HSC BIOLOGY MODULE 7

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'Germ theory of disease'

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Biology

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'Germ theory of disease'

The belief that disease and decay are the product of living organisms was unknown to people, who still believe in spontaneous generation: that living organisms could arise from inanimate materials such as rotting meat.

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Infectious disease

Caused by another organism or infective agents (pathogen).

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Non-infectious disease

Doesn't involve the transfer of pathogens between the host.

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Prions

An abnormal protein that is capable of causing degenerative diseases of the nervous system that do not contain any genetic material. Pathogenic prions cause disease by inducing abnormal folding patterns in the normal proteins that they come in contact with. The abnormal proteins are deposited within the central nervous system and other organs. Diseases caused by pathogenic prions are known as transmissible spongiform encephalopathies (TSEs). They are called 'spongiform' diseases because the brain tissue is full of holes, resembling a sponge.

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Example: mad cows disease.

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Viruses

It is a non-cellular pathogen and consist of a protective protein coat (called a capsid) that encloses the genetic material, which may be DNA or RNA - this is the infectious part of the virus that contain RNA are known as retroviruses. The viral protein coat contains chemicals that allow the virus to attach to the surface of the host cell. Once the virus has attached to a cell, it enters and takes over the cell's reproductive mechanisms, making many copies of itself. The host cell becomes so full of copies of the virus that it dies and bursts, releasing the new viruses, which repeat the replication process in other cells.

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Example: influenza.

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Bacteria

Are single-celled prokaryotic organisms that have a cell wall but no membrane-bound nucleus or organelles. They reproduce asexually through binary fission and can be classified according to shape; spherical = coccus, rod - shaped = bacillus, spiral = spirillum, comma -shaped = vibrio.

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Bacteria that cause disease produce toxins or chemicals that are harmful to the host's body or damage the host tissue directly. Some bacteria can form endospores (a tough, waterproof external layer) which allows the bacteria to resist heat, chemicals and desiccation.

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Example: whooping cough.

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Protozoa

Are single-celled eukaryotic organisms that are classified along with algae and slime mould in the kingdom protista . They have a membrane-bound nucleus, membrane-bound organelles and cell membrane, but no cell wall. Often reproduce by the process of binary fission. Most are free-living and do not cause disease.

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Type of classification: flagellates which propel by a long whip-like tail called a flagellum, amoebae use projections of the cytoplasm to move around, ciliates have many hair-like projections, called cilia, which propel to protozoan bu beating rapidly , sporozoa are protozoans that do not have structures for motion and reproduce by releasing spores.

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Example: malaria (anaphalis)

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Fungi

Are eukaryotic organisms; they are heterotrophic - have a cell wall with no chlorophyll. Most are saprophytic. They live on dead plant and animal materials . They can be unicellular or multicellular - some fungi like yeast reproduce asexually whilst other fungi may reproduce sexually and asexually.

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Fungi infections can be; cutaneous = outer layer skin, subcutaneous = beneath the skin surface, systemic = affecting internal organs

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Example: tinna

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Macroparasite

Visible to the naked eye. Multicellular eukaryotic organisms. Some cause disease directly, whilst others act as vectors . They are often classified into two groups based on where they live: endoparasites; live inside the body or ectoparasites; live on the outside of the body

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Parasitic arthropods - they are invertebrates that have an exoskeleton and a segmented body; including insects and arachnids. They are ectoparasites. They cause skin irritations, act as vectors for other pathogens and contribute to blood loss and concurrent infections and treatment involves generally involves chemicals. Fleas, ticks, lice, mites, flies, mosquitoes are all examples

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Example: tick or tapeworm

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Direct transmission of infectious disease

Person-to-person contact, droplet spread.

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Indirect transmission of infectious disease

Airborne transmission, contaminated objects, food and drinking water, animal-to-person contact, vector-borne.

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Vector transmission

Indirect transmission of pathogens. Occurs through arthropods such as certain species of mosquitoes, sandflies, ticks, fleas and flies, or through infected aquatic snails. It usually involves a bite from an arthropod that is bloodsucking and transmits the pathogen during a meal, although in some cases, animals swallow the arthropod in the act of grooming themselves.

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Airborne - adaptations to facilitate the transmission of pathogens

Can remain suspended in the air for long periods of time, resists drying out , can cause sneezing or coughing, able to tolerate a wide range of oxygen concentrations.

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Waterborne - adaptations to facilitate the transmission of pathogens

Can colonise and proliferate in water, modified outer surface structures to allow motility, able to tolerate high salinity, many can't be destroyed simply by boiling water.

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Faeco-oral - adaptations to facilitate the transmission of pathogens

Very stable in varied environments, antimicrobial resistance gene.

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Soil-borne - adaptations to facilitate the transmission of pathogens

Form endospores to resist desiccation, stable in the environment under a range of conditions, grow mainly in the root zone, only a few bacteria are soil-borne pathogens of plants.

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Blood-borne - adaptations to facilitate the transmission of pathogens

Takes advantage of altered features or red cells to facilitate growth and development.

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Vertical / sexual - adaptations to facilitate the transmission of pathogens

Transmission across placenta, uterine invasion, unprotected sexual activity, consumption of placenta but other animals in the wild, aerosolised from afterbirth.

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Koch

Developed the agar plate technique for culturing microorganisms. From his research, he determined that each disease is caused by specific microorganisms; a principle he used to identify the specific microorganisms that were responsible for a disease.

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Koch's postulate - The same microorganism must be presented in every diseased host. The micro-organism must be isolated and cultured in a laboratory and accurately described and recorded. When a sample of the pure culture is inoculated into a healthy host, this host must develop the same symptoms as the original host. The micro-organism must be able to be isolated from the second host and cultured and identified as the same as the original species.

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Pasteur

Is credited with creating the science of microbiology. He identified microbes as the agents responsible for spoilage during the production of wine, beer and vinegar, leading to pasteurisation.

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His germ theory; support through his swan-neck flask experiment.

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He contributed to the development of vaccines for fowl cholera. He predicted that spontaneously generated was incorrect and microbes are present in the air and that food spoils when these microbes land and become active; disproved spontaneous generation; microbes were airborne, developed the world's first attenuated vaccine, demonstrated that fermentation was caused by living organisms; yeast, invented pasteurisation; heating liquids at high temps to kill micro.

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Adaptations of pathogens to facilitate their transfer

For an organism to cause disease it must; enter the host, multiply in host tissues, resist or not stimulate hot defence mechanisms, damage the host.

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Prions - adaptations to facilitate adhesion to and invasion of the host by a pathogen

Secreting factors that enable prions to invade follicular dendritic cells in lymphoid tissue. From lymphoid tissue, they invade nervous tissue through the autonomic nerves and travel to the brain, 'piggyback' other proteins such as ferritin to facilitate movement.

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Viruses - adaptations to facilitate adhesion to and invasion of the host by a pathogen

Adhesion - must enter the nucleus of the host cell to facilitate replication of the viral genome. Viral surface proteins adhere to host cell surface receptors and co-receptors.

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Invasion - enveloped viruses are encoded within an envelope endosome formed from the host cell membrane as they move into the cell. Non-enveloped viruses form a pore in the host cell membrane and deliver the viral genome through it.

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Bacteria - adaptations to facilitate adhesion to and invasion of the host by a pathogen

Adhesion - pili and fimbriae, adhesion of the surface of the bacterial cell can resist washing action of secretions, translocation of bacterial proteins cause host cell membrane engulfment of bacteria.

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Invasion - enzymes break down cell contents, capsules resist phagocytosis by host, chemical strategies to destroy host immune defences, toxins are secreted to the damage host cell.

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Protozoan - adaptations to facilitate adhesion to and invasion of the host by a pathogen

Microtubule protrusion into host cell facilitates entry and formation of a vacuolar membrane gives protection from lysosomes. In receptor-mediated attachment, recruits lysosomes fuse with the cell membrane and pathogen enter vacuole made of lysosomal membrane and then deactivate lysosomal enzymes.

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Fungus - adaptations to facilitate adhesion to and invasion of the host by a pathogen

Adhesion - assisted by cell wall or capsule molecules that permit adhesion to host cell

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Invasion - thermotolerance; heat shock proteins are synthesised to cope with the body temperature, converts to parasitic yet when exposed to heat, cell wall and capsules protect fungi from being attacked, secretion of hydrolytic enzyme damages host cell and provides nutrients to fungus, evasion mechanisms; capsule production, suppression of cytokine production and reduce fungicidal power of macrophages.

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Macroparasites - assisted by cell wall or capsule molecules that permit adhesion to host cell

Hookworms; secrete immunomodulatory proteins that reduce host cell immune response. Ticks; molecules are secreted when a tick bites into the skin to prevent vasoconstriction, preventing the host from forming a clot and initiating the inflammatory response.

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Plants response to pathogens

Defences may be passive or active.

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Passive defence - plants

Two major types; physical barriers and chemical barriers.

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Physical barriers include things like thick cuticles, cell walls and small stomata, bark and vertical hanging leaves - inhibit pathogen entry.

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Chemical barriers include things like the presence of chemical compounds in the tissue of plants, enzyme production and chemical receptors - inhibit pathogen entry.

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Active defence - plants

Three major groups; pathogen recognition, rapid active response (minutes to hours), delayed active response (days).

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Plant diseases

Typically caused by various fungi and bacteria - examples include panama disease is caused by a fungus (Fusarium oxysporum) and potato blight is caused by a fungus (Phytophthora infestans).

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Fire blight - plant disease

A plant disease of pome fruit (apples, pears), found in nearly every apple producing country except Australia and Japan. It is caused by the bacterium Erwinia amylovora. It attacks blossoms, leaves shoots, branches, fruit and roots resulting in tissue death and bacterial ooze droplets on infectious tissues. Spread through rain, wind, insects and pruning tools. Prevention is the best management for it, done through: on-farm biosecurity to prevent entry, establishment and spread of pests and diseases amd ensuring all staff and visitors are instructed in and adhere to on-farm hygiene practices.

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Animals response to pathogens

Innate immunity is present at birth and is genetically determined. It response to pathogens are non-specific and include the first line of defence and the second line of defence. All cell and defences involved are genetic and not adapted to specific infections.

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Animal diseases

Typically caused by various viruses and bacteria - examples include newcastle disease is a viral disease and anthrax is caused by a bacterium (Bacillus anthracis).

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Foot and mouth disease - animal disease

FMD is a highly contagious disease of cloven-hoofed animals (cattle, sheep, goat and pig). It also is excluded from Australia as a result of strict quarantine. FMD is caused by the foot and mouth disease virus (FMDV). It causes fever and blisters in the mouth and hooves, learning to serve production losses as even though the infected animal recovers, it is left debilitated.

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Cause and effects of disease in agricultural production

Infectious disease in agriculture - two types of plant/animal disease;

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endemic; disease constantly present within a country or region

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exotic; disease that is introduced.

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An interplay of three factors contributes to the development of infectious disease; host, pathogen and environmental factors.

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Factors that contribute to the risk of infectious disease: increased mobility of the human population, rise of intensive and industrial type agriculture, changing patterns of land use, climate change, antimicrobial resistance, pesticide resistance, loss of genetic diversity, increase in hobby farmers, increase in use of aquaculture as marine and freshwater animal population decrease.

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Fungi - cause of infectious diseases in plant in agriculture

Reservoirs of fungal spores exist in contaminated seeds, farm machinery, soil and nearby weeds, and are generally transmitted by wind, water and contact with the reservoirs through normal farming operations. Fungi enter plants through their stomata or any other opening caused by mechanical damage to the plant, such as pruning and insect bites. They damage the plant by destroying conducting tissues and absorbing nutrients.

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Insects and mites - cause of infectious diseases in plant in agriculture

Insects and mites cause direct damage to plant tissue and act as vectors for other pathogens.

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Bacteria - cause of infectious diseases in plant in agriculture

Reservoirs of pathogenic bacteria may occur in soil, weeds and seeds.

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Humans can also harbour bacteria on their hands and equipment from previous work with a contaminated crop of plants. However, bacteria only multiply and spread when certain conditions are met. These include humid, warm weather, overcrowding of plants, inappropriate soil conditions (water, nutrients, pH and salinity) and poor air circulation.

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Nematodes - cause of infectious diseases in plant in agriculture

The nematode attacks plant roots, creating galls and lumps. The plants subsequently wilt, turn yellow and die. The eggs of these nematodes can persist in the soil for a year and reinfect the next crop. The infestation can be dealt with by repeated cultivation of the soil and exposure to the sun, combined with the removal of residual root material after harvesting to reduce reservoirs of the eggs.

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Viruses - cause of infectious diseases in plant in agriculture

Viruses are stable in the environment and can persist in plant material left over after cropping. They can also form a reservoir on contaminated equipment. Increased plant densities and frequent handling of plants by humans appear to play a role in its transmission.

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Phytoplasma - cause of infectious diseases in plant in agriculture

They are transmitted from plant to plant by insect vectors and inhabit phloem tissue.

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Abiotic factors that cause disease include...

Temperature variation, light availability, chemical agents, water quantity and quality, nutrients available in the soil.

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Effects of infectious disease in plants

Biological effects on the plant, the social and economic effects on the farmer, the social and economic effect on the economy.

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Effects of infectious diseases in farm animals

Death of affected animals, loss of appetite, the onset of weight in a short period of time, economic loss to the farmer, reduction of production and therefore reduction in profit, human illness and disease, the low growth rate in young animals, loss of fertility.

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The first line of defence

Consists of a range of physical and chemical barriers that are non-specific and prevent the entry of pathogens.

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Physical pathogens

Mucous membrane, cilia and peristalsis: Mucous membrane lines the respiratory, urinary and digestive tracts.

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A layer of ciliated epithelial cells lines this mucous, secreting mucus that lubricates the layer below and traps any debris or microbes that enter the body This movement directs the and any trapped microbes out and away from the body's organs; i.e. sneezing.

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Chemical pathogens

Stomach acid/enzymes, lysozyme, alkali pH in the small intestine

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Chemical - Stomach acid and small intestine: Lysosomes within the saliva attack and perforate cell walls of many bacteria. The stomach contains hydrochloric acid which kills most microbes, preventing them from entering your body via food or water.

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Innate immune system - 2nd line of defence

Provides non-specific resistance that attempts to destroy any invading pathogens. Adaptations; inflammation, phagocytosis, fever, cell death.

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2nd line of defence - Inflammation

Small messenger proteins, called cytokines send signals from a site of injury to the bloodstream, where defence molecules and cells move to the site of injury ready for the pathogenic onslaught. This extra volume of blood causes swelling, heat and redness - symptoms of inflammation.

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2nd line of defence - Fever

The cytokines signal the hypothalamus in the brain to increase the body's temperature by a couple degrees, which can then damage the enzymes in bacteria or unravel the RNA folds of a virus.

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2nd line of defence - Phagocytosis

The process by which one cell engulfs another; they recognise and engulf smaller pathogenic invading cells. They have special enzymes (released from the lysosome) that help breakdown the pathogen.

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2nd line of defence - Cell death

A cluster of cells may surround the pathogen and damage tissue, sealing off the pathogen from other areas of the body.

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white blood cells (phagocytosis) - Neutrophils

The first to move to the site of infection to inactivate pathogens. They are short acting and then self-destruct after a few days. They are used in the body to fight acute (short, severe) infections. An increase in circulating neutrophils in the blood is indicative of an active site of inflammation in the body.

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