Module 7 IQ1

Question 1

Disease

Answer 1

Any condition that impairs the normal functioning of a living thing.

Question 2

Infectious disease

Answer 2

Any disease caused by a pathogenic organism.

Question 3

Non-infectious disease

Answer 3

A disease that is not caused by a pathogen and is not contagious.

Question 4

Pathogen

Answer 4

Any organism that causes disease in another.

Question 5

Microorganism

Answer 5

A microscopic organism, especially a bacterium, or fungus.

Question 6

Macroorganism

Answer 6

Any organism that can be seen with the naked eye (or with a simple lens).

Question 7

Transmission

Answer 7

A method of transmission is the movement or the transmission of pathogens from a reservoir to a susceptible host. Once a pathogen has exited the reservoir, it needs a mode of transmission to the host through a portal of entry. Transmission can be by direct or indirect contact or through airborne transmission.

Question 8

Host

Answer 8

A host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensal guest (symbiont). The guest is typically provided with nourishment and shelter.

Question 9

Vector

Answer 9

An insect that transmits pathogens by biting; an example is an Anopheles mosquito transmitting Plasmodium, which causes Malaria.

Question 10

Symptoms

Answer 10

A feature of a disease that is apparent to the patient.

Question 11

Direct Contact

Answer 11

Transmission by direct contact occurs when there is physical contact between the host and a non-infected organism. There is horizontal (non-related) and vertical (mother to child). Types of Direct contact include; person to person contact which occurs when an infected individual touches or exchanges bodily fluids with another individual, for e.g touching, kissing, sexual contact; and droplet spread which is the result of droplets spray caused by talking, coughing, sneezing. Examples include; HIV/AIDS and Herpes simplex virus (HSV).

Question 12

Indirect Contact

Answer 12

Transmission occurs from a reservoir created by the host outside itself, such as contaminated materials and surfaces or objects (fomite). Types of indirect contact include airborne transmission - some pathogens remain suspended in the air for many hours outside of the body - contaminated objects, food and drinking water (improper treatment/handling), animal to person contact (scratch/bite or handling waste) and vector borne. Examples of disease transmitted in this way include; Measles virus and Influenza.

Question 13

Vector Transmission

Answer 13

A type of transmission that occurs through arthropods (mosquitoes, sandflies, ticks, fleas and flies). It involves a bite from an arthropod that is bloodsucking and transmits the pathogen during a meal. Vector diseases are common in warm, humid parts of the world. Examples include; Malaria & Canine and feline heartworm.

Question 14

Epidemic

Answer 14

An epidemic is an outbreak of (a usually) infectious disease that spreads rapidly to a large proportion of a population in a relatively short period of time.

Question 15

Pandemic

Answer 15

An epidemic that spreads to an entire nation or to other countries. E.g Corona Virus.

Question 16

Describe how you conducted a practical investigation relating to the microbial testing of water or food samples.

Answer 16

Independent - source of water
Dependent - Number & type of microbes
Controlled - Temp of incubator, amount of water used to inoculate, type & amount of agar, type and size of petri dish, time in incubator.

1. Obtain 3 samples of water: samples of water: tap, bottled, pond.
2. Collect four sterilised nutrient agar plates.
3. Seal one for a control.
4. Wipe down work area with methylated spirits, set up bunsen burner.
5. Sterilise inoculating loop by heating on bunsen flame until red hot.
6. Dip the loop into the first sample of water, quickly open the lid of the Petri dish at an angle of no more than 45°. Using a gentle zigzag motion spread the water over the surface of the nutrient agar without digging into the agar, and close the lid as soon as this has been completed.
7. Seal the Petri dish carefully using sticky tape applied to the edge so that there are no gaps. Label.
8. Repeat step 5-7 for all 3 samples.
9. Place your labelled nutrient agar plates upside down into the incubator set at 30°C.
10. Observe change - count no. of bacterial colonies.

Question 17

Inoculation

Answer 17

Introduce (cells or organisms) into a culture medium.

Question 18

How does disease transmission occur during an epidemic?

Answer 18

Factors that influence disease transmission:

• virulence (severity) of the Virus strain
• population density; increased proximity between infected and non infected individuals.
• population mobility in an affected area; more movement = more spread
• host exposure and susceptibility; the more exposure to the disease (e.g no ppe, masks etc) the more likely one is to get infected
• cultural beliefs and behavioural practices
• A continual source of the disease (a reservoir) such a contaminated water supply (cholera), rats and fleas (bubonic plague) or an infected person (AIDS).
• Social and environmental factors such as a contaminated water supply. Limited or no water supply and poor sanitation facilities
• An increase in the amount of a pathogen or new strains of the pathogen that can spread from animals to humans.
• Globalisation increases the spread of disease around the world.
• Delays in diagnosing a disease and isolation or quarantining of infected persons by health authorities.

Question 19

Describe a prion and its key disease mechanism, giving an example.

Answer 19

The term “prion” refers to abnormal, non-cellular pathogenic agents that are transmissible and are able to induce abnormal folding of specific normal cellular proteins called prion proteins that are found most abundantly in the brain. Prions are a defective form of a protein molecule which do not contain DNA or RNA. The functions of these normal prion proteins are still not completely understood. Scientists believe prions likely spread between animals through body fluids like feces, saliva, blood, or urine, either through direct contact or indirectly through environmental contamination of soil, food or water. An example is the degenerative brain disorder Creutzfeldt-Jakob disease.

Question 20

Describe a virus’s key disease mechanism, giving an example.

Answer 20

Viruses are microscopic parasites that lack the capacity to thrive and reproduce outside of a host body, they are non-cellular. They contain DNA, RNA and a protective coat. For transmission of a virus to occur, a virus must enter a host through a portal of entry, replicate or disseminate within the host, and be transmitted to a new host through a portal of exit.
Pathogenic mechanisms of a viral disease include:
(1) implantation of the virus at the portal of entry
(2) local replication
(3) spread to target organs (disease sites)
(4) spread to sites of shedding of virus into the environment.

• Viruses are sometimes able to cross skin barriers by being in the saliva of a biting insect such as dengue fever caused by dengue viruses carried in the bite of an Aedes mosquito and Yellow fever (Flavivirus) in Aedes mosquito.
• Some viruses have viral envelopes which can help the virus to avoid the host immune system e.g herpes simplex virus, chickenpox and influenza.

Common contagious viral diseases include the flu, the common cold, HIV. An example in plants is the Tomato spotted wilt virus.

Question 21

Describe Bacteria’s key disease mechanism, giving an example.

Answer 21

Bacteria are cellular prokaryotic pathogens. They reproduce by binary fission, and have a short generational time, meaning large amounts of them can be produced quickly.

• Some bacteria are obligate, meaning they cannot live or reproduce outside a cell, thus they invade the cells of other organisms, causing diseases.
• Some have a polysaccharide capsule which can act as a virulence factor making them more effective at causing disease.
• parasitic relationship formed by producing toxins/chemicals harmful to host’s body or by damaging host tissue directly –> allows them to continue causing disease.
• Transmission- direct or indirect
• Some bacteria can form an endospore (tough, waterproof external layer) and lie dormant in env for yrs.
• Bacteria may aggregate forming mucus-like structure (biofilm) –> can bind to living tissue and other surfaces –> enhance their defences against antibiotics.

Animal:
Bordetella pertussis causes whooping cough

Plant:
Bacterial soft rot (Pseudomonas spp., Erwinia spp.)

Question 22

Describe Fungi’s key disease mechanism in animals, giving an example.

Answer 22

Fungi are eukaryotic organisms that have a cell wall composed of chitin, and are heterotrophic (produce their own food). They can be unicellular or multicellular and reproduce asexually or sexually. Most fungal infections are opportunistic, affecting those with weakened immune systems or concurrent diseases. Many are dermatophytes.
- Transmission through close contact with organism or contaminated objects

Animal:
Tinea, caused by epidermophyton or trichophyton.
Thrush caused by a yeast, Candida Albicans.

Question 23

Describe Fungi’s key disease mechanism in plants, giving an example.

Answer 23

Sources of fungal infections for plants are infected seed, soil, crop debris, nearby crops and weeds. Fungi are spread by wind and water splash, and through the movement of contaminated soil, animals, workers, machinery, tools, seedlings and other plant material. They enter plants through natural openings such as stomata and through wounds caused by pruning, harvesting, hail, insects, other diseases, and mechanical damage.

Example: Botrytis rots or grey mould.

Question 24

Describe a Protozoans key disease mechanism, giving an example.

Answer 24

Protozoa are unicellular eukaryotes, classified in the kingdom Protista. They usually reproduce through binary fission. 1-300 micrometres size. They are motile.

• Transmission can occur through contaminated water, e.g giardia lamblia causes giardiasis
• Transmission can occur through insect bites, e.g malaria and trypanosomiasis.

Animal:
Malaria - Plasmodium parasites.

Plant:
Phytomonas, a kinetoplastid

Question 25

Describe an Ectoparasites key disease mechanism, giving an example.

Answer 25

Live outside the host’s body. Ectoparasites infest the skin of human beings and other animals. They usually suck blood and inject toxins while feeding. Blood loss => disease. They can also act as vectors for the transmission of other pathogens.

Animals:
Fleas - they can transmit tapeworms in dogs and cause allergic reactions, Ticks - ixodes holocyclus.

Plant:
Nematode diseases. Many important ectoparasites feed on plant roots—dagger nematodes

Question 26

Describe an Endoparasites key disease mechanism, giving an example.

Answer 26

Live inside the host’s body. Intercellular parasites are those that inhabit the spaces of the body of the host. Intracellular parasites are endoparasites that live within the cell of the host. They usually reproduce using eggs (e.g helminths) - the rapid spread of parasitic eggs –> larvae hatch from eggs and mature in host –> or eggs may be deposited into env - allows the disease to continue to spread

Animals:
Heartworms in dogs - type of parastic helminth (block blood flow)

Plant:
Root-lesion nematodes (Pratylenchus species)

Question 27

Koch’s postulates

Answer 27

Criteria for establishing a causative relationship between a microorganism and a disease;

1. Associate - The same micro-organism must be present in every diseased host.
2. Isolate - The micro-organism must be isolated and cultured in the laboratory and accurately described and recorded.
3. Re-isolate - When a sample of the pure culture is inoculated into a healthy host, this host must develop the same symptoms as the original host.
4. Inoculate - The micro-organism must be able to be isolated from the second host and cultured and identified as the same as the original species.

(e.g anthrax bacillus - in the sheep)

Question 28

What are the issues with Koch’s postulates?

Answer 28

Step 2: Not every pathogen is able to be grown in pure culture.
eg 1. bacteria that causes syphilis and leprosy are very difficult to grow in artificial media.
eg 2: Viruses - are intracellular parasites and not able to be grown in pure culture, they have to be grown in cells.
Step 3 and 4: Not always ethical, especially with humans. You can’t go take a healthy human and inject them with a serious pathogen and see that they get a disease. This is why we use animal models however there are some pathogens that affect humans that don’t readily affect animals. Making it difficult to carry out these steps.

Question 29

Pasteur’s experiments - pasteurisation

Answer 29

He found that micro-organisms (bacteria) were responsible for wine, beer and vinegar spoilage. Led to a better understanding of the need for hygiene, sanitation and water treatment to prevent transmission of diseases through indirect contact.
–> he discovered that heating these solutions long enough would kill the contaminating bacteria that was present after fermentation. (pasteurisation)
This practice was soon adopted as a way of preventing the transmission of pathogens which cause diseases such as tuberculosis.

Question 30

Pasteur’s experiments - ‘swan-necked flask’ experiments

Answer 30

He used flasks with long, drawn-out necks that were not sealed.

1. Meat broth was boiled in the flasks and, as they cooled, the air was drawn in from outside.
2. Any micro-organisms present in the air did not reach broth as they were trapped in the narrow neck and the curve of the glass.
3. No bacterial/fungal growth observed.
4. Bacterial growth occurred if the curve of the flask was broken off and the contents exposed to the air.
5. Tipping a flask to allow the solution in it to reach the curve where the microorganisms were trapped resulted in bacterial growth occurring.

This led to an understanding that it is not possible to contract an infectious disease without first having a pathogen transferred between hosts. This helped to uncover the causal relationship between microorganisms and disease. Discredited spontaneous generation.

Question 31

Pasteur’s experiments - Anthrax

Answer 31

Pasteur demonstrated that anthrax - a disease of cattle, sheep and horses, was caused by a rod-shaped bacterium known as Bacillus anthracis. He developed the technique of using a weakened strain of the disease and injecting (inoculating) animals with it.
In a classic experiment, he took 50 sheep, inoculated 25 with this weakened form of the disease. Several days later he inoculated all 50 with a strong dose of the disease. He predicted that the 25 untreated sheep would die from anthrax, and they did. The other 25 survived. Today we call this vaccination.

Question 32

Pasteur’s experiments - silkworms

Answer 32

• Discovered microbes were the cause of a disease in
silkworm eggs that was threatening the silk industry.
• Developed a method (the cellular egg production) to enable preservation of healthy silkworm eggs.

Question 33

Pasteur’s experiments - Vaccine for chicken cholera

Answer 33

• Grew bacteria that caused cholera in chickens.
• Injected chicken with cholera bacteria cultures that
had been left idle for some time.
• Found that these chicken could then be injected
with fresh chicken cholera bacteria and not become
diseased.

Question 34

Pasteur’s experiments - Vaccine for Rabies

Answer 34

• Could not identify the microbe that caused rabies
• Experimented by injecting animals with body fluids from other animals with rabies and found he could prevent rabies from developing in them.
• Successfully used this vaccine on humans (the initial test subjects had been bitten by rabid animals and were expected to soon die.) though he used a milder version of the vaccine which had been passed through rabbits (came from their fluid) in order to allow the humans bodies to recognise the disease but not be severely affected by it.

Question 35

Virulence factor

Answer 35

Molecule produced by a pathogen that increases its effectiveness in invading the host and causing disease

Question 36

Incubation period

Answer 36

The time that elapses between exposure of a person to a pathogen and when symptoms first become apparent.

Question 37

Germ theory of disease

Answer 37

Infectious diseases are caused by microbes.

Question 38

Spontaneous generation

Answer 38

The supposed production of living organisms from non-living matter, as inferred from the apparent appearance of life in some supposedly sterile environments.

Question 39

What was the effect of Pasteurs contributions to vaccine development?

Answer 39

He established the principle of immunity and developed an effective way to prevent infectious disease. This led to an understanding of the role of immunisation in preventing the transmission of many infectious diseases.

Question 40

What were some of Koch’s contributions?

Answer 40

Developed the agar technique for growing micro-organisms –> still used today to culture disease causing micro-organisms.
Identified the microorganism responsible for causing anthrax by isolating and culturing a bacteria found in the blood of sheep that had died from anthrax. He then injected healthy sheep with this bacteria. –> This added further weight to the germ theory of disease as it showed that a micro-organism grown outside the body caused a disease.
Demonstrated that specific microorganisms are responsible for causing specific diseases. Used this principle to develop Koch’s postulates –> still used today to identify causal agents.
Identified the bacteria responsible for tuberculosis and cholera –> led to management strategies for these diseases.

Question 41

Chain of infection

Answer 41

A host that is susceptible
A pathogen that can transmit disease
A mode of transmission

Question 42

endospore

Answer 42

a tough structure surrounding some bacteria that protects them, from environmental stresses during dormant periods

Question 43

Assess the causes and effects of diseases on agricultural production - plant diseases.

Answer 43

Revise table

Question 44

Assess the causes and effects of diseases on agricultural production - animal diseases.

Answer 44

Revise table

Question 45

Compare the adaptations of different pathogens that facilitate their entry into and transmission between hosts.

Answer 45

Revise table