Module 7: Infectious Disease

In this article, we will go through HSC Biology Infectious Disease. We examine a variety of infectious diseases as well as their treatment, prevention and control.

You will look at how the immune systems of plants and animals respond to infectious disease as well as medical and agricultural developments that draw on the work of scientists.

The following areas will be covered during the Module:

• Topic 1: Causes of Infectious Diseases
• Topic 2: Responses to Pathogens
• Topic 3: Immunity
• Topic 4: Prevention, Treatment and Control

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Topic 1: Causes of Infectious Diseases

Diseases in plants and animals can be infectious or non-infectious. In this module, we will focus on infectious diseases.

Infectious diseases are caused by pathogens that enter the body and can be transmitted from one organism to another.

Pathogens can either be microscopic or macroscopic and are classified below.

• Bacteria: Unicellular prokaryotes that have a cell wall but lack membrane-bound organelles.
• Viruses: Non-cellular and non-living agents that require a host to reproduce. Consist of nucleic acids (DNA or RNA) and a protein coat only.
• Fungi: Unicellular or multicellular eukaryotes. Possesses a cell wall made of chitin.
• Prions: Misfolded proteins that are non-living and non-cellular.
• Protozoa: Unicellular eukaryotes that lack a cell wall.Macro-parasites: Parasites visible to the naked eye.

Discovery of pathogens

Before the invention of the microscope and the discovery of pathogens, various theories existed to explain the spread of disease.

One such theory, spontaneous generation, maintained that living things could spontaneously generate from non-living matter.

The work of scientists Robert Koch and Louis Pasteur disproved spontaneous generation and replaced it with Germ Theory – that diseases are caused by tiny microbes.

Louis Pasteur

Louis Pasteur carried out several experiments proving that microbes in the air cause contamination in food and are also responsible for causing diseases in humans.

His swan-neck flask experiment (shown below) proved that microbes in the air are responsible for food contamination.

This led to the process of food pasteurisation to destroy microbes using heat.

Robert Koch

Robert Koch also contributed to understanding infectious diseases.

He established the link between specific microbes and specific diseases by formulating Koch’s Postulates that state:

1. The microorganism must be present in diseased organisms and absent in healthy organisms.
2. The microorganisms must be isolated from the diseased organism and grown in pure culture.
3. The cultured microorganism must produce the same disease upon inoculation into a healthy organism.
4. The microorganism must be re-isolated and re-cultured form the inoculated organism. Upon culturing, it should be identical to the original isolated microorganism.

Culturing techniques

Since microbes (e.g. fungi and bacteria) are invisible to the naked eye, specific culturing techniques are used to grow them in large enough numbers to study them.

This requires:

1. Inoculating nutrient agar plates with a sample (e.g. from food or water).
2. Incubating the plates at a constant temperature e.g. 30 °C.
3. Observing and recording the number of microbial colonies after the incubation period
4. Repeating the experiment at least three times and averaging the results.

Sterile techniques

Sterile techniques are important to prevent contamination.

A Bunsen burner should be used to sterilise the inoculating loop and once the plate is inoculated it must remain sealed to prevent the release of harmful pathogens into the environment.

Chain of infection

The chain of infection shows how infectious diseases result from interactions between the pathogen, the host and the environment.

• Reservoirs are where an infectious agent lives and grows e.g. in a human host.
• Portals of exit are where the pathogen leaves the reservoir to infect other organisms e.g. through the mouth (coughing).
• Modes of Transmission are ways that the pathogen can be transferred from the reservoir to susceptible host such as physical contact, droplets or air.
• Portals of Entry are the ways that pathogens enter the host e.g. mucous membranes.
• Susceptible hosts are hosts that are vulnerable to infection e.g. they have an immune deficiency.

Modes of transmission

Modes of transmission include direct, indirect and vector transmission.Modes of transmission include direct, indirect and vector transmission.

• Direct contact transmission includes physical contact between an infected and a susceptible individual. Examples include hand shaking or coughing directly on someone.
• Indirect transmission occurs through touching of contaminated objects, airborne transmission (e.g. airborne virus) or animal to person (handling of animal faeces).
• Vector transmission is a form of indirect transmission where typically the pathogen is transmitted through a bite of an insect e.g. mosquitoes carrying the Plasmodium parasite that causes malaria in humans.

Pathogens can infect almost every part of the human system. Our innate and adaptive immune responses (discussed later) protect us against pathogens.

Pathogens such as bacteria and viruses have developed adaptations to facilitate their entry into and transmission between hosts.

One such adaptation is the use of vectors to bypass the skin barrier.

For example, mosquitoes bite the skin injecting the pathogen directly into the blood.

Bacterial adaptations

Bacterial adaptations include:

• Formation of a sticky biofilm to provide protection against host’s immune response.
• Production of kinase enzymes allow Staphylococcus bacteria mobility in the blood stream by dissolving blood clots.
• Production of toxins that damage the tissue and weaken the immune system.
• Fimbriae appendages that adhere to surfaces such as the urinary tract which prevents the bacterium from being flushed out.
• Flagellum that enhance the swimming ability of bacteria e.g. burrowing into stomach mucous membrane.

Viral adaptations

Viral adaptations include antigenic shift and antigenic drift.

The immune system can learn to identify specific pathogens by responding to the specific shape of antigens on the pathogens surface.

1. Antigenic shift: two different viruses combine, creating a new virus with different antigens.
2. Antigenic drift: the virus accumulates random mutations which can also change the shape of the antigens so that the immune system does not recognise them.

Similar mutations have occurred in the genome of the virus that causes COVID-19.

Pathogens take advantage of the host immune response to exit the host and infect other organisms.

Influenza virus exits the host through respiratory droplets during coughing or sneezing and stays in the air infecting other individuals.

Plant diseases

Plant diseases caused by pathogens can be classified according to whether they are Biotrophic pathogens that feed on living plant tissue or necrotrophic pathogens that produce toxins which kill plant cells before they feed on them.

Plant diseases are typically caused by fungi, viruses, bacteria and macroparasites.

Oak mildew is an example of a fungal infection that results in powdery blotches and produces a range of effects on the plant.

This includes inhibition of photosynthesis and chlorophyll production (chlorosis), blocking of stomata and wilting.

The lack of genetic diversity in crop plants make the entire crop susceptible to a single pathogen.

This in turn affects agriculture by reducing crop yield and causing major economic loss in the agricultural industry.

Animal diseases

Animal diseases spread quickly through livestock as intensive farming keeps many animals together in a small space.

An example includes mad cow disease which is caused by prions that damage the cow’s central nervous system.

This disease can spread amongst animals and on to humans through consumption of infected meat.

Once the disease is detected, all potentially infected cows must be destroyed to prevent further spread.

This affects the agricultural industry by causing major economic loss and often results in banning of livestock exportation from affected regions.

During your class, you will look at various examples of plant and animal diseases, infection mechanisms and responses to infections.

Topic 2: Responses to Pathogens

Plants only have an innate immune response to protect themselves from pathogens.

Hypersensitive response

An example is the hypersensitive response which detects pathogens and kills the infected cells through apoptosis.

Physical defences such as trichomes protect the plant from predation by insects or large herbivores.

Trichomes are spines found on plants such as the Gympie Gympie Tree of Queensland.

Leaf dropping

Leaf dropping is another mechanism to eliminate the pathogen.

Leaf dropping is carried out in three steps:

1. Nutrients are resorbed from the leaf.
2. A protective layer of lignin forms at the site of leaf detachment.
3. Cells at the detachment site are digested by enzymes to cause leaf dropping.

Defensive chemicals

Plants can produce specific defensive chemicals such as caffeine which is toxic to fungi and insects.

If a pathogen defeats the plant’s physical defences and enters the cell, it can be detected by Pattern Recognition Receptors (PRR).

This triggers an immune response leading to thickening of the cell wall to prevent further spread.

In the next topic we will look at how animal cells undergo physical and chemical changes in response to pathogens.

Topic 3: Immunity

In animals, exposure to a pathogen triggers an immune response which leads to physical and chemical changes in the cells and tissues.

Animals have innate and adaptive immune responses that are divided into three lines of defence:

• First line (Innate): Comprised of physical and chemical barriers.
• Second line (Innate): Involves phagocytosis and inflammation.
• Third line (Adaptive): Response of specialised B-cells and T-cells that are specific to the pathogen.

First line of defence (Innate)

The table summarises the physical and chemical barriers that prevent pathogen entry into the body.

Second line of defence (Innate)

When the first line of defence is breached, the body resorts to the second line of defence to prevent the pathogen from infecting nearby tissue cells.

Third line of defence (adaptive)

The third line of defence is an adaptive response that is specific to the pathogen encountered.

It is composed of white blood cells called B-cells and T-cells.

B-cells attack pathogens outside of cells, whereas, T-cell attack pathogens inside of cells.

B-Cells

B-cells mature in the bone marrow and can become plasma cells that produce specific antibodies that inactivate a pathogen.

Some B-cells become memory B-cells that remain in the body for a long period of time and will respond quickly if the same pathogen is encountered again.

The process of creating memory B-cells during the initial antigen encounter and using these cells to create a more rapid response upon subsequent encounter is called the Primary and Secondary Response respectively.

These responses depict acquired immunity against the pathogen.

T-cells

T-cells mature in the thymus gland and carry out cell-mediated responses against pathogens that have infected the body cells.

There are four different types of T-cell:

• Helper T-cell: Regulates interaction between B and T-cells by secreting cytokines (chemicals).
• Cytotoxic (Killer) T-cell: Kills infected cells by releasing toxic granzymes that destroy the cell.
• Suppressor (Regulatory) T-cell: Inactivates B and T-cells once the pathogen is destroyed.
• Memory T-cell: Remains in the body and reactivates quickly with future infections of the same pathogen. They provide acquired immunity (just like Memory B-cells).

Topic 4: Prevention, Treatment and Control

Infectious diseases can spread very quickly amongst populations leading to epidemic or pandemic events.

An epidemic is an outbreak that affects many people in a given region, whereas a pandemic is a more global outbreak across different countries or continents.

New diseases are more likely to become epidemics or pandemics, with the majority of new diseases in humans originating from animals.

Case study: malaria

Malaria has been both an epidemic and pandemic disease which is caused by Plasmodium protozoa that are transmitted into secondary hosts (humans) through the bite of a female mosquito vector.

Disease spread is examined through collecting incidence and prevalence data. Incidence refers to new cases while prevalence is the proportion of a population with a disease during a certain time.

In order to reduce incidence and prevalence of Malaria, prevention and control methods target the mosquito vector due to their role in disease transmission.

This includes:

• Use of medications such as Chloroquine for early treatment intervention.
• Decreasing mosquito larvae by draining off swamps to remove breeding grounds.
• Using insecticides such as DDT to kill adult mosquitos.
• Limiting of the number of bites by using fly screens and protective clothing.

These methods have been successful for eradication of malaria in many countries (shown below).

During your class you will interpret given data to calculate incidence and prevalence rates.

General procedures

General procedures can be employed to reduce disease spread, including:

• Hygiene practices such as washing hands or use of disinfectants to eliminate pathogens.
• Quarantine of infected individuals or animals.
• Public health campaigns to raise awareness amongst the population.
• Use of pesticides to destroy disease vectors.
• Genetic engineering to alter the genotype of organisms and produce disease resistant organisms.
• Vaccination to gain acquired immunity

Vaccinations

Vaccinations continue to be crucial.

Herd immunity can be achieved by vaccinating the majority of the population to protect vulnerable individuals such as the elderly.

Antibiotics and Antivirals

Antibiotics and Antivirals can be used as treatment for the control of infectious diseases.

Antibiotics

Antibiotics work by stopping bacteria from replicating by preventing cell wall, nucleic acid or protein synthesis.

Different antibiotics work on different bacteria. Antibiotics reduce disease spread by eradicating bacteria more quickly.

However, antibiotic resistance due to misuse is a growing problem that reduces effectiveness.

Antivirals

Antivirals work on viruses by blocking their action at various points during infection, including:

• Entry of the virus to the cell.
• Reverse transcription of viral RNA into DNA.
• Translation of viral DNA into proteins.
• Integration of viral DNA into the genome of the cell.

Antivirals cannot cure the disease but only reduce duration of the illness by assisting the immune system to fight against the virus.

Tamiflu is an antiviral that can shorten influenza symptoms by one to two days if taken within 48 hours of disease onset.

As a preventative measure it can also decrease likelihood of sickness by 55%. Similar to antibiotics, antiviral resistance is a growing problem due to mutations.

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