Module 7 Flashcards
What is a disease?
A disorder of structure or function in a human, animal or plant, which has a known cause and a distinctive group of symptoms, signs or anatomical changes.
What causes an infectious disease?
Caused by an infective agent known as a pathogen.
Differentiate between signs and symptoms:
A sign is an objective measurement that can be made by others, e.g. temperature, rash, wounds.
Symptoms are self reported factors by patients that are not directly observable, e.g. pain, fatigue, headache.
What is a disorder?
A disease with no known cause
What is a communicable disease?
A disease that can be transmitted from plant to plant or animal to animal
What is a virulence factor?
A pathogen has a range of strategies to achieve transmission from host to host, and enable effective infection (entry + reproduction)
For transmission to occur, what must occur?
1) a susceptible host
2) a mode of exiting the infected host
3) survive the transmission - direct, indirect or vector
4) a mode of entry into new host
The likelihood of an organism developing an infectious disease is based on:
- the pathogenicity of the microbe (the no. of pathogens)
- the defence capabilities of the host
What are the modes of transmission?
A) General transmission
- wind, water (vehicle transmission)
- mosquitoes, fleas (animal vectors)
B) Human to human transmission:
- Direct contact (droplets)
- Indirect contact (fecal-oral, airborne)
What is direct method of disease transmission?
Involves the individuals physically transferring the pathogens to another individual.
- occurs when there is physical contact between host and non-infected organism
- person-to-person: touching, kissing, sex
- droplet: droplets containing the infectious agent land on nearby people (coughing, sneezing)
What is indirect mode of transmission?
A disease is passed from an infected person to another person without direct contact. It involves objects being contaminated with pathogens (fomites).
- fecal-oral: faeces are transmitted from an infected.person to another person by mouth
- airborne: infectious agent enters the air when an infected person coughs/sneezes/breathes, remains in the air for a period of time, and them comes in to contact with another person.
What is vehicle transmission?
Involves the spread of pathogens by contaminated air, food or water.
What is vector transmission?
involves animals assisting in the transfer of pathogens between individuals.
- biological vectors: transfer the pathogen from one individual to another, but the pathogen also undergoes part of its lifecycle as a vector i.e. mosquito
- mechanical vector: physically transfer the pathogen from one person to another, without being infected by themselves e.g flies
What is a fomite?
An object or substance that carries infection.
What are some classifications of bacteria?
- single-celled prokaryotic organism with a cell wall but no membrane-bound organelles
- reproduce asexually by binary fission, generation time varies between 10 mins-24 hours, larger than viruses but smaller than protozoans
- some bacteria have a polysaccharide capsule which can act as a virulence factor, making them more effective in disease
- bacteria may be aerobes, anaerobes or facultative aerobes (can switch between both)
What is the transmission of bacterial disease?
e.g Meningococcal Meningitis
- transmission may occur directly through close contact with infected host organism, or indirectly with a bacterium-contaminated object
- some bacteria form an endospore (tough, waterproof external layer) and lies dormant in the environment for years, which can resist heat, chemicals and desiccation.
- bacteria may aggregate, forming a mucus-like structure called a biofilm, they can bind to living tissue, enhancing their defence against antibiotics.
Classification of fungi:
eukaryotic organisms, cell wall made from chitin, are heterotrophic (not capable of making their own nutrients), and are saprophytic (live on dead organic matter), may be unicellular or multicellular, may produce asecually or sexually.
What is the transmission of fungal disease?
e.g. Thrush
- fungal infections can be cutaneous (outer skin layer), sub-cutaneous (beneath the skin surface) or systematic (affecting internal organs)
- most fungal infections are opportunistic (targeting those with a weak immune system)
- may be transferred through direct contact or with contaminated objects
What is the classification and transmission of protozoa?
E.g. malaria, giardiasis
- can be transmitted by insect bites (malaria), or transmitted via faecal-oral route (giardiasis)
- single-celled eukaryotic organism, membrane bound organelles with no cell wall, usually produce by binary fission, most are motile, flagellum propel, cilia propel the protozoa by beating rapidly, pseudopods (projections of the cytoplasm) move around
What are macro-organisms (macroparasites)?
Visible to human eye, can cause disease directly or acts as a vector in the transmission of disease, classified into endoparasites and ectoparasites.
What are endoparasites?
Macroparasites that live inside the host’s body, e.g. elephantiasis is caused by a filarial worm.
What are ectoparasites?
Macroparasites that live on the outside of the body, usually blood sucking, e.g. mosquitoes, fleas, leeches. Some inject toxins while feeding which cause inflammation, it can also act as vectors for transmission of other pathogens.
Transmission of helminths
All helminths reproduce using eggs, which may be deposited in the environment, to be picked up by another host through unsanitary drinking or infected soil. Larvae then hatch from the egg and mature in the host.
What are viruses?
Non-cellular, can only reproduce in a host cell, contain genetic information in the form of nucleic acids
Classification of viruses:
E.g. Corona Virus, Glandular fever, AIDS
Consists of a capsid (protective protein coat) which encloses the (DNA/RNA) - the infectious part of the virus. Some viruses also have a lipid membrane surrounding the capsule.
- Viral protein coat contains chemicals which allow the virus to attach to the surface of the host cell. Once it enters the cell, it invades the cell’s reproductive mechanisms, making many copies of itself. The host cell then becomes so full that it dies and bursts, releasing all the copies of the virus, which repeat the replication process in other host cells.
Classifications of prions:
- abnormal protein capable of causing degenerative diseases of the nervous system
- pathogen prions cause disease by inducing abnormal folding patterns in the normal proteins which they come in contact with.
How can prions be transmitted?
E.g. Kuru in humans, Scrapie in sheep
- ingesting tissue containing infectious prions e.g. nervous tissue
- result of surgery where contaminated equipment was used
- inheriting the mutated genes that code for the infectious prion
What was the historical understanding of the causes of disease?
Prior to the works of Koch and Pasteur, the cause of disease and decay was explained by the theory of spontaneous generation (life arose spontaneously from non-living things). Miasma theory was that ‘bad’ air caused diseases, which was accepted since ancient times. Pasteur was able to disprove this theory by establishing the ‘germ theory of disease’ (microbes cause disease)
Who was Robert Koch?
Scientist who established the link between a specific microbe and specific diseases. He developed the agar plate for growing microorganisms.
What are Koch’s postulates?
- Identify – the same micro-organism must be present in every diseased host (and
not present in healthy individuals). - Isolate & culture – the micro-organism must be isolated and grown in pure culture
in the laboratory and accurately described and recorded. - Inoculate – a healthy host inoculated with a sample of the cultured micro-organism
must develop the same symptoms as the original host - Re-isolate & identify – the micro-organism must be able to be isolated from the
second host and cultured and identified as the same as the original species.
Who was Pasteur and what did he do?
- invented the process of pasteurisation, as he found that bacteria caused wine, beer and vinegar spoilage
- developed vaccines for anthrax and rabies
- applied germ theory to show that silk worm disease associated with specific parasites, saving the silkworm industry
What are the types of plant/animal diseases of concern in Australia’s agriculture?
- Endemic diseases: diseases consistently present within a country/region e.g. anthrax in sheep and cattle
Exotic diseases: introduced
What are the 3 factors that contribute to the development of infectious disease?
Host factors, pathogen factors and environmental factors
Host factors explain:
Susceptibility to disease, access to pathogen, concurrent disease or poor nutrition leading to weakened immune response, drought/heatwave stress on the host
Pathogen factors explain:
Pathogen’s availability, its ability to transfer between hosts, virulence factors including adhesion and invasion of host tissues, and successful establishment inside host tissues
Environmental factors explain:
overcrowding and lack of hygiene leading to a build-up of wastes, which provide a suitable environment for pathogen reservoirs; a favourable environment within the host for pathogens to establish and cause disease
What are some factors contributing to the risk of disease?
- increased mobility of human populations -> travellers, imports
- risk of intensive and industrial-type agriculture
- changing pattern of land use > humans are upsetting the ecosystem balance, which creates risk of disease jumping barriers from animals to humans
- climate change (change in rainfall patterns may favour formation of reservoirs of pathogens in soil)
- pesticide resistance / antimicrobial resistance (overuse/misuse of antibiotics)
- loss of genetic diversity (inbreeding and monoculture practices reduced resilience in a population)
What are the causes of disease in plants in agriculture?
Fungi: reservoirs of fungal spores exist in contaminated seeds, farm machinery, soil and nearby weeds, and are generally transmitted by wind, water, etc.
Insects: cause direct damage as well as act as vectors for pathogens
What are abiotic factors that cause disease?
- temperature variation
- light availability
- chemical agents
- water quality and quantity
- nutrient availability in soils
What are the effects of infectious diseases in plants?
- biological effects on the plant
- reduced yield of grains, pastures, fruit, veggies
- loss of trading opportunities
- economic loss for the farmer
What are the effects of infectious diseases in farm animals?
- death of infected animals
- loss of appetite and weight over a short period of time
- economic loss to the farmer
- loss of international trade
- human illness and disease
What are the adaptations of pathogens to facilitate their transfer?
For an organism to cause disease it must:
- enter the host
- multiply in host tissues
- resist host defence mechanisms
- damage the host
- pathogens have developed many adaptations to gain entry and persist their hosts
- virulence factors and evolutionary strategies
Distinguish between pathogen adhesion and invasion:
Adhesion is the 1st step to establish infection by attaching to host cells or tissues e.g. strep. pyogenes uses M protein to attach to epithelial cells in the throat and helps them resist being washed away by saliva.
Invasion is the 2nd step, once pathogen penetrates host cells, tissues enable it to spread deeper into body. E.g. strep. pyogenes breaches the epithelial layer and invades tissues using enzymes
Adhesion and invasion strategies in viruses:
Adhesion: viral surface proteins adhere to host cell surface receptors
Invasion:
Adhesion and invasion strategies in bacteria:
Adhesion: pili and fimbriae resist washing action of secretions, bacterial cells form a biofilm
Invasion: enzymes break down cell contents, capsules resist phagocytosis by host cells, toxins are secreted to damage host cells
Adhesion and invasion strategies in fungus:
Adhesion: assisted by cell wall or capsule
Invasion: hydrolytic enzymes break down host tissues, allowing invasion
Use an example to describe the link between pathogen adaptations that facilitate transmission, and the mode of transmission of a pathogen between hosts:
Example: Influenza virus
Mode of transmission: Indirect via respiratory droplets released when a person coughs, sneezes of talks. These droplets are inhaled by others, or can settle onto surfaces (fomites) where they may be picked up.
Adaptations:
1) virus has proteins that allow it to adhere to receptors in respiratory tract
2) replicates quickly in respiratory tract causing host to expel it via coughing as it irritate mucus membranes
3) lipid envelope stops it during out on surfaces too quickly
