect nets and insect repellent, which are moderately
effective.
- Dengue Fever vaccines activate memory B and T cells in individuals to fight the virus when
they are exposed to it and thus, prevent symptoms. This is the most effective form of
prevention; however, they are harder to access.
Microbial Testing of Food and Water Samples
Streak Plate Method
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This method is used rather than the spread plate method because It requires less materials
and is quicker.
First, the inoculation loop is sterilised with a Bunsen burner and allowed to cool down,
before being used to collect a microbial sample from the food/soil/water to be tested (the
inoculum).
Then, it is used to streak one quadrant of the plate in a zig zag fashion, before doing the
same to the other three quadrants.
The inoculation loop should be sterilised with the Bunsen flame before each streak.
The streak pattern in each quadrant should also slightly overlap with those of the
neighbouring quadrants.
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The Agar plate is then sealed and incubated for 24-48 hours
Sterilisation Technique of Inoculation Loop (Accuracy)
- Pass the inoculation loop at a 45 degree angle through the flame of a gas burner until the
entire length of the wire becomes orange from the heat. This incinerates any contaminants
on the wire.
- Do not place the loop on the workbench or any other surface otherwise it may become recontaminated.
- Let the loop cool for a few seconds before using it to collect the sample to avoid killing any
microbes.
Incubation Process (Accuracy)
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The agar plates should be incubated for 24-48 hours so that there is enough time for
microbial colonies to form but not enough time for the microbes to use up available
nutrients and excrete toxic waste that could lead to the death of the colonies.
The agar plates should be incubated upside down so that liquid from any condensation that
occurs would not get into the agar and disturb the development of microbial colonies.
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Validity
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Use the same type of Agar Plate in each trial to ensure the nutrients the microbes are
cultured in are the same.
Use the same plating technique for each trial: (the streak plate technique).
Incubate the agar plates at 34℃ for 24-48 hours to ensure the microbes are grown in
uniform conditions.
All agar plates used should also be sealed with sticky tape to ensure no other microbes enter
and only the microbes collected from the sample is being cultured.
Safety Precautions
Precaution
Reasoning
Washing hands with soap and water.
This prevents any microorganisms, which
could potentially be pathogens, from
entering the body and causing illness.
Tuck in ties and tie up long hair before
operating the Bunsen burner.
To minimise the risk of the spreading of the
flame to yourself, other group members
and other areas of the workspace.
Wear safety glasses.
To prevent the water from the test tube
that contains the inoculum from splashing
into eyes, as again, they could potentially
be pathogens.
Dispose used agar plates by ensuring they
are fully sealed before placing them into a
yellow biohazard bag and putting it in the
rubbish bin.
To prevent the spread of pathogens to
others and to the environment after the
experiment is conducted.
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Observation of Results
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Different types of colonies/microbial growth can be distinguished by their different colours
or patterns they form on the plate.
Plant Disease – Panama Disease
Causes
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Caused by the Fusarium Oxysporum Fungi: The most recent strain is called TR4.
After it infects a plant, the soil beneath it is contaminated for a very long period of time
afterwards so bananas cannot be grown on it again.
Panama disease has wiped out the Gros Michel variety of banana in the 1950s. After this,
Cavendish Bananas were the only variety of banana to be grown worldwide, becoming a
monoculture. Due to the lack of genetic diversity, it is highly susceptible to Panama disease.
Effects on Agricultural Production
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The disease causes blockage of water-carrying vessels in the banana plant, which causes
leaves to wilt.
As the banana plant loses its leaves, it is unable to photosynthesise as efficiently and carry
out other processes.
Therefore, the production rate, yield, and profit per banana plant decreases. Eventually, the
banana plant dies, leading to a reduction in profits.
As a result, banana farmers lose their jobs, which also leads to less money for the economy.
National and global shortages in bananas will also follow.
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Thus, the effect on agricultural production is negative overall.
Animal Disease – Foot Rot
Causes
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Foot Rot is a disease which affects the feet of sheep, damaging the connective tissues
between the horn and flesh of their hooves. In severe cases, deeper structures of the foot
such as the joints may also become damaged.
It is caused by the Dichelobacter Nodosus bacteria. These bacteria usually occur in wet soil,
where they can be present for up to ten months. They infect sheep by entering through cuts
or bruises in the foot.
When inside the hooves of sheep, the bacteria can be present for years.
When sheep walk around, they can spread the bacteria into soil, where it can be transferred
to other sheep.
Foot Rot is a seasonal disease as most cases are recorded during wet seasons, where the
bacteria become abundant in mud and soil.
Effects on Agricultural Production
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Symptoms for the sheep include: painful swelling in the foot area as well as discharge,
bleeding, abscesses and an unpleasant smell.
The sheep may also experience lameness, limiting its mobility.
When a flock of sheep is infected with Foot Rot, their health decreases, and as a result, the
quality of their product (like wool or mutton) is also decreased, leading to shortages.
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A decrease in yield decreases profits from the farm, which decreases contributions to the
economy.
For example, in 1988, Foot Rot cost the sheep farming Industry in NSW $24 - $48.
Thus, the effect on agricultural production is negative overall.
Adaptations of Pathogens
Viruses
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Projections on the outer layer of the viral particle are used to cling onto the outer
membrane of a host cell as well as act as a “protein key” that tricks the cell into letting in the
virus via endocytosis.
The virus also tricks the cell into producing thousands of copies of the virus, increasing its
ability to transfer its genetic material and infect other body cells.
Viruses also stimulate sneezing and coughing in their infected host to facilitate transmission
between hosts.
Some viruses are able to survive in watery reservoirs for long periods of time, increasing
their ability to enter susceptible hosts
All of the following adaptations apply to the influenza virus, which you can use as an
example.
Bacteria and Protozoa
- Bacteria example: E-Coli. Protozoan example: Giardia. Both these pathogens have all the
following adaptations.
- Flagella- tails they use to propel themselves forward to where they need to. (our body cells)
- Pili- hairs on the outside of bacteria which can find receptors on the surface of our body
cells, stick to them and release chemicals to break them down and access nutrients. These
are the bacteria’s modified projections.
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Bacteria may also produce a chemical called biofilm to protect themselves as they undergo
binary fission and produce many copies of themselves.
Just like viruses, they have also evolved to cause coughing and sneezing in their infected
hosts so they can be more easily transmitted between hosts
They also have the ability to travel survive in both watery reservoirs for long periods of time,
however, they can also travel within these reservoirs, so they are able to move to areas
where they are more likely to access a new host.
E
Responses
to Pathogens
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Inquiry Question
2: How does a plant or animal respond to infection?
C
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l
Case Study:
Myrtle Rust (Fungal Infection)
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Description of Myrtle Rust
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Myrtle Rust (scientific name: Austropuccinia Psidii) is a fungal pathogen that affects
Myrtaceae Plants (including Eucalyptus Trees, Bottle Brushes and Tea Trees.).
It is an infection that attacks new growth, including leaves, shoot tips and the young stems
of plants.
It usually starts as a small purple spot, which turn into bright, yellow spores. The spores are
eventually dispersed and transported by wind to other plants, where they continue to attack
new growths.
This makes it difficult for the plant to grow.
Prevalence: Myrtle Rust was first detected in 2010 and has since spread across Eastern
Australia.
Responses from Myrtaceae Plants
- Innate defences include:
▪
The bark on the outside of Myrtaceae plants.
▪
They also have thick cell walls composed of pectin.
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▪
Once infection has occurred, additional structural proteins are deposited that
reinforce the plant’s cell walls at the site of infection.
▪
Eucalyptus oils produced by eucalyptus trees only are anti-bacterial and anti-fungal.
Specific defences (not to be confused with adaptative defences)
▪
Oxidative bursts are a mechanism of Myrtaceae plants that floods the site of
infection with oxygen, causing the myrtle rust fungus to burn up as it would overfire
all of its processes while managing the oxygen.
▪
The plant may also produce antimicrobial peptides, which are specific amino acids
that minimise the impact of the myrtle rust fungus.
Physical Defences Against Infection
Skin and Mucus Membranes
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Cilia
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Skin is made up of tightly packed cells that form a protective layer against pathogens from
entering the body.
The outermost layer of the skin is constantly being shed, removing any pathogens with it.
Additionally, cells lining all openings of the body secrete a layer of mucous which traps
pathogens and other foreign particles and prevent them from entering the body.
These are part of the body’s first line of defence against invasion.
Cilia are tiny hairs which line the air passages in the body, including the nose and throat.
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They prevent infection and push pathogens away from the lungs by moving in a wave like
motion at 12 beats per second.
Physical Removal from the Body
- Both Diarrhea and Vomiting are reflex actions coordinated by the brain in response to the
prescence of pathogens in the gut.
- Both processes are able to quickly expel microorganisms quickly from the Gastrointestinal
tract.
- Additionally, the body many also respond with increased urination to help flush out
pathogens.
- These processes are both part of the body’s 2nd line of defence.
Chemical Defences Against Infection
Urine
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Urine is sterile until it reaches the lower urethra.
Antimicrobial peptides in the urinary tract prevent infection by preventing bacteria from
binding to epithelial (surface) cells in the region.
These are part of the body’s 1st line of defence against invasion.
Secretions
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Secretions include: sebum, sweat, saliva and tears.
An enzyme called lysozyme is present in most bodily secretions. It is able to break down
bacterial cell walls to defend against infection.
Saliva and tears are two bodily secretions that additionally contain antimicrobial peptides.
These are part of the body’s 1st line of defence.
The Lymphatic System
- The lymphatic system produces white blood cells, of which there are many types that each
have different roles in the immune system against pathogens.
- The lymphatic system also drains pathogens into lymph nodes using lymph fluid, where they
can be neutralised and deactivated by other cells in the immune system.
- The Lymphatic system is part of the body’s 2nd line of defence.
Inflammation
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Inflammation is one of the first responses of the immune response to infection or injury.
Signs of inflammation include redness, swelling, heat and pain.
The purpose of inflammation is for the body to eliminate the pathogen. By increasing blood
flow to the area, more white blood cells can be brought to the area, and by increasing the
temperature, this may inhibit the function of the pathogens.
When tissues are damaged due to infection or injury, hormones called histamines are
released which increase the permeability of blood vessels at the site of infection, allowing
white blood cells to travel more easily to the site. Histamine is also responsible for most of
the signs of inflammation mentioned above.
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Inflammation is part of the body’s 2nd line of defence.
Phagocytosis
- Phagocytosis is when a cell changes its shape to engulf pathogens and cellular debris.
- White blood cells in the human immune system that do this include macrophages,
neutrophils and dendritic cells.
- Once pathogens are enclosed within these phagocytes, they are broken down by enzymes.
- Phagocytosis is part of the body’s 2nd line of defence.
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Immunity
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Inquiry Question
3: How does the human immune system respond to exposure to a pathogen?
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s Immune System
The Innate
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General aInformation
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- The
innate immune system provides non-specific protection against pathogens by
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o
responding
in a generic manner to all foreign invaders and has a rapid response rate.
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- Innate
responses are part of the body’s 2nd line of defence, activated after pathogens have
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penetrated the 1st line of defence.
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Monocytes
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Monocytes are leukocytes that normally circulate throughout the blood.
Upon infection, they travel to the site of infection where they differentiate into
macrophages and dendritic cells.
Note: A leukocyte is another word for white blood cell and the word you should use in exam
responses.
Macrophages and Dendritic Cells
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Macrophages and Dendritic cells can be found at the site of infection as well as elsewhere in
the lymphatic system and swallow pathogens via phagocytosis.
Macrophages can do this over and over again, unlike Neutrophils, which self-destruct once
they perform phagocytosis on a pathogen.
Once a Macrophage swallows a pathogen, it breaks down the pathogen and displays the
pathogen’s antigens on its surface and act as an antigen presenting cell.
They also release cytokines, proteins that signal other cells in the immune system to act.
Dendritic cells have similar functions to Macrophages as they also perform phagocytosis and
antigen presentation.
However, macrophages are more commonly associated with removing pathogens via
phagocytosis while Dendritic cells are more commonly associated with antigen presentation.
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Neutrophils
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e
- Neutrophils
are another type of phagocyte;
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n however, they self-destruct once they consume
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and
break down a pathogen- pus is basically
just a pile of dead neutrophils.
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r cells that become present at the site of infection
- They
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first
innate
response
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rh just like macrophages, also release cytokines.
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They are also a type of granulocyte, which is a leukocyte that promotes scab formation and
clotting.
Basophils and Mast Cells
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Both these cells are normally in circulation around the blood and are granulocytes.
Once infection occurs, mast cells and basophils move to the site of infection where they
release histamine, the chemical responsible for beginning the inflammation process and
clotting cycle.
Natural Killer Cells
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Natural Killer cells are one of the three types of lymphocytes, a type of leukocyte that also
includes T cells and B cells.
Unlike them, Natural killer cells do not need to recognise a specific antigen before attacking
infected cells.
Natural killer cells kill infected cells by binding to it, releasing cytotoxins and causing
apoptosis (programmed cell death), before going on to kill more infected cells.
The Adaptive Immune System
General Information
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The adaptive immune system provides specialised protection against pathogens which enter
the body, and thus needs time to develop upon primary exposure to a pathogen.
However, because immunological memory is part of the adaptive immune system, response
upon secondary exposure to the pathogen is stronger and faster.
Helper T Cells
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All T cells make up the body’s cell-mediated immune response.
All T cells (except for memory T cells) are produced in the bone marrow and mature in the
Thymus. After they mature, they circulate throughout the blood and lymphatic system.
Each helper T cell in the body have different receptors (some texts call them antibodies as
well) for different antigens.
Helper T cells start the adaptive immune response when an Antigen Presenting cell (APC)
from the innate immune system finds the correct helper T cell in the lymphatic system with
a matching receptor to the target antigen.
The APC would then bind to that helper T cell and activate it.
Once activated, the helper T cell would divide rapidly into more helper T cells, memory T
cells and suppressor T cells. The helper T cell clones all release cytokines, which are used to
activate cytotoxic T cells.
A (Killer) T Cells
Cytotoxic
n
- Once
activated by helper T cells, the role of cytotoxic T cells is to kill the body’s infected cells
A
- Infected
body cells display the pathogen’s antigens on their surface, which cytotoxic T cells
P
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bind to.
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- Cytotoxins
such as perforin and granzymes are released by the cytotoxic T cell into the
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m
infected
cell to facilitate apoptosis, deactivating the pathogens inside the infected cell in the
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process.
After this, they detach and go on to kill more infected body cells.
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A (Suppressor) T Cells
Regulatory
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Once the immune response has achieved its purpose, the suppressor T cells controls the
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oimmune response by supressing the activity of other T cells.
- tThis is done by releasing cytokines which inhibit the function of effector T cells. (below)
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- xNote: effector T cells are an umbrella term form both helper and cytotoxic T cells.
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Naive Bl Cells
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- oAll B cells in the body make up the humoral/antibody mediated immune response.
- cAll B cells are produced and mature in the bone marrow, after which, they circulate
a
throughout the blood and lymphatic system. Before they are activated, they are known as
l
lNaïve B cells.
- eEach Naïve B cell has membrane bound antibodies on its surface, with the type of antibody
dvarying from cell to cell. These surface antibodies are also known as B cell receptors.
C
- DWhen a Naïve B cell encounters a pathogen with antigens that match the antibodies/
8receptors on its surface, it takes in the pathogen and displays its antigen on its surface.
- +A corresponding helper T cell that has already been activated would then bind with the
T
cpresented antigen and activate the naïve B cell, causing it to divide rapidly into Plasma Cells
eand Memory B cells with the exact same antibodies that match with the antigens on the
ltarget pathogen.
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Plasma Cells
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Plasma cells deactivate the target pathogens in the body by producing the corresponding
antibodies, which bind to the antigens on the pathogen.
Plasma cells can produce thousands of antibodies per second.
Memory B Cells and Memory T Cells
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Memory B cells and T cells, which are produced when their respective lymphocytes are
activated, continue to circulate throughout the body even after the initial infection is
resolved and provide the body immunological memory.
They are antigen specific, ensuring a prompt response should the same pathogen reinfect
the organism.
A graph of the concentration of antibodies in the body over time is shown below, after
primary and secondary exposures to an antigen.
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Initial concentration of antibodies is zero because there are no memory cells for the
pathogen in question.
The increase in concentration is due to the increase in plasma cells once a naïve B cell has
been activated to defend the body against the pathogen.
The 1st peak of antibody levels occurs as the pathogen is removed from the body after initial
exposure.
Antibody concentrations then decrease, but not back to zero, because memory B cells have
been produced, which remain in the body after infection.
Upon secondary exposure to the pathogen, antibody concentrations rise even higher
because memory cells are present, allowing the body to produce large amounts of antibody
very quickly.
As a result, the 2nd peak of antibody levels is higher than the 1st because there are more
plasma cells secreting antibodies than the first time.
After the removal of the pathogen from the body after secondary exposure, the antibody
concentration is higher than the 1st time because more memory B cells are formed.
Antigen-Antibody Interactions
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All cells in the body, not just pathogens, have antigens on their surface.
After embryonic development, the body learns to recognise the antigens on the body’s own
cells as ‘self’ and as a result, they are not attacked by cells from the immune system.
However, when donor organs or tissues are transplanted into the body, the
receptors/surface antibodies on immune cells will recognise the antigens on these structures
as “non self” and as a result will attack them via an immune response.
Thus, the patient who receives donor organs will require immune suppressants for the rest
of their life. This stops their immune system from attacking the donated organ but leaves
the patient more susceptible to actual pathogens that cause illness.
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One way antibodies released by plasma cells disable pathogens is through neutralisation,
which is when they bind to all the antigens on a pathogen for the purpose of inhibiting their
functioning.
Another way is through opsonisation, which is when they bind to the antigens on a pathogen
for the purpose of tagging them for elimination by phagocytes, and also to make the
phagocytosis process easier as without antibodies, the negatively charged membrane of the
pathogen and the phagocyte could repel each other
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Summary of the Immune System
Below is a diagram summarising the how the immune system responds after primary
exposure to a pathogen.
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Note: an interleukin is a type of cytokine.
Prevention, Treatment and Control
Inquiry Question 4: How can the spread of infectious diseases be controlled?
Factors that Limit the Spread of Infectious Diseases
Local Factors Include:
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Immunisation to create herd immunity within populations.
Hygiene Practices such as:
▪
washing hands regularly
▪
covering coughs and sneezes
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cleaning surfaces regularly
▪ staying at home if you are sick.
Educating the public with knowledge about diseases and ways to prevent them.
Regional Factors
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Ensuring a population has a clean water supply.
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Ensuring a population has adequate sanitation facilities such as toilets and disposal facilities.
Continued surveillance of possible outbreaks of disease.
Rapid identification of the microbial cause of a disease if an outbreak occurs.
Implementing appropriate responses to an outbreak at a regional level such as:
▪
Isolation of sick people
▪
Treatment of sick people
▪
identification of high risk groups (like the elderly)
▪
the provision of supplies that prevent further transmission (like masks)
Global Factors
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Implementing appropriate responses to an outbreak at a global level such as:
▪
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Travel bans
▪ Quarantining of international travellers
Communication between countries and the World Health Organisation about the
progression of a disease, new findings and data so all countries can strategize the
combatting of the disease.
Methods for Preventing Disease Spread
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Use COVID-19 as an example for all of these.
Hygiene Practices
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Washing hands, cleaning wounds and preparing food properly are processes that minimise
the likelihood of pathogens entering the body.
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Quarantine
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Quarantine refers to the isolation of an individual for a period of time to prevent the spread
of a disease to their community.
This allows for the passing of the infectious period or for symptoms to disappear in the
individual so that they may be safe to interact with other people again.
Quarantining is also used for diseased plants and animals in agriculture.
Vaccination
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Vaccination triggers a small scale immune response in the body using the target pathogen’s
antigen so the body creates memory B cells and T cells for a target pathogen and is thus
more equipped to fight it upon secondary exposure to it.
Vaccination reduces the likelihood of infection, serious illness and the transmission of the
disease to others.
Public Health Campaigns
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Public Health Campaigns lead to the reduction of disease causing habits and healthier
choices in the community, and thus prevents the spread of disease.
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This can also be done by educating the public about the causes and impacts of diseases.
Pesticides
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In Agriculture, by using pesticides to prevent insects from acting as a vector for diseases that
may be spread among crops, crop damage and loss of yield is minimised.
Genetic Engineering
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Altering the genome of organisms may make them less susceptible to a disease or less
susceptible to be a vector for a disease.
Example: CRISPR was recently used to genetically engineer Anopheles Mosquitos to remove
their host factor (FREP1) gene. This disallowed plasmodium parasites to survive in the gut of
mosquito, and as a result, Malaria cannot be transferred to humans from these mosquitoes.
The Effectiveness of Pharmaceutical Treatments
Antivirals
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Antivirals can’t destroy viruses, and only inhibit their reproduction by targeting them at a
number of stages in their life cycle.
Antivirals block viruses by:
▪
Preventing them from entering body cells by interfering with their ability to bind to
receptors on the body cell’s surface.
▪
Or preventing them from creating extra copies of themselves inside the cell by
blocking them from releasing their viral genome or blocking the transcription of
their viral genome.
▪
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Or preventing them from being released by their host cell by blocking molecules
found on the surface of viruses.
Advantage: Antivirals Prevent the reproduction of viruses.
Disadvantage: Antivirals cannot kill viruses.
Disadvantage: The use of Antivirals can lead to antiviral resistance as viruses reproduce
rapidly and thus mutate rapidly.
Disadvantage: Research and Development of antivirals is expensive, and thus, they are also
expensive to buy.
Therefore antivirals can be considered ineffective for controlling the spread of a viral
disease.
Antibiotics
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Antibiotics work by either killing the bacteria or inhibiting the growth of bacteria. The
processes which they use to do these things include:
▪
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Decreasing the membrane permeability of body cells.
▪ Interfering bacteria’s ability to synthesise proteins.
Advantage: Inhibits the bacteria’s ability to infect people.
Disadvantage: The recent overuse and misuse of antibiotics has led to antibiotic resistance.
However, antibiotics can still be considered effective for controlling the spread of bacterial
diseases, for now.
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Case Study: Management and Quarantine in COVID-19 in Australia
Overview
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COVID-19 is an infectious respiratory disease caused by the SARS-CoV-2 virus.
The three main modes of transmission are: inhaling airborne droplets containing the virus,
direct contact with people with the virus, and contact with a fomite contaminated with the
virus.
It was first recorded at the end of 2019 and is ongoing.
Environmental Controls
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Providing of facilities for barrier nursing (strict infection control conditions), water and
hygiene controls, hand hygiene and safe waste management, provisions and wearing of
PPE and correct training.
Throughout 2020-2021, The NSW government mandated that everyone wear a mask in
public spaces and public transport to limit the spread of the disease.
Also throughout 2020-2021, The NSW government ordered lockdowns during the autumn
and winter months in an attempt to slow the spread of the disease.
Frequent hand sanitation was recommended.
PCR testing was recommended to anyone who showed symptoms of COVID.
RAT test kits were made widely available in 2021 and their use was encouraged if symptoms
developed.
Hospitals included portioned off areas for COVID patients to limit the spread of the virus.
The vaccination of the majority of Australians throughout 2021 reduced the impact of the
virus on individuals.
Quarantine
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Initially, infected people and close contacts had to self-isolate for 14 days, or until
symptoms disappear. This was later changed to 7 days.
Travel Bans to and from Australia were also enacted during parts of 2020 and 2021 to
prevent the further spread of the virus in and out of the country.
When international travel was allowed, travellers had to quarantine for a week to protect
national health.
Evaluation of Effectiveness
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Throughout 2021 until now, Australia had one of the highest vaccinated rates in the world,
creating herd immunity in the country. Additionally, Australia also had a relatively low rate
of infection throughout 2020-2021, compared to other countries. Therefore, the
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environmental management and quarantine methods used throughout this pandemic can
be considered effective.
Historical and Current Strategies for Predicting and Controlling Disease
Historical Strategies
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Many Ancient Civilisations, including the Mayans and Chinese believed used Astrology to
predict diseases, including observing the alignment of stars and planets and the presence of
certain constellations in the night sky. This was ineffective at predicting diseases because
there is no link between celestial objects and diseases on Earth.
John Snow’s epidemiological technique in the 19th century mapped out the locations people
who got cholera lived in London, as well as all the water pumps in the city. He observed that
many sick people lived around a faulty water pump where cholera resided, allowing him to
effectively control the Cholera outbreak.
European cultures in the middle ages attempted to control the spread of diseases by
balancing out the supposed 4 humours of patients, using methods such as bloodletting. This
led to wounds which increased the patients’ likelihood of further infection, and as a result, s
an ineffective method for controlling diseases.
Current Strategies
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Computer models use real time data on population density, travel and trade, climate change
and agricultural practices to simulate outbreak events with great precision. As a result, they
are able to guide disease-control organisations on policy making during an outbreak, making
them effective at controlling disease.
Web Based Surveillance: The number of people searching the web for keywords related to
particular diseases are monitored, as it is shown that there is a correlation between these
searches and cases of disease. This has been effective at predicting outbreaks as predictions
can be made on average 1-2 weeks before disease-control organisations.
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Aboriginal Medicine
Bush Medicine
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Aboriginal Bush Medicine includes the use of plant materials to create herbal medicines and
included the following:
Eucalyptus Oil was used to treat muscle aches, fevers and chills.
Tea Tree Leaves were ground into a paste to treat wounds.
Witchety Grubs were ground into a paste to treat burns and soothe skin.
Desert Mushrooms were used to treat sore mouths and lips by sicking on the body of the
mushroom.
The Incident with Smokebush in Western Australia
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Smokebush is a plant native to Western Australia that Aboriginal people have traditionally
used as a medicine.
In the 1980s, Smokebush was discovered to contain an active molecule that could combat
the HIV virus, and as a result, the US and WA governments filed patents that gave them
exclusive rights to the plant for AIDS treatment.
This demonstrates the disregard of the Indigenous Traditional Ecological Knowledge, which
had been exploited for financial gain.
These kind of patents also prevent Indigenous people from using their own Traditional
Ecological Knowledge from their own culture without a paid licence to the companies or
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organisations involved, highlighting a significant flaw in the Australian and US patenting
systems.
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