Pathogens and Infectious Diseases: Further Exploration (HSC SSCE Biology): Revision Notes

Pathogens and Infectious Diseases: Further Exploration

Introduction to non-cellular pathogens

Non-cellular pathogens are infectious agents that lack cellular structure. Unlike bacteria, fungi, and protozoa, these pathogens cannot carry out metabolic processes independently. The two main types of non-cellular pathogens are viruses and prions. Understanding these unique disease-causing agents is crucial because they require different control strategies compared to cellular pathogens.

In Sydney during winter 2017, hospitals experienced a surge in patients with severe vomiting and diarrhoea. The outbreak particularly affected aged care facilities and childcare centres. The culprits were Norovirus and Rotavirus, both viral pathogens spread through direct contact with infected individuals. This real-world example demonstrates how viral infections can rapidly spread through vulnerable populations.

Viruses: characteristics and structure

What are viruses?

The word 'virus' comes from a Latin term meaning 'slimy liquid or poison'. Scientists first recognised viruses as pathogens in the 1890s when studying tobacco mosaic virus. Viruses occupy a unique position between living and non-living things, possessing characteristics of both.

Living characteristics:

• Contain genetic material (nucleic acids)
• Pass on hereditary information to offspring
• Can evolve and adapt

Non-living characteristics:

• Not composed of cells
• Cannot carry out metabolism independently
• Can be crystallised like chemicals

Virus structure

Viruses are extremely small, measuring only $100-200$ nm in diameter. They can only be viewed using an electron microscope. All viruses share a basic structural plan consisting of:

Core components:

• Genetic material: Either DNA or RNA (never both)
• Capsid: A protective protein coat surrounding the genetic material
• Envelope (in some viruses): A lipid membrane derived from the host cell that surrounds the capsid

Types of virus structures

Different viruses have distinct structural forms. The main types include:

Icosahedral viruses (e.g., cold virus, $25$ nm): Twenty-sided symmetrical structure with a geometric appearance.

Spherical viruses (e.g., mumps virus, $100$ nm): Round shape with envelope surrounding the capsid.

Helical viruses (e.g., tobacco mosaic virus, $350$ nm): Rod-shaped or filamentous structure with genetic material arranged in a helix.

Complex viruses (e.g., bacteriophages, $50$ nm): Intricate structures with specialised parts like tail fibres and base plates.

How viruses replicate

Viruses cannot reproduce on their own. They hijack the host cell's reproductive mechanisms through a process called the lytic cycle:

Diseases caused by viruses

Viruses cause numerous diseases in humans, animals, and plants:

• Influenza (flu)
• Measles
• AIDS (caused by human immunodeficiency virus, HIV)
• Herpes
• Glandular fever
• SARS (severe acute respiratory syndrome)
• Tobacco mosaic virus disease in plants

Treatment challenges

Prions: a unique pathogen

The discovery of kuru

What are prions?

A pathogenic prion is an abnormal protein capable of causing degenerative diseases of the nervous system. Prions are fundamentally different from all other pathogens:

• No genetic material: Unlike viruses, bacteria, fungi, and protozoa, prions contain no DNA or RNA
• Smallest pathogens: Even smaller than viruses
• Pure protein: Consist only of misfolded protein molecules

Prion structure and mechanism

The key difference between normal and pathogenic prions lies in their protein folding:

Normal prion protein: Contains amino acids arranged in alpha helix structures - spiral, coiled shapes that are loosely packed.

Infectious prion protein: Contains amino acids arranged in beta-pleated sheets - flat, densely packed layers stacked together.

How prions cause disease

Pathogenic prions cause disease through a domino-like effect. When an infectious prion comes into contact with a normal prion protein, it induces the normal protein to change its shape and become infectious too. This creates a chain reaction:

1. Infectious prion encounters normal prion protein
2. Normal protein refolds into the pathogenic shape
3. Both proteins are now infectious
4. Each infectious prion converts more normal proteins
5. Abnormal proteins accumulate in nervous tissue

The accumulating abnormal proteins are deposited within the central nervous system and other organs, causing progressive damage.

Effects on brain tissue

Prion diseases are called transmissible spongiform encephalopathies (TSEs) because the brain tissue of infected individuals becomes full of holes, resembling a sponge:

The images show normal brain tissue compared to tissue affected by prion disease. The white spaces represent holes where brain tissue has been destroyed by accumulating abnormal prions.

Characteristics of TSE diseases

How prions are transmitted

Prion diseases can be contracted through several routes:

Dietary exposure:

• Ingesting tissue containing infectious prions, particularly nervous tissue and brain
• Eating contaminated meat products

Medical procedures:

• Surgery using contaminated equipment that was not properly sterilised
• Receiving contaminated growth hormone injections from infected animals
• Receiving corneal transplants from previously infected organ donors

Genetic transmission:

• Inheriting the mutated gene that codes for the infectious prion form

Spontaneous formation:

• Random conversion of normal prion proteins to infectious forms (very rare)

Diseases caused by prions

Important prion diseases include:

In humans:

• Creutzfeldt-Jakob disease (CJD): Rapidly progressive and fatal brain disease
• Variant CJD: Linked to consuming beef from cattle with BSE ('mad cow disease')
• Kuru: Transmitted through mortuary cannibalism in Papua New Guinea
• Fatal familial insomnia: Spontaneous transformation of protein in the brain

In animals:

• Bovine spongiform encephalopathy (BSE): 'Mad cow disease' in cattle
• Chronic wasting disease: Affects deer, elk, and moose in North America
• Scrapie: Affects sheep and goats

Modes of transmission of infectious disease

Understanding how diseases spread is critical for controlling outbreaks. Transmission involves carrying or transferring a pathogen from an infected host to a non-infected organism.

Pathogen reservoirs and carriers

Pathogens often exist in reservoirs - environments or living hosts where they can survive outside an active infection. The ability of a pathogen to survive outside a host relates directly to its transmission mode.

Environmental reservoirs:

• Spore-forming bacteria like anthrax can exist for years in soil where infected animal fluids have contaminated the ground
• Water sources can harbour various pathogens
• Contaminated surfaces and objects

Living reservoirs:

• The human gut can harbour disease-causing organisms
• Animals can carry diseases that affect humans
• Intermediate hosts in parasite life cycles

Human carriers:

Complex life cycles in parasites

Macroparasites often have complex life cycles involving multiple hosts and transmission stages. The parasite must successfully complete each stage and be transmitted between hosts to reach sexual maturity.

The chain of infection

Three modes of transmission

All disease transmission falls into three main categories:

Direct contact - Transfer through physical contact with infected skin or body secretions

Indirect contact - Transfer through non-living objects or environmental sources

Vector transmission - Transfer through another organism, typically an arthropod

Direct contact transmission

Direct contact transmission occurs when there is physical contact between an infected host and a non-infected organism. This transmission can be classified as:

Horizontal transmission: Contact between organisms of the same generation, or between organisms that are not parent and child.

Vertical transmission: Contact between offspring and parent, such as from mother to baby during childbirth.

Types of direct contact

Physical contact that can transmit disease includes:

• Touching infected skin
• Sexual contact
• Kissing
• Contact with nasal or oral secretions
• Biting
• Direct contact with blood or other body fluids
• Direct contact with open wounds
• Prenatal transmission: Before birth or during pregnancy
• Perinatal transmission: Around the time of birth, including during delivery

Diseases spread by direct contact

Common examples include:

• Skin infections: Ringworm and impetigo spread through touching infected skin
• Cytomegalovirus (CMV): Transmitted through saliva and other body fluids
• Glandular fever: Spread through saliva (the 'kissing disease')
• HIV/AIDS: Transmitted through blood and sexual contact
• Herpes simplex virus (HSV): Spread through direct contact with infected areas

Indirect contact transmission

Indirect contact transmission occurs when the host and another organism have no direct physical contact. Instead, infection occurs from a reservoir created by the host outside itself.

Types of indirect transmission

Airborne transmission:

• Coughing or sneezing releases droplets containing pathogens
• These droplets can travel up to $8$ metres through the air
• Other people inhale the contaminated droplets

Surface contamination:

• Touching infected surfaces or objects
• A fomite is any object or substance that carries infection
• Examples include door handles, utensils, towels, and medical equipment

Contaminated food or water:

• Pathogens survive in food or water sources
• Consumption leads to infection

Medical equipment:

• Infected surgical instruments not properly sterilised
• Surgical instruments are generally sterilised using saturated steam under pressure in an autoclave

Vector transmission:

• Discussed in detail in the next section

Diseases spread by indirect contact

Important examples include:

• Measles virus: Spreads through infected droplets in the air
• Gastroenteritis: E. coli bacteria transmitted through contaminated food and water
• Toxoplasmosis: Protozoan Toxoplasma gondii spread through infected cat droppings
• Legionnaires' disease: Over 50 species of Legionella bacterium transmitted through contaminated water in cooling towers
• Influenza: Exposure to droplets and biological matter containing virus particles

Vector transmission

Vector transmission is a specialised form of indirect transmission involving living organisms that carry pathogens from one host to another.

Common vectors

Arthropod vectors:

• Certain species of mosquitoes
• Sandflies
• Ticks
• Fleas
• Flies

Other vectors:

• Infected aquatic snails
• Mammals such as fruit bats (Hendra virus) and pigs (Menangle virus)
• Sometimes infected plants and fungi

How vector transmission works

Most vector transmission involves arthropods that feed on blood. The typical process is:

1. Vector bites or feeds on infected host
2. Pathogen enters vector's body
3. Pathogen may or may not replicate in the vector
4. Vector bites new host
5. Pathogen is transmitted during feeding

Environmental and social factors

Several factors influence the transmission of these diseases:

Environmental factors:

• Vector diseases are most common in warm, humid climates
• These conditions favour insect survival and reproduction
• Female Anopheles mosquitoes (malaria vectors) require water to lay eggs
• Climate change may expand the range of disease vectors

Social factors:

• Housing quality affects exposure to vectors
• Access to screens, insect repellents, and bed nets
• Public health infrastructure and disease surveillance
• Education about disease prevention

Diseases spread by vectors

Important vector-borne diseases include:

• Chagas disease: Transmitted by triatomine bugs
• Malaria: Spread by female Anopheles mosquitoes
• Dengue fever: Transmitted by Aedes mosquitoes
• Leishmaniasis: Spread by sandflies (now found in Australian macropods)
• Schistosomiasis: Transmitted by aquatic snails
• Onchocerchiasis: Spread by blackflies
• Canine and feline heartworm: Dirofilaria immitis transmitted by mosquitoes
• Hendra and Nipah viruses: Carried by fruit bats

Case study: equine influenza virus outbreak

This case study demonstrates how understanding transmission is critical for disease control.

Background

Equine influenza (horse flu) is an exotic disease in Australia - it is not normally found here. If it became enzootic (endemic in an animal population), it would devastate the Australian horse industry. The 2007 outbreak demonstrates the importance of rapid response and biosecurity measures.

Cause and symptoms

Pathogen: Equine influenza virus is an orthomyxovirus affecting horses and donkeys. It does not infect humans.

Virus strains:

• Equine-1 (H7N7)
• Equine-2 (H3N8)

Clinical signs:

• Fever ($38.5°\text{C}$ or higher)
• Watery nasal discharge
• Hacking cough
• Loss of appetite
• Muscle pain
• Depression
• Laboured breathing

Transmission pathways

Management of the outbreak

Once diagnosis was confirmed, rapid action prevented catastrophic spread:

Immediate response:

Movement restrictions: The NSW Chief Veterinary Officer imposed a state-wide lockdown on all horse movements, which eventually became nation-wide. This prevented further spread of the virus to unaffected areas.

Coordination centre: A management centre coordinated activities at disease control headquarters in Orange and Menangle, NSW.

Quarantine measures: Horse properties throughout NSW were quarantined, preventing any animals from leaving or entering.

Disease mapping: The spread was carefully tracked. By late August, the virus had reached the NSW Central Coast and Hunter Valley. Areas with high horse populations (such as Dubbo) experienced the fastest disease spread.

Outbreak trajectory

The epidemic curve shows the pattern of new cases over time:

• Cases increased rapidly in the first few weeks
• Peak incidence occurred around weeks $4-6$ with approximately 1000 cases per week
• Numbers declined steadily after the peak
• By week 15, very few new cases were reported

Control of future outbreaks

The Australian Veterinary Association concluded that mass vaccination is not a justifiable option for several reasons:

Why vaccination is not recommended:

• The equine influenza virus mutates similarly to human influenza viruses
• Vaccination would not provide immunity against new strains
• Even vaccinated horses can spread the virus indirectly
• Vaccination might delay disease detection by masking symptoms

Recommended control measures:

This case study illustrates how understanding transmission pathways allows for effective disease control through breaking the chain of infection.

Identifying microbes in food and water

Microbes are present everywhere in our environment - on our bodies, in the air, on surfaces, in water, and in food. While most microbes are harmless or even beneficial, some can contaminate food and water, causing illness.

Detection challenges and solutions

Individual microbes are too small to see with the naked eye. However, when provided with suitable conditions for reproduction and growth, many microbes cluster together, forming visible colonies.

Conditions needed for colony growth:

• Moisture
• Nutrients
• Warmth (typically around $30°\text{C}$)

Laboratory methods:

Colony characteristics

After incubation, visible colonies appear on the agar surface. These colonies can be identified based on distinctive features:

Form (basic shape):

• Circular: Round, even edge
• Irregular: Uneven, asymmetric shape
• Filamentous: Thread-like projections extending outward
• Rhizoid: Root-like, branching appearance

Elevation (cross-sectional shape):

• Raised: Slightly elevated above agar surface
• Convex: Dome-shaped, rounded top
• Flat: Level with agar surface
• Umbonate: Raised centre with flatter edges
• Crateriform: Depressed centre, crater-like

Margin (edge pattern):

• Entire: Smooth, even edge
• Undulate: Wavy edge
• Filiform: Thread-like projections at edge
• Curled: Curved, spiral edge pattern
• Lobate: Lobed, finger-like projections

Additional features:

• Colour: Varies widely - white, yellow, orange, pink, or other colours
• Surface texture: Smooth, dull, shiny, wrinkled, or rough
• Size: Diameter of the colony

Distinguishing bacterial and fungal colonies

Bacterial colonies typically appear:

• Smooth and glossy
• Coloured (various colours)
• Relatively small
• With defined edges

Fungal colonies typically appear:

• Furry or fuzzy
• Large
• Often white or grey
• With spreading, diffuse edges

By examining these characteristics, microbiologists can identify the types of microbes present in food or water samples, helping to ensure safety and detect contamination.

Remember!