Infectious diseases - bio

What is a disease?

Any condition that interferes with how an organism (or any part of it) functions

What is an infectious disease?

An illness resulting from the invasion of an organism’s tissues by disease causing agents/organisms, and can be transmitted from one host to another

What are pathogens

Microorganisms capable of causing illness.

They cause infectious diseases and spread from one host to another

E.g. bacteria, etc.

What is a zoonotic disease?

Infectious diseases that can be transmitted between vertebrate species

Can be caused by wildlife hunting, e.g. HIV, and live animal markets, e.g. COVID-19

Why is it important to understand the nature of diseases?

• To devise effective prevention strategies

• To implement appropriate measures to safeguard public health

• Essential for identifying and combating infections effectively

• Researchers can develop targeted treatments and preventative measures to control the spread

What is susceptibility?

The likelihood of developing a disease

If the susceptibility of an organism is high, it means that their ability to resist the disease is low

What is virulence?

The ability/measure of a pathogen to cause severe disease within its host

“Disease causing capacity”

What are symptoms?

The effects the pathogen has on the host

What is the incubation period?

The time between the infection and symptoms

What are the types of pathogens?

• Bacteria

• Fungi

• Protists

• Viruses

What is bacteria?

Single-celled organisms (prokaryotic) without a true nucleus or membrane-bound organelles, they form colonies

What are some characteristics of bacteria?

• Circular DNA and simple cellular structure: cell wall, cell membrane, cytoplasm, DNA, ribosome

• Can be categorised based on their shape: cocci, bacilli, spirilia, and their response to gram staining (gram positive and gram negative)

• Some have flagella for movement and pili for attachment

What is fungi?

Eukaryotic organisms with complex cellular structure and distinct organelles, including nucleus, mitochondria, and golgi apparatus

What are some characteristics of fungi?

• Cell walls that contain chitin

• Exists in various forms, e.g. single-celled yeast and multi-cellular molds and mushrooms

• They reproduce through spores; asexually or sexually produced

What are protists?

Diverse group of eukaryotic microorganisms, ranging from single-celled to multicellular.

What are some characteristics of protists?

• Primarily aquatic and can be both free-living and parasitic

• Exhibit a variety of structural characteristics: cilia, flagella, and pseudopodia (for movement and feeding)

• Some protists, like plasmodium, are responsible for causing significant diseases, like malaria.

What is a virus?

Unique entities that straddle the line between non-living and living as they require a host cell to replicate (classified as non-cellular and not living)

What are some characteristics of viruses?

• Consists of genetic material (DNA or RNA) enclosed in a protein coat (capsid)

• Some have additional outer envelopes, derived from the host cell’s membrane, studded with spike proteins

• They lack cellular structures and organelles, making them dependent on host cellular machinery for reproduction

What are the 8 diseases (studied in class)?

• Jarrah dieback

• Malaria

• Ross river virus

• Influenza

• Chytridiomycosis

• Tuberculosis

• Crown gall

• Viral disease of honeybees

what is phytophthora cinnamomi (Jarrah dieback)

A disease caused by the water mould protist phytophthora cinnamomi

This plant pathogen affects plants in Australia, such as Jarrah, banksia, and grass trees by attacking their root system

In WA more than 40% of native plants are susceptible to the disease, (soilbourne), particularly in the SW of WA

• Has potential to be more devastating to the environment than salinity

• Field detection of the disease is difficult until the plant is dead

• The plant communities destroyed cannot be restored and could be lost forever

• Threats to biodiversity, economies, and local communities

• The impact on the plant is swift after transmission

• It is soilbourne and travels faster when it rains

What is the lifecycle of phytophthora cinnamomi?

• Healthy roots become infected as the zoospores swim through the soil water, and attach to root tip cells of susceptible host plants, e.g. Jarrah trees

• Infection begins on the root tips and grows rapidly through the root system. The mycelial threads (from zoospores, growing long, thin, microscopic filaments of cells - hyphae) grow and absorb the plants nutrients

• Sporangium and chlamydospores grow on the mycelium, these release zoospores and once they swim to and infect a plant, they produce long-lived chlamydospores which can survive unfavourable conditions

• When conductive conditions return, the chlamydospores germinate to produce mycelia and sporangia that can infect host roots - they remain in the dormant stage for long periods of time until rain occurs

• Chlamydospores germinate (when it rains), releasing zoospores

• The zoospores are released into the soil and penetrate the roots of a host plant

• The cycle continues

What is the method of transmission of phytophthora cinnamomi?

• Usually indirect through vehicles, e.g. cars and shoes carrying the contaminated soil

• Soilbourne and spreads faster when it rains

• Can be direct from infected to susceptible plants by root-to-root contact

What is the effect of phytophthora cinnamomi on the host?

• Known as “root rot”

• Zoospores adhere to and infect the roots, producing mycelium and rapidly killing it

• Phytophthora mycelium draws the nutrients from plant cells, fueling further growth and reproduction of the pathogen, killing plant cells in the process

• The entire plant dies if the root system becomes extensively infected or if the pathogen reaches the stem/trunk base of the plant, cutting off water and nutrients to the leaves and branches

• Growth of protist reduces the ability of the plant to absorb water and nutrients, death may follow

What is the treatment for phytophthora cinnamomi disease?

• There is no known cure as the impacts are negative, permanent, and irreversible

• Translocation to re-establish in the wild have failed

• Aerial spraying of targeted phosphite treatment: more effective as management not really treatment

What are prevention methods for phytophthora cinnamomi disease?

• Developing area specific management actions to prevent further spread

• Understanding the effects of the pathogen/disease

• Reducing risk of human vectoring with upgrading hygiene and infrastructure

• Raising awareness and ensuring that people aren’t carrying the disease unknowingly

What is malaria?

Caused by unicellular protists from the Plasmodium genus and is life threatening

• There are 6 plasmodium species that can cause Malaria (Falciparum is responsible for the most severe cases)

What is the life cycle of malaria?

1. Zygotes of the malarial parasite plasmodium develop into sporozoites (relatively immature form of plasmodium) in the gut of female Anopheles mosquitoes, then migrate into their salivary gland

2. Sporozoites enter the human blood-stream and more into the liver, this is the mosquito vector blood feed

3. Sporozoites reproduce asexually in liver cells

4. Merozoites (product of asexual reproduction of sporozoites) move into the bloodstream and enter the red blood cells, where they reproduce asexually

5. Merozoites released from red blood cells can infect and multiply in other red blood cells

6. Some merozoites can form male and female gametocytes, these are released into the bloodstream

7. Blood is sucked from an infected human by a female mosquito. The gametocytes travel to the mosquitoes gut where they mature into gametes and fuse to form zygotes (sexual reproduction). The zygotes penetrate and burrow through the wall of the mosquitoes stomach and forms cysts

8. Sporozoites form within the cysts and migrate to the salivary glands of the mosquito, ready to infect a new host

   The cycle continues

What is the method of transmission of malaria

• Indirectly to a host by the bite of a female anopheles vector. The mosquito is the vector that completes the life cycle of the Plasmodium pathogen

• The pathogen requires the mosquito and host

• Only female Anopheles mosquitoes eat blood meals as it aids the development of their fertilised eggs after mating, as a consequence they only transmit malaria

What effect does malaria have on its host?

• Lysis: when infected red blood cells rupture and release merozoites and their metabolic wastes into the bloodstream. Malarial headaches, chills, and a burning fever.

• Ongoing rupture of red blood cells leads to anaemia (lack of red blood cells), lowering amount of oxygen that is transported to the cells. If untreated, may develop into enlargement of liver and spleen

• When transmitted the pathogen, there are no obvious symptoms until the red blood cells are affected

• Children have weaker immunity to the disease and are more likely to develop severe complications, dying from the disease

• Fever

• Lethargy

• Nausea

• Muscle pains

• Cyclical chills

• Sweating

• Pulmonary distress (reduction in number of blood cells)

What is the treatment for malaria?

Anti-malarial drugs: chemical compound used to treat malaria

• Artemisinin: most rapid treatment and targets the young ring stage of the parasite inside red blood cells, preventing from mating further and being released into the blood stream. 90% of patients make a full recovery but it is still expensive for individuals in malaria endemic areas

RTS,S Vaccine: uses recombinant protein to alert the hosts immune system of the parasite, producing antibodies and cells that attack the parasite, preventing it from infecting and developing inside the human liver

Expand on drug resistance and its problem to malaria?

It is influenced by the use of cheap/counterfeit anti-malarial drugs, monotherapies, poor public health infrastructure, and unnecessary use of anti-malarial drugs

The World health organisation recommends combination therapies, such as artemisinin based as it presents less risk of resistance (therefore no dangerous strains developing) as multiple compounds are used to target the parasite

What are prevention strategies for malaria?

• Health education programs (raising awareness)

• Mosquito nets: physical barrier against mosquitoes at night. Insecticide treated nets are most effective, as it also provides a chemical barrier, but must be installed properly (risk of resistance)

• Insecticides and indoor residual spraying (IRS): targets nervous system of insect and reduces transmission to other people

• Biological control: relies on natural interactions, such as predation, and can be important to manage amount of disease carrying insects. E.g. fish eating mosquitoes

How does and will climate change affect the distribution of malaria?

• Changing rainfall patterns and rising global temperatures are already affecting the geographic distribution of certain infectious diseases - Malaria is shown towards higher altitudes in Ethiopia and Colombia in warmer years

• Excessive monsoon rainfall and high humidity was identified as a major influence that enhanced mosquito breeding and survival

• If the global temperature increases by 2-3 degrees Celsius, it would increase the number of people who are at risk of malaria by 3-5% (several hundred million). Also, seasonal duration of malaria would increase in many current endemic areas

• Environmental change of agricultural intensification, dams, canals, and irrigation, all contribute to more breeding sites for mosquitoes and crop insecticides leading to vector resistance.

What is the ross river virus (RRV)?

A small encapsulated single-stranded RNA alphavirus endemic to Australia, Papua New Guinea, and other islands in the South Pacific - caused by viral infection

Zoonotic disease (can travel from marsupial to human - via mosquito vector)

It depends on the location of mosquitoes, which tend to like wet areas

The virus prefers warmer conditions

What is the lifecycle of RRV

• Mosquito takes blood feed containing viruses from marsupials

• The virus replicates inside the mosquito and travels to its salivary glands

• The same female mosquito vector takes another blood feed on a human host and injects its saliva containing viruses into the bloodstream. Primary replication occurs in the skeletal muscles of the human host (where the virus replicates)

• The human-mosquito-human/marsupial transmission is unlikely but possible

• The virus can circulate around marsupials, that act as the reservoir for the virus, via various mosquito vectors

What is the method of transmission for RRV?

• Transmitted by vector: Southern Saltmarsh mosquito

• Other mosquitoes may also serve as vectors

• Female mosquitoes infected with the virus feed on animals (kangaroos and wallabies), and humans - this transfers the virus into the bloodstream and allows it to enter the host’s red blood cells

What is the RRV effect on its host?

• The blood cells that have been infected eventually rupture

• Most patients develop arthritis in their fingers, wrists, and knees

• Joint pain, depression, and fatigue can persist for months after becoming ill

• Other symptoms: Fever, chills, muscle aches, rash, swollen lymph nodes, and more

What is the treatment for the RRV?

• There is no treatment or cure; the symptoms can be alleviated through pain medication (e.g. Panadol) and rest (sleeping more, etc.)

What is the prevention strategies for RRV?

• Prevention is key

• Long clothing (light coloured) and loosely fitted clothes treated with repellent

• Effective use of insect repellents - 20 minutes after sunscreen

• Avoid mosquito prone areas, especially in mornings and evenings

• Flyscreens/bed nets that are properly fitted

• Manage environment, e.g. remove water sources that are breeding grounds for mosquitoes

• Broadscale of spraying insecticides

What is influenza?

Single-stranded RNA viruses that are usually transmitted from a respiratory tract, through the air inside airborne droplets, when an infected host coughs or sneezes

The infectious agents that cause influenza are the influenza A, B, or C viruses - it is a viral infection

Has lipid envelope with protein spikes, and relatively unstable at room temperature

There is a species barrier due mostly to structure of HA protein

HA receptor binding determines the species which different types of influenza are able to infect to different forms of the receptor present on the host cell - provides a considerable species barrier between birds and humans, with pigs being able to be infected by both types of virus and spread to humans

What is the lifecycle of influenza?

• The influenza viruses bind to the surfaces of epithelial cells, typically in the nose throat, and lungs of mammals, and intestines of birds (portal of entry)

• The cell then imports the virus through endocytosis

• Through a series of stages the viral RNA and RNA polymerase are released into the cytoplasm

• The RNA polymerase is then transported to the nucleus of the cell where it begins transcribing viral RNA

• New viruses are assembled in the cells cytoplasm and the viruses are released from the host cell

• Protein synthesis occurs and the host cell dies causing the virus to be released from the cell

• If the host coughs, sings, or sneezes, they release airborne droplets through the mouth and can infect another person (portal of exit)

What is the mode of transmission of influenza?

• Direct transmission: an infected person sneezes mucus directly into the eyes, nose, or mouth of another person

• Airbourne: when someone inhales the aerosols produced by an infected person coughing, sneezing, or spitting

  • Can get into respiratory tract - can spread from about 6 feet away

• Through hand-to-eye, hand-to-nose, or hand-to-mouth transmission, either from contaminated surfaces or direct personal contact, e.g. a hand shake

• There are enough infectious virions in a small droplet to start a new infection

What is the effect of influenza on its host?

• Infectious about 1 day before symptoms, and 5-7 days after symptoms (may be longer for children)

• Incubation period is short, with symptoms appearing 1-4 days after infection

• High fever with sudden onset and extreme fatigue

• Mixture of symptoms of common cold and pneumonia: body ache, headache, and fatigue

• Complications can lead to: viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems (like asthma or heart problems)

What is the treatment for influenza?

• Bed rest

• Fluids

• Pain relief

• Antiviral medication - may reduce severity (e.g. Tamiflu)

What are prevention strategies for influenza?

• Yearly vaccination - as the strain develops the year prior’s vaccination becomes redundant and not useful for combatting the new strain

• Basic hygiene, e.g. covering mouth and nose, washing hands, avoiding public spaces, social distancing, etc, to prevent further spread

What is the viral disease of honeybees?

There are 24 known viruses that affect honey bees, all of which have different effects but may appear to assist in a major problem known as Colony collapse disorder (CCD)

What is Sacbrood disease?

Caused by the sacbrood virus and causes infected larvae to die just before pupation, failing to shed and having a build up of fluid

The virus can be carried by vector nurse bees when contaminated food is fed to the brood of the larvae

The virus replicates inside the larval cells, causing larvae to display unusual behaviour - they turn on their backs and lie stretched out with their heads lifted (once they die they turn light to dark brown)

When humans clean out the dead larvae they can cause the viruses to be disturbed and transported to other worker bees

What is Deformed wing virus (DWV)?

A disease caused by the deformed wing virus, usually transmitted indirectly by the Varroa mite

Has not reached Australia, due to biosecurity measures

In heavily infected varroa colonies, 100% of adult worker bees can be infected with DWV

Some can be asymptomatic, and others have severe symptoms such as:

• Early death of pupae

• Deformed wings

• Shortened abdomen

• Cuticle discoloration in adult bees - which die in 3 days, causing the colony to eventually collapse

What is Colony Collapse Disorder (CCD)?

When the majority of worker bees in a colony disappear and leave behind a queen, plenty of food, and a few nurse bees to care for the remaining immature bees and Queen

It severely threatens human food supply and economic stability by causing widespread losses of honey-bee colonies (which pollinate the crops we eat), as well as impacting the natural ecosystem.

What is the life cycle of the viral disease of honeybees?

• The varroa mite punctures the bee and allows viral disease to enter the bee and its host cells - adult bee with varroa feeds on hemolymph and the mite enters the larva, moving beneath the larva and into bee food

• Replication occurs in the cytoplasm of the host cell

• RNA of the virus is injected into the genome of the host and translated into a polypeptide

• The mite can infect all stages of the bee lifecycle, from eggs to adults

• The mites transfer via close contact between the bees

What is the method of transmission of the viral disease of honeybees?

• The varroa mites are the vectors and spread the diseases between the bees - the mites then feed off the bees and travel with them, often moving onto an uninfected bee

• The mites bite the honey bees, they puncture the skin and this allows the viral diseases to enter the bee - it then enters the bees host cells

What is the effect of the viral disease of honeybees on the host?

• Wing deformities (DWV)

• Colony Collapse disorder (CCD)

• Sacbrood disease

• Shortened lifespans

• Altered or unusual social behaviour

• Increased mortality

• Reduced foraging activity

What is the treatment for the viral disease of honeybees?

• There is no treatment once the bee is infected

What is the prevention for the viral disease of honeybees?

• Replacing brood combs regularly

• Replacing the Queen bee with a resistant strain of bee

• Not breeding from stock showing signs of viral infection

• Minimising nutritional stress of a colony - providing sugar syrup and pollen supplements

• Reducing use of sprays, such as nicotinamides, which reduce bee numbers and add stress to hives

• Quarantine zones

• Removal or destruction of infected hives

• Pre-treatment of hives with pesticide to target the mite

What is tuberculosis?

Caused by mycobacterium tuberculosis, an acid (mycolic acid) fast aerobic bacterium responsible for causing a disease known as tuberculosis

Mycolic acid makes these bacteria impermeable to a lot of strains

Respiratory disease

What is the life cycle of tuberculosis?

• Tuberculosis causing bacteria enters the lungs and is picked up by the macrophages (cells of the immune system) - enters through airborne droplets

• The host ingests the bacteria

• If there are too many bacteria or the macrophage is not strong enough, TB bacteria will breed (multiply) inside of the macrophage

• The macrophages and bacteria form a complex on the lungs, with the bacteria able to remain dormant for years - patients are not contagious and are asymptomatic

• In 5% of infected people, the complexes progress to active infection. The granuloma breaks down and releases bacteria into the body system

• The TB bacilli reproduce quickly, forming a cavity in the tissue where the body’s immune system cannot reach them

• From the cavity, they can quickly spread through the tissue and the person develops symptoms of active TB

• The infected person coughs, transmitting the airbourne droplets, and the cycle continues.

What is the method of transmission of tuberculosis?

• Through the lungs spread through the air when a person with active TB coughs, sneezes, sings, or speaks.

  *A person with latent TB cannot transmit their bacteria because it is still controlled by their immune system

What is the effect of tuberculosis on its host?

Objectively identifiable signs:

• Bloody cough

• Fever

• Scarring of the lungs

Symptoms:

• Fatigue and chest pain

What is the treatment for tuberculosis?

• Treated with a combination of antibiotics taken for about 6-9 months, depends on doctors note - if medication is not taken as prescribed, the bacteria can develop antibiotic resistance to these drugs leading to more dangerous strains (“multi-drug resistant TB” or “extensively drug resistant TB”)

• X-rays

• Blood tests

• Skin test: harmful pieces of tuberculosis causing bacteria are injected into the skin and if within 3 days small red swelling occurs, they are considered to have latent TB infection, active TB disease, or have received vaccine for TB (no reaction may mean no disease or really weak immune system)

What are prevention strategies for tuberculosis?

• Basic hygiene (covering mouth/nose when coughing, etc.)

• Immunisation: BCG vaccination (protects against severe forms of TB)

• Isolating people (or quarantine) with active infections

• Avoid close contact with those infected

What is crown gall?

Disease in plants caused by Agrobacterium tumefaciens and is predominately a soilbourne disease

Unlike phytophthora, it requires a portal of entry; a wound on a plant

Known to affect 140 different plant species (e.g. agricultural plants)

Agrobacterium is a well-known vector that can transfer DNA between itself and plants, therefore why it has become an important tool for genetic engineering

Stonefruit and more agricultural species of plants are more susceptible to the disease

What is the lifecycle of crown gall?

• The bacteria lives in the soil and invades the host through a portal of entry; a wound

• The galls develop by inserting a plasmid into the plant genome which produces a growth hormone - parts of deceased vines become weakened and die; grafts develop at injuries or graft unions.

  • The DNA crosses into the plant cells cytoplasm and into the nucleus, incorporating itself into the genome and producing tumours

  • The DNA carries genes for the biosynthetic enzymes for production of plant hormones - this provides a nitrogen source for the bacteria that most other micro-organisms can’t use, giving them a selective advantage

• Plant cells divide uncontrollably to form the tumours

• Bacteria will multiply and be protected in the gall

• The bacterium can cause necrosis on the roots and persist in vine debris in the soil when the vine is removed - being released into the soil

• In spring, the bacterium can be detected in sap of bleeding vines

• The bacteria is released into the soil and infects the next susceptible host plant

What is the method of transmission of crown gall disease?

• Spread through the soil, the bacteria swim through the soil to the roots

• Soil disturbance increases the movement of the bacteria

• Infects the plant through a wound, usually at ground level or on the roots

• Many nursery plants are infected through grafting and budding scars - it can also enter the plant through mechanical injury to crown or roots, these injuries can be caused by cultivation equipment, insects, or animals

What is the effect of crown gall on the host?

• The plant host has tumours (galls) growing on their roots, stems, or branches

• These tumours can restrict the movement of water and nutrients through the plant

• Plants may appear stressed or have stunted growth

• Additional stress, as well as restriction of vital nutrients and water, may lead to plant death

What is the treatment for crown gall?

• There is no treatment or cure

• The infected plant and surrounding soil must be removed and disposed of

What are prevention strategies for crown gall?

• Prevention is key

• Do not plant any plants in soil that has been previously infected

• Treat wounds on susceptible plants with biological control (bacteria similar to bacterium)

• Sterilise the soil and sanitise all tools/equipment

• Any plants with galls or suspicious swellings at the graft union or near soil level should be destroyed

What is chytridiomycosis?

An infectious disease in amphibians caused by the chytrid Batrachochytrium dendrobatidis - nonhyphal zoosporic fungus

It has been linked to dramatic population declines, or even extinctions of amphibian species around the world

The fungus is capable of causing sporadic deaths in some amphibian populations and 100% mortality in others

Lead to the extinction of at least 4 frog species in Australia

Has contributed to 30% of the global decline of amphibian species

What is the life cycle of chytridiomycosis?

• The fungus predominately affects the outermost layers of the skin containing keratin - the spores burrow into the skin and develop - swimming zoospores infect the amphibian skin and attach by microtubule roots

• When the frog reaches a threshold of 10,000 zoospores they are unable to breathe, hydrate, osmoregulate or thermoregulate correctly

• Zoospores encysts (reabsorbs the flagellum and develops a cell wall).

• The body (zoosporangium) becomes more complex and starts producing new zoospores

• Zoosparangium develops one or more discharge papillae (tubes where new zoospores will be released)

• The cap on the discharge papillae dissolves and mature zoospores are released into the external environment - ready to infect susceptible host and re-infect existing host

*Chytrids lack hyphae, so instead they grow roughly spherical smooth-walled zoosporangium

What is the method of transmission of chytridiomycosis?

• Travels (swims) in moist soils and water

  • The fungus (temperature sensitive) lives in the soil and reproduces asexually only

  • Their zoospores have flagellum which allow them to be motile and swim through water

• Infects host when the skin comes into contact with infected soil or water

• Also contracted from direct contact with infected frogs, human hands/tools or other water animals such as ducks

• Highly contagious

• Indirect transmission is more common (waterborne)

What is the effect of chytridiomycosis on the host?

• Reddening of the ventral skin

• Convulsions with extension of hind limbs (epilepsy)

• Accumulations of sloughed skin over the body

• Slight roughening of the surface with minute skin tags

• Small ulcers or hemorrhage

• Behavioural changes: lethargy, failure to seek shelter, failure to flee, loss of righting reflex, and abnormal posture

• Unable to breathe, hydrate, osmoregulate or thermoregulate correctly

• As the pathogen invades the skin, other layers of keratin are damaged, disrupting: respiration, salt regulation, and osmoregulation

• Amphibians are highly reliant on their skin for processes such as oxygen and carbon dioxide exchange, water absorption, pathogen defense, and electrolyte transportation - therefore the impact is extensive

What is the treatment for chytridiomycosis?

• No effective way of treating the infection

• Trials of treating with salt in infected water has shown promising results

• Antifungal drugs can be effective - but harder for wild populations

What are prevention strategies for chytridiomycosis?

• Prevention is key to controlling the disease

• Monitoring the effects of the disease

• Establishing disease free population of endangered frogs

• Reduce the risk of spread from infected to uninfected areas

What do pathogens need to do to best survive and reproduce?

• Invade the host (have a portal of entry)

• Exploit a nutrient rich area of the host

• Avoid host defense mechanisms

• Replicate

• Exit (have a portal of exit) and transmit to new hosts (mode of transmission)

What is a resevoir?

Site where a pathogen primarily resides and possibly multiplies and grows, they can be living or non-living

Give examples of portals of entry:

• Mucous membranes: Surface membranes moistened with slimy, sticky, and viscous mucus. Found in the human respiratory, gastrointestinal and reproductive tracts (secretes mucus)

• Skin, wounds, eyes, and ears

• Wounded skin: pathogen can penetrate the barrier and use the wound as a portal of entry, e.g. crown gall

*Many pathogens enter the host via the same portal that they exit - e.g. TB enters through respiratory system and infects others from out of the respiratory system

Give some examples of portal of exit:

• Bite/blood feed/saliva

• Digestion (elimination)

• Respiratory system (coughing and sneezing)

• Blood contact

• Reproductive system (e.g. some honeybee viruses passing from queen to worker bee)

What is transmission?

How the pathogen is transferred from the host and/or reservoir to a susceptible host

Without a mode of transmission, the pathogen will die when the host dies

What are the contact types of transmission ?

• Direct: physical touch between infected and susceptible host via skin or body fluids, e.g. Herpes (cold sores)

• Close contact by respiratory droplets: infected host transmits large airbourne droplets a short distance to the susceptible host. The droplets are too heavy to travel suspended in the air currents - pathogen enters mucous membranes, e.g. Influenza

• Fomite: contact with a contaminated inanimate object, the source of contamination is the infected host. A susceptible host will then touch their nose/mouth (mucus membrane) or skin wound, e.g. Tinea (shower)

What are other forms of transmission?

• Airbourne via aerosols: infected hosts may transmit pathogens into the air when coughing, sneezing, or talking. Respiratory droplets (mix of saliva and mucous) shrink to droplet nuclei when water evaporates, leaving light-weight droplet nuclei able to be transmitted. When the nuclei are carried along air currents they are aerosols and can enter the respiratory tracts of susceptible hosts when inhaled

• Vector: living things, usually blood sucking animals, transmit diseases by ingesting pathogens during blood-feed from an infected host and infect a susceptible host during a second blood feed - pathogens replicate inside vectors, e.g. Malaria

• Vehicle/waterborne/soilborne/foodborne: Pathogens are transmitted by moving a common inanimate vehicle, e.g. muddy contaminated tires. Run off from hills can infect areas below, e.g. phytophthora dieback

What interrelated factors impact the spread of disease?

• Growth of pathogen population

• Density of host population

• Mode of transmission

• Also globalisation, urbanisation and climate change

Expand on growth of pathogen population:

• An increase in population can lead to an increase in the spread of disease

• Pathogens like viruses have high rates of replication, which increases their chance of spread

Expand on density of host population:

• An increase in density means an increased rate of spread of the disease

• Transmission may be relatively unaffected by host density and relatively more affected by vector density

• High density populations that have low health care means effective transmission for the pathogen - and vice versa

How does urbanisation relate to density of the host population?

• Urbanisation: Movement of people from rural areas to towns and cities

• Led to higher density living with more people in a particular area at the same time

• More infected individuals will come into contact with susceptible individuals

• Poor living conditions leads to the spread of diseases

Expand on mode of transmission:

• Worriesome infectious agents are those that are very contagious and virulent

• If there are multiple modes of transmission but few hosts or pathogens, then there is a limiting factor and spread will be limited

What is globalisation?

The process where the world is becoming increasingly interconnected as a result of massively increased trade, economics, travel, and cultural exchange

How does globalisation affect the spread of infectious disease?

• The efficiency, speed, and reach of modern transport puts people at risk of the emergence of new strains of familiar diseases and from completely new diseases

• Human migration and tourism is leading to more cases of the movement of both disease vectors and the diseases they carry

What is antibiotic resistamce?

• Either through mutation or gene transfer, a bacterium can gain a gene that provides resistance against the antibiotic

• When the antibiotic is taken again, all but the resistant bacteria die, which then survive and reproduce, passing their resistance to their offspring

• After many generations, a new population has evolved

• This is linked to natural selection as they pass advantageous traits to replicated bacteria

• Observable in a shorter amount of time which leads to the evolution of bacteria

• It is important to only use antibiotics against bacterial infections, otherwise you are introducing antibiotics into your body when you do not need them

What are management strategies used to control the spread of infectious diseases?

• Quarantine

• Immunisation (herd immunity)

• Disruption of pathogen life cycle

• Medications - antibiotics and antivirals

• Physical preventative measures

What is quarantine?

Period of isolation by potentially infected individuals to prevent the spread of a contagious disease

This is until the incubation period and clinical signs and symptoms have passed (and/or negative test results), the organisms suspected of carrying a disease are no longer isolated from local, susceptible populations

What is biosecurity?

A set of strategies that support the prevention of, response to, and recovery from diseases that affect our economy, environment, and health

Used by our biosecurity officers to stop goods and individuals from carrying disease into healthy susceptible populations - Border force

Australia has such strict laws due to the unique flora and fauna, and agricultural species (crops)

What are the measures that are put into place to protect Australia’s agricultural industry and flora + fauna?

1. Inspection of all material brought into Australia

2. Materials and organisms (particularly plants, e.g. soil, seeds, animal/plant products) that display impact of an infectious disease are destroyed or kept in quarantine sections

3. Monitoring for vectors entering Australia (e.g. Varroa mite)

4. Northern Australia on high alert for infectious disease (due to proximity to South-East Asia)

5. Any items, such as tools, are treated before movement between regions

What is immunisation?

The act of building protection from infectious disease by natural infection or use of vaccine - developing further resistance when invaded by the same specific pathogen

Vaccines stimulate active immunity = production of specific antibodies in a susceptible host during its response to a specific pathogen, promoting the formation of memory cells - the immune system is able to recognise the disease and protect the body

Active immunity: natural exposure and infection, and artificial vaccination

Passive immunity: natural maternal antibodies, and artificial monoclonal antibodies

What is Herd immunity?

When a high enough proportion of a population (threshold proportion) is immune to an infectious disease, the fraction who are not immune are (to a large degree) protected from transmission

This can stop diseases from spreading and prevents outbreaks

When a high percentage of the threshold of the population are vaccinated it is difficult for the infectious disease to be spread as there are not many people who can be infected - this is an effective management strategy for preventing epidemics

It also protects those vulnerable members of the population who are unable to be vaccinated, e.g. old people, pregnant women, etc.

But it does not protect against all vaccine preventable diseases, e.g. tetanus

What are the principles of Herd immunity?

1. Critical (high enough) proportion of the host population becomes immune to a specific disease

2. Immunity is usually by an artificial vaccine (or can be gained naturally by recovery from a disease) causing the formation of specific antibodies and memory cells against a specific pathogen

3. This limits the spread of disease as there are 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 those who cannot be vaccinated, and treatment that suppresses the immune system (vaccines containing live viruses)

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

What is the disruption of pathogen life cycle?

• Generally involves a reservoir, portal of exit from infected host, a mode of transmission, a portal of entry, replication, and a susceptible host

• Complex analysis of the biology of the organisms life cycle is crucial to meeting the aim of stopping its spread

• E.g. targeting mosquito larvae - for Malaria

• Scientists hope to eliminate and eradicate diseases by targeting the persistence (ability of pathogen to survive for long periods of time in reservoirs), methods of replication, and modes of transmissions of the pathogen

What are antibiotics?

• Medications that treat bacterial infections and are known as antimicrobial chemicals that inhibit or destroy bacteria targeting structures/processes (only present in bacteria)

• Misuse occurs when antibiotics have been prescribed and full course has not been taken

• Bacteria resistant to antibiotics survive and quickly reproduce, passing their resistance gene

What are antivirals?

Antimicrobial chemicals that inhibit the ability of viruses to replicate by minimising the symptoms and infectivity, shortening the duration of the illness

Examples of antivirals acting on various target points:

1. Inhibit binding/attachment

2. Inhibit entry/penetration (blocks protein channels in the host membrane)

3. Inhibit transcription factors to viral DNA

4. Prevents release of newly assembled virus

What are examples of physical preventative measures?

• Handwashing: can prevent individuals from contracting infections, particularly those spread by fecal/oral routes - most effective with warm water and antiseptic handwash. This kills pathogens, including washing clothes

• Using filtered clean water and food: water should be treated before supplied - pathogens can be carried long distances in water - if unclean water is used to wash food, infectious agents can be transmitted and consumed

• Sanitation: the safe disposal of human excreta as it prevents the oral entry of many infectious diseases, slowing the spread significantly - about 2.6 million people lack this

• Sneeze and cough into elbow: the elbow can act as a barrier to any airborne droplets that exit an infected host during a cough or sneeze - if the hand is used instead it could become a fomite or vehicle and provide pathogen with a mode of transmission

Why are contemporary models in disease epidemiology important?

• They predict disease spread, show the effectiveness of interventions, and find epidemic frequency and location

• This is done through supercomputing - allowing for accuracy and speed

• This helps public health officials make informed decisions

• The models can predict where and when disease outbreaks are likely to occur based on geographical, climatic, and demographic data