C12- Communicable Disease

Question 1

What is meant by communicable disease?

Answer 1

A disease caused by pathogens that can be passed from one organism to another.
Can be the same or different species.

Question 2

Define pathogen and vector with examples.

Answer 2

1. Pathogen - infective, disease causing micro-organisms. Each has their own particular characteristics.
   - bacteria, viruses, fungi, protoctista.
2. Vectors- carry pathogens from one organism to another.
   - water and insects.

Question 3

What are bacteria and how can they be classified?

Answer 3

Prokaryotes. Do not have a membrane bound nucleus or organelles.
- a small proportion of bacteria are pathogens which cause communicable disease.
Can be classified by:
1. Their basic shape.
2. Their cell walls.

Question 4

Explain the shapes of bacteria.

Answer 4

Can be:

• rod shaped (bacilli).
• spherical (cocci).
• comma shaped (vibrios).
• spiralled (spirilla)
• corkscrew (spirochaetes)

Question 5

Explain how bacteria are classified by their cell walls.

Answer 5

The two main types of bacterial cell walls have different structures and react differently with Gram staining.

1. Gram positive bacteria (MRSA):
   - stained with crystal violet.
   - purple/blue under light microscope.
2. Gram negative bacteria (E.coli):
   - stained with safranin.
   - red under light microscope.

Question 6

Advantage of classifying bacteria by cell walls?

Answer 6

The type of cell wall affects how the bacteria react to different antibiotics.(compound that kills/inhibits growth of bacteria)

Question 7

Viruses.

Answer 7

Are non living infectious agents (alot smaller than bacteria).

• structure includes some genetic material (DNA/RNA) surrounded by protein.
• all naturally occurring viruses are pathogenic.
• bacteriophages = viruses that attack bacteria.

Question 8

How do viruses work?

Answer 8

1. Invade living cells, where their viral🧬 is inserted into the host DNA.
2. Reproduce rapidly and evolve by adapting to their host cell.
3. Use the host cell to make more viruses and then burst out of the cell, destroying it and spreading to infect others.

Question 9

How do protoctista work?

Answer 9

Take over cells and break them open as the new generation emerge.
- do not take over genetic material.
- they digest and use the cell contents as they reproduce.
(Eg. Proctists that cause malaria)

Question 10

How do fungi work?

Answer 10

Digest living cells and destroy them.

• combined with the response of the body to the damage caused by the fungus gives the symptoms of disease.
• some produce toxins which affect the host cells and cause disease.
• grow by budding.

Question 11

How do bacteria work?

Answer 11

Most produce toxins that poison/damage host cells in some way:
- some toxins break down the cell membranes.
- some damage/inactivate enzymes.
- some interfere with host cells genetic material do cells cannot divide.
These toxins are a by-product of the normal functioning of the bacteria.

Question 12

Ways of direct transmission of disease between animals?

Answer 12

1. Direct contact:
   - kissing or STIs
   - skin to skin contact (athletes foot/ ring worms)
   - microbes from faeces transmitted on the hands.
2. Inoculation:
   - break in the skin.
   - animal bite (rabies).
   - puncture in the wounds/sharing needles.
3. Ingestion:
   - taking in contaminated food/drink. Or transferring pathogens from hand to mouth.

Question 13

Ways of indirect transmission of disease between animals?

Answer 13

1. Fomites:
   - inanimate objects eg. Bedding,socks,cosmetics.
2. Droplet infections:
   - minute droplets of saliva and mucus are expelled when you talk cough or sneeze. These could contain pathogens and when breathed in by a healthy person can infect them.
3. Vectors:
   - transmit disease from one host to another. Often animals like mosquitoes, rat fleas transmit bubonic plague. Water also acts as a vector (diarrhoeal diseases).

Question 14

Factors affecting the transmission of communicable disease in animals.

Answer 14

• overcrowded living and working conditions.
• poor nutrition.
• compromised immune system ( taking immunosuppressant drugs / aids or HIV)
• poor disposal of waste (provides breeding sites for vectors).
• climate change (introduces new vectors and diseases)
• culture/ infrastructure.
• socioeconomic factors (lack of education, insufficient public warning).

Question 15

Factors affecting transmission of communicable diseases in plants.

Answer 15

• overcrowding (increases likelihood of contact).
• poor mineral nutrition (reduces resistance).
• damp, warm conditions ( increase survival and spread of pathogens).
• climate change.
• planting varieties of crops that are susceptible to disease.

Question 16

Direct transmission of pathogens between plants.

Answer 16

Involves direct contact of a healthy plant with any part of a diseased plant.
Eg. Ring rot, tobacco mosaic virus, tomato and potato blight, black Sigatoka.

Question 17

Indirect transmission of pathogens between plants.

Answer 17

1. Soil contamination: infected plants often leave pathogens/reproductive spores in the soil which can infect the next crop.
2. vectors:
   - wind= bacteria, viruses and spores can be carried in the wind.
   - water= spores swim in the surface of water on leaves. Raindrops carry them.
   - animals= insects and birds carry pathogens and spores from plant to plant when they feed.
   - humans= transmitted by hands, clothing, fomites, farming particles and transportation of crops.

Question 18

Ring rot.

Answer 18

A bacterial disease of potatoes, tomatoes and aubergines caused by gram positive bacterium.

• damages leaves, tubers and fruit.
• can destroy up to 80% of the crop and there is no cure.
• once bacterial ring rot infects a field, it cannot be used to grow potatoes for 2 years min.

Question 19

Tobacco mosaic virus (TMV)

Answer 19

Virus that affects tobacco plants and many other species.

• damages leaves, flowers and fruit.
• stunts growth and reduces yields. Can lead to almost total crop loss.
• resistant crop strains available but no cure.

Question 20

Potato/tomato blight.

Answer 20

• caused by fungus like protoctist.
• the hyphae penetrate host cells, destroying leaves, tubers and fruits.
• no cure but resistant strains, careful management and chemical treatments can reduce infection risk.

Question 21

Black sigatoka.

Answer 21

A banana disease caused by fungus.

• attacks and destroys the leaves.
• hyphae penetrate and digest the cells, turning the leaves black.
• can cause a 50% reduction in yield.
• no cure. Fungicide treatment can control the spread of the disease. Resistant strains are being developed.

Question 22

Tuberculosis (TB).

Answer 22

Bacterial disease of humans, cows, pigs, badgers and deer.

• damages and destroys lung tissue and suppresses the immune system, so the body is less able to fight off other diseases.
• rise in HIV/aids also caused increase in TB as those people are more likely to develop TB.
• curable by antibiotics. Preventable by improving living standards/ vaccine.

Question 23

Non-specific defences of the body to keep pathogens out.

Answer 23

1. skin covers the body- has a skin flora of healthy microorganisms that outcompete pathogens for a space on the body. Also produces sebum, an oily substance that inhibits growth of pathogens.
2. Body tracts lined by mucous membranes- secrete mucus to trap microorganisms and contain lysozymes, which destroy bacterial/fungal cell walls. Mucus also contains phagocytes which remove remaining pathogens.
3. Lysozymes in tears and urine and the acid in the stomach also prevent pathogens entering.
4. Expulsive reflexes- coughs and sneezes eject pathogen-laden mucus from gas exchange system. Vomiting/ diarrhoea expel contents of the gut along with infective pathogens.

Question 24

Blood clotting and wound repair.

Answer 24

When platelets come into contact with collagen in skin or the wall of damaged blood vessels, they adhere and begin secreting several substances.
Most important are: thromboplastin and serotonin.
The clot dries out, forming a hard, tough scab that keeps pathogens out.
-Epidermal cells below scab start to grow, sealing the wound permanently.
-damaged blood vessels regrow.
- collagen fibres deposited to give new tissues strength.
- once new epidermis reaches normal thickness, scab sloughs off and wound is healed.

Question 25

What is thromboplastin and serotonin?

Answer 25

Thromboplastin- an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot.
Serotonin- makes smooth muscle in the walls of the vessels contract so they narrow and reduce supply of blood to that area.

Question 26

What is the inflammatory response?

Answer 26

A localised response to pathogens resulting in inflammation at the site of a wound.
- inflammation is characterised by pain, heat, redness, and tissue swelling.
- mast cells are activated in damaged tissue and release chemicals called histamines and cytokines.
If the infection is widespread, this response can cause a whole body rash.

Question 27

Explain the role of histamines and cytokines in the inflammatory response.

Answer 27

• histamines make the blood vessels dilate, causing localised heat and redness. The raised temp, helps prevent pathogens reproducing.
• histamines make blood vessel walls more leaky so blood plasma is forced out, once forced out of the blood it is knows as tissue fluid. This fluid causes swelling and pain.
• cytokines attract white blood cells (phagocytes) to the site. They dispose of pathogens by phagocytosis.

Question 28

What are the non specific defences of the body to get rid of pathogens?

Answer 28

1. Fevers.

2. Phagocytosis.