Infectious Diseases

Infectious diseases

• A disease is any condition that interferes with how an organism functions

• Diseases are described as infectious (communicable) if they are are caused by a pathogen. A pathogen is a disease causing agent, they can be cellular or non cellular

Non-Cellular Pathogens

• A non - cellular pathogen is not comprised of cells.

Non - cellular: Viruses

• Main example of non - cellular pathogens are viruses. They are made up of a protein coat that contains genetic material (DNA or RNA)

• Viruses do not contain any cellular machinery and are unable to reproduce without host cells

  • Obligate parasites

• To be able to achieve reproduction, the virus binds to a hosts cell and is able to insert its genetic material. This hijacks the cells organelles and forces them to make copies of the viruses coat and nucleic acid. Overtime the virus number increases to a point where it will release viruses into the host to repeat the cycle.

  • This is called the lytic phase

Viral replication - Eukaryotes

1. Attachment

2. Entry of virus nucleic acid (DNA/RNA) into the host cell

3. Viral DNA/RNA enters the nucleus of the eukaryotic host cell

4. Viral DNA/RNA directs the host cell to replicate it and make copies of viral proteins via translation

5. New viral DNA/RNA and proteins assemble at the hosts cell membrane

Viral replication - Bacterial cell

1. Virus particle bind to the wall of host cell and viral DNA enters the cells cytoplasm

2. Viral DNA directs host cell machinery to produce viral proteins and copies of viral DNA

3. Viral proteins are assembled into coasts; DNA is packaged inside

4. Tail fibres and other components are added to coats

5. Host cell undergoes lysis and dies. Infectious virus particles are released

Diseases caused by Virus

1. Ross River

2. Influenza (Type A or B)

3. Honeybee Viruses

4. Australian bat lyssavirus

Cellular Pathogens

• The following pathogens are comprised of cells either unicellular or multicellular. Cells are the basic unit of life, they can interact with their environment, grow and reproduce

Cellular: Bacteria

• Are prokaryotic cells

• Reproduce via binary fission

• No membrane bound organelles

• Circular chromosomes and plasmids

• Some bacteria have a flagellum for mobility

• Some have a bacterial capsule which enables adhesion to tissue or cells

• Some bacteria are able to form endospore → this is a dormant stage where the bacteria can exist for long periods awaiting favourable conditions

• Reproduce via binary fission

• Unicellular

• Other bacteria reproduce via budding or spores

• Either way bacteria asexual reproduction is a rapid process where many generations can pass in a short period of time

Binary fission stages

1. Single chromosome condenses

2. The single chromosome replicates along with some plasmids

3. The chromosomes attach to the cell membrane on opposite poles and are pulled apart as the cell elongates

4. A cleavage furrow forms and develops the cell membrane, separating the two cells

5. A new cell wall forms and the division is complete

Bacteria - Shapes

• Bacteria are often defined cellular shape and cell membrane complexity

• Spherical (coccus/cocci)

• Rod (bacillus/bacilli)

• Spiral

• Vibrio

Bacteria - How they cause disease?

• Once inside a host, bacteria divide rapidly

• Causes disease by:

  • Directly damaging tissues

  • Producing toxins that harm cells

• Some only affect nearby cells while other travel to different parts of the body

• Much larger than viruses

Bacteria - Examples

1. Tuberculosis

2. Crown Gall

Cellular: Fungi

• Fungi are eukaryotes with membrane - bound organelles and a nucleus

• Cell walls made of chitin, not cellulose

• Most fungi do not have a flagella

• Chytrid have a flagella, making them mobile

• Single celled are larger than bacteria

Fungi - Hyphae + Mycelium

• Most fungi grow long, thin filaments that absorb nutrients

  • Increased SA for absorption

• A network of filaments is called a hyphae (singular = hypha)

• Strong chitin walls around each cell

• Hyphae intertwine to form mycelium, which is the main body

• Mycelium can penetrate host tissues to obtain nutrients

Fungi - Reproduction

• Reproduce via spores

• Spores can be produced asecually and secually

• Mature mycelium forms sporangia, which release spores

• When spores lang on a moist food source, they germinate and grow into a new mycelium

Fungi - Example

• Some fungi are pathogenic, causing disease in plants and animals. However, not all cause disease

• A common example in frogs is chytridiomycosis

• As a fungus grows on skin, it produces spores

Cellular: Protists

• Mostly unicellular group of eukaryotic organisms with a membrane bound nucleus

• Many are free living, while others are parasitic

• Can reproduce sexually or asexually, depending on environments

• Protists are structurally very diverse

Protists - Animal-like protists

• Plasmodium is an animal - like protist (protozoan)

• Lacks chlorophyll and a cell wall

Protists - Plant-like protists

• Phytophthora is a plant like protist

• Has a cellulose wall

• belongs to the stramenopiles which have a flagella for movement

• Filaments for nutrient absorption

• Filaments for nutrient absorption, giving a fungus - like appearance

• Classified as an oomycete

Pathogen terminology:

• Pathogen:

  • A disease causing agent

• Pathogenicity:

  • A qualitative measure if an agen can cause a disease

• Virulence:

  • A quantitative scale of a pathogens capacity to cause disease

• Communicable/Infectious:

  • Can be passed on from one host to another

• Transmission:

  • The movement of a pathogen from a host to a new one. Can be direct or indirect

• Symptoms:

  • The subjective signs of infection

• Susceptibility:

  • The likelihood of developing a disease. High susceptibility = low resistance

Zoonoses/Zoonotic

• Pathogens that move from one vertebrate group to another

• Occurs due to constant exposure and the pathogen mutates to infect

• Once the change occurs the new vertebrate group is very susceptible and have higher rates of infection

Incubation period

• Time lag is due to many reasons, for example, may have to divide many times to reach sufficient numbers to cause disease

• Toxins produced by bacteria as waste products of metabolic activity may take time to accumulate to a level that affects the host

• Often contagious before onset of symptoms

Pathogen Life Cycle

• Mode of transmission

• Portal of entry

• Susceptible host

• Infected host reservoir

• Site for replication of infectious agent

• Portal of exit

• Some infectious diseases have another possible reservoir

Modes of Transmission

• Direct

  • Direct

  • Close

  • Reservoir

• Indirect

  • Vecrtor

  • Airbrone droplets

  • Soil/waterborne

PATHOGENS TO MEMORISE

1. Malaria

2. Tuberculosis

3. Ross River Virus

4. Phytophthora Cinnamomi (Dieback)

5. Chytridiomycosis

6. Influenza

7. Corw Gall

8. Bee disease *

Disease Transmission

• All pathogens require a mode of transmission - a way to move from one host to another

• Can occur via a reservoir

• Can be direct or indirect

Direct transmission

• The transfer of pathogen from an infected host to a susceptible host

• Close contact - close proximity between individuals and contact with bodily fluids or skin of the host. Direct contact occurs when tough with an infected host or fluids

• Close is within 1.5m

Indirect transmission

• The transfer of a pathogen from an infected host to a new host via a vector, inanimate object or through air

• Through vectors

• Airborne over larger distance landing on surfaces

• Soil/waterborne acting as a reservoir

Reservoir

A reservoir is an organism or habitat in which a pathogen can reside and replicate beofre entering a susceptible host

Vector

A vector is an agent that transmits pathogens from one host to another - mosquito

Levels of infection

1. Sporadic

2. Endemic

3. Epidemic

4. Pandemic

Factors affecting disease spread

• Spread of disease is the transmission to a susceptible host in a new area or into new populations

• Dependent on several factors

• Outside of these factors others impacting can be globalisation, climate change and host susceptibility

Factors affecting disease spread - Growth of pathogen population

• The higher the amount of pathogens present the higher the rate of infection, therefore greater spread

• Occurs when a pathogen has a fast reproductive cycle. Can also depend on the vector and the way they interact with hosts or their reproduction cycle

Factors affecting disease spread - Density of host population

• The higher the populations density, the higher the spread of disease. Higher densities means more organisms in a given area. When pop. density in high, more susceptible individuals are more likely to make contact with an infected individual.

Factors affecting disease spread - Mode of transmission

• The mode a pathogen uses to infect a host determines its spread and ability to affect more individuals

• Some pathogens are highly contagious and others are highly virulent impacting the spread in a pop.

• Some pathogens have multiple modes of transmission, increasing the capacity to infect more individuals

Factors affecting disease spread - Globalisation

• Globalisation is the process by which the world is becoming increasingly interconnected as a result of increased trade, economics, travel and cultural change

• he efficiency, speed and reach of modern transport puts people and populations at greater risk.

• This has a greater effect when a population has no previous experience with a particular pathogen/disease/strain as if the pathogen gets into the new population. The rate of infection and impact is significant

Factors affecting disease spread - Climate change

• The distribution of mosquito borne diseases are being impacted by global climate changes. Climate change has created conditions that are favourable for mosquito lifecycle and spreading the disease to new areas

Factors affecting disease spread - Antibiotic resistance

• Bacteria are living things which evolve

• Antibiotics are factors that impact bacteria survival, therefore bacteria can evolve to be resistant to antibiotics

• If resistant strains develop they are harder to manage, more likely to spread and therefore have higher rates of infection

Factors affecting disease spread - Poor living conditions

• Poor sanitation and contaminated water → increases transmission of waterbrone diseases

• Overcrowding → increases person to person contact especially airborne

• Poor hygiene and limited healthcare makes prevention and treatment more difficult

• Poor nutrition weakens immune system

• Poor housing and close contact with animals

• Limited electricity to treat water and kill pathogens

Factors affecting disease spread - Lack of adequate healthcare provisions

• Limited health care → increases vulnerability

• Fewer vaccinations → lower herd immunity, allowing easier disease spread

• Limited medicines → harder to treat infections

Pathogen management strategies

• Control measures are a set of strategies that reduce the incidence and duration

1. Quarantine

2. Immunization + Herd immunity

3. Disruption of a pathogen life cycle

4. Medications

Pathogen management strategies - Quarantine

• Quarantine: Separating people who may have been exposed to a disease from the healthy population during the incubation period

• Isolation of Individuals:

  • Prevents potentially infected people from spreading the pathogen to susceptible people

  • People remain isolated until:

    • Incubation period has passed

    • Testing shows no infection

  • Can involve restricting movement, medical monitoring and isolation

• Strict border laws/Biosecurity:

  • Australia has strict quarantine and biosecurity laws because being an island helps prevent exotic diseases and pests entering

  • Potentially infected materials can be confiscated, destroyed, treated or quarantines

  • Food, plant and animal products may be restricted as they can introduce pathogens

Pathogen management strategies - Immunization + Herd immunity

• Immunization is the act of protecting an individual from a disease by the use of a vaccine, it describes the process of an individual developing immunity to a pathogen

• A vaccine is a treatment containing dead, weakened or inactive forms of a pathogen that stimulates a specific immune response. The forms administered often cannot replicate and therefore pose no immediate harm to the organism

• Vaccines develop immunity by inducing a response to the part of the pathogen called an antigen. During infection, antigens exist on the surface of a pathogen and trigger a response, after the response in an organism, it will develop memory cells which are able to be recalled if similar antigens appear.

Herd Immunity

• Once a high proportion of a population develops immunity the smaller, susceptible proportion are protected due to herd immunity

• High proportion threshold varies based on the pathogen. herd immunity does reduce the rate of spread and infection, aiming to protect those not vaccinated

• Principle is that in enough are immune, an infected individual is unlikely to make contact with a susceptible individual and that pathogen cannot be transmitted.

Herd Immunity - Principles

1. A critical proportion of host population becomes immune to a specific disease

2. Immunity is usually by an artificial vaccine (or through recovery) causing the formation of specific antibodies and memory cells to a specific pathogen

3. Limits the spread of the disease as there are too few susceptible people to sustain the spread. The pathogen cannot reproduce at a high enough rate to sustain its population

4. Infected hosts carrying the pathogen are more likely to come into contact with immune individuals, reducing the possibility of transmission and reducing the risk for susceptible people

5. The higher the proportion of immune individuals, the greater the protection

6. Protects people who cannot be vaccinated, such as those anaphylactic and pregnant women, or those undergoing a treatment that suppresses immune response

7. The proportion of the population who need to be immune to create herd immunity depends on the virulence of the pathogen.

Pathogen management strategies - Disruption of a pathogen life cycle

• Understanding the life cycle of a pathogen can help scientists work out how to prevent and control disease spread

• *Refer to the pathogen life cycle from earlier*

• Many from the life cycle and be targeted to break cycle and decrease pathogen survival

• When one of three interrelated factors influencing the disease spread stops. Three factors are:

  • A susceptible host in sufficient density

  • Growth of a virulent pathogen population

  • Transmission

Disruption of a pathogen life cycle - Malaria

• Mosquitoes have been targeted as a control measure, but high activity has caused low effectiveness

• Targeting the human host through early detection and medication has potential but due to increased poverty it has prevented this approach from breaking the cycle

• Insecticides are most effective, but resistance has evolved

• Simple measures like nets are effective but low education has lowered effectiveness

Pathogen management strategies - Medications

1. Antibiotics

2. Antivirals

Medications - Antibiotics

• Antimicrobial chemicals that inhibit or destroy bacteria. They target structures or processes only present in bacteria

• They often target prokaryotic structural features of bacteria so there is little effects on other organisms due to eukaryotes having a different structure.

• Drugs used to treat bacterial diseases can be grouped into categories based on their modes of action

  • Streptomycin: Disrupts protein synthesis

  • Penicillin: Disrupts cell wall growth

  • Clotrimazole: Disrupts cell membrane permeability

Antibiotics targets

1. Protein synthesis prevention → prevents cells from producing proteins

2. DNA replication→ prevent cell from replication

3. Destruction of cell membrane and cell wall

4. Block metabolism, therefore death

Antibiotic resistance

• Antibiotics have been overused and misused

• Misuse occurs when antibiotics have been prescirbed and the full course is not taken

• Resistant bacteria have evolved rapidly. Antibiotics provide a survival advantage to bacteria populations with antibiotic resistance. Resistance occurs through random mutations

• They reproduce and ovetime entire populations can become resistant

• Bacteria can also complete gene transfer, increasing resistance spread in a pop. Horizontal gene transfer occurs through conjugation and transformation

Medications - Antivirals

• Are antimicrobial chemicals that inhibit the replication of viruses

• Viruses are difficult to manage due to being obligate parasites that require host cells

Antivirals targets

1. Inhibit entry into host cells

2. Inhibit binding to host cells

3. Inhibit transcription by blocking transcription factors or binding to viral DNA/RNA

4. Prevent the release of the viral cells from the host cells.

Physical Preventative Measures

1. Handwashing:

   • Regular and effective handwashing methods reduce disease transmission

   • Most handwashing uses an antiseptic or antimicrobial agent to prevent the pathogen from existing

2. Clean water:

   • Treating water to reduce the existence of pathogens that are transmitted

3. Sanitation:

   • The safe removal of waste products

   • Can be linked to clean water

4. Elbow sneezing:

   • To prevent spread people sneeze into their elbow to reduce certain pathogens from spreading

5. Barriers:

   • Physical barriers to reduce the contact with pathogens or vectors. Masks, shields and mosquito nets

Virus lifecycle

• Protein coat

• Nucleic acid

• Non-cellular

• Very small

• Obligate parasites

Deformed WIng Virus

• DWV is transmitted by Varroa mite. The ite is a vector and moves the pathogen into the host population.