Module 7

Question 1

What is a disease?

Answer 1

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.

Question 2

What causes an infectious disease?

Answer 2

Caused by an infective agent known as a pathogen.

Question 3

Differentiate between signs and symptoms:

Answer 3

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.

Question 4

What is a disorder?

Answer 4

A disease with no known cause

Question 5

What is a communicable disease?

Answer 5

A disease that can be transmitted from plant to plant or animal to animal

Question 6

What is a virulence factor?

Answer 6

A pathogen has a range of strategies to achieve transmission from host to host, and enable effective infection (entry + reproduction)

Question 7

For transmission to occur, what must occur?

Answer 7

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

Question 8

The likelihood of an organism developing an infectious disease is based on:

Answer 8

• the pathogenicity of the microbe (the no. of pathogens)
• the defence capabilities of the host

Question 9

What are the modes of transmission?

Answer 9

A) General transmission
- wind, water (vehicle transmission)
- mosquitoes, fleas (animal vectors)

B) Human to human transmission:
- Direct contact (droplets)
- Indirect contact (fecal-oral, airborne)

Question 10

What is direct method of disease transmission?

Answer 10

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)

Question 11

What is indirect mode of transmission?

Answer 11

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.

Question 12

What is vehicle transmission?

Answer 12

Involves the spread of pathogens by contaminated air, food or water.

Question 13

What is vector transmission?

Answer 13

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

Question 14

What is a fomite?

Answer 14

An object or substance that carries infection.

Question 15

What are some classifications of bacteria?

Answer 15

• 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)

Question 16

What is the transmission of bacterial disease?

e.g Meningococcal Meningitis

Answer 16

• 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.

Question 17

Classification of fungi:

Answer 17

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.

Question 18

What is the transmission of fungal disease?

e.g. Thrush

Answer 18

• 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

Question 19

What is the classification and transmission of protozoa?

E.g. malaria, giardiasis

Answer 19

• 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

Question 20

What are macro-organisms (macroparasites)?

Answer 20

Visible to human eye, can cause disease directly or acts as a vector in the transmission of disease, classified into endoparasites and ectoparasites.

Question 21

What are endoparasites?

Answer 21

Macroparasites that live inside the host’s body, e.g. elephantiasis is caused by a filarial worm.

Question 22

What are ectoparasites?

Answer 22

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.

Question 23

Transmission of helminths

Answer 23

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.

Question 24

What are viruses?

Answer 24

Non-cellular, can only reproduce in a host cell, contain genetic information in the form of nucleic acids

Question 25

Classification of viruses: E.g. Corona Virus, Glandular fever, AIDS

Answer 25

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.

Question 26

Classifications of prions:

Answer 26

- 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.

Question 27

How can prions be transmitted? E.g. Kuru in humans, Scrapie in sheep

Answer 27

- 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

Question 28

What was the historical understanding of the causes of disease?

Answer 28

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)

Question 29

Who was Robert Koch?

Answer 29

Scientist who established the link between a specific microbe and specific diseases. He developed the agar plate for growing microorganisms.

Question 30

What are Koch's postulates?

Answer 30

1. Identify – the same micro-organism must be present in every diseased host (and not present in healthy individuals). 2. Isolate & culture – the micro-organism must be isolated and grown in pure culture in the laboratory and accurately described and recorded. 3. Inoculate – a healthy host inoculated with a sample of the cultured micro-organism must develop the same symptoms as the original host 4. 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.

Question 31

Who was Pasteur and what did he do?

Answer 31

- 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

Question 32

What are the types of plant/animal diseases of concern in Australia's agriculture?

Answer 32

- Endemic diseases: diseases consistently present within a country/region e.g. anthrax in sheep and cattle Exotic diseases: introduced

Question 33

What are the 3 factors that contribute to the development of infectious disease?

Answer 33

Host factors, pathogen factors and environmental factors

Question 34

Host factors explain:

Answer 34

Susceptibility to disease, access to pathogen, concurrent disease or poor nutrition leading to weakened immune response, drought/heatwave stress on the host

Question 35

Pathogen factors explain:

Answer 35

Pathogen's availability, its ability to transfer between hosts, virulence factors including adhesion and invasion of host tissues, and successful establishment inside host tissues

Question 36

Environmental factors explain:

Answer 36

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

Question 37

What are some factors contributing to the risk of disease?

Answer 37

- 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)

Question 38

What are the causes of disease in plants in agriculture?

Answer 38

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

Question 39

What are abiotic factors that cause disease?

Answer 39

- temperature variation - light availability - chemical agents - water quality and quantity - nutrient availability in soils

Question 40

What are the effects of infectious diseases in plants?

Answer 40

- biological effects on the plant - reduced yield of grains, pastures, fruit, veggies - loss of trading opportunities - economic loss for the farmer

Question 41

What are the effects of infectious diseases in farm animals?

Answer 41

- 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

Question 42

What are the adaptations of pathogens to facilitate their transfer?

Answer 42

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

Question 43

Distinguish between pathogen adhesion and invasion:

Answer 43

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

Question 44

Adhesion and invasion strategies in viruses:

Answer 44

Adhesion: viral surface proteins adhere to host cell surface receptors Invasion:

Question 45

Adhesion and invasion strategies in bacteria:

Answer 45

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

Question 46

Adhesion and invasion strategies in fungus:

Answer 46

Adhesion: assisted by cell wall or capsule Invasion: hydrolytic enzymes break down host tissues, allowing invasion

Question 47

Use an example to describe the link between pathogen adaptations that facilitate transmission, and the mode of transmission of a pathogen between hosts:

Answer 47

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