12.3 - Transmission of communicable diseases

Question 1

Direct transmission (A): Direct contact

Answer 1

• Kissing or any contact with body fluids of another person, for example, bacterial meningitis and many sexually transmitted diseases
• direct skin-to-skin contact , for example, ring-worm, athlete’s foot
• microorganisms from faeces transmitted on the hand, for example, diarrhoeal diseases

Question 2

Direct transmission (A): Inoculation

Answer 2

• Through a break in the skin, for example during sex (HIV/AIDS)
• from an animal bite, for example, rabies
• through a puncture wound or through sharing needles, e.g. septicaemia

Question 3

Direct transmission (A): Ingestion

Answer 3

• Taking in contaminated food or drink, or transferring pathogens to the mouth from the hands, for example, amoebic dysentery, diarrhoeal diseases

Question 4

Indirect transmission (A): Fomites

Answer 4

• Inanimate objects such as bedding, socks, or cosmetics can transfer pathogens, for example, athlete’s foot, gas gangrene and staphylococcus infections

Question 5

Indirect transmission (A): Droplet infection (inhalation)

Answer 5

• Minute droplets of saliva and mucus are expelled from your mouth as you talk, cough or sneeze
• If these droplets contain pathogens, when healthy individuals breathe the droplets in they may become infected, for example, influenza and tuberculosis

Question 6

Indirect transmission (A): Vectors

Answer 6

• A vector transmits communicable pathogens from one host to another.
• Vectors are often but not always animals, for example, mosquitoes transmit malaria, rat fleas transmit bubonic plague, dogs, foxes and bats transmit rabies
• Water can also act as a vector of disease, for example, diarrhoeal diseases

Question 7

Transmission between animals and humans (A)

Answer 7

• Example: The bird flu strain H1N1 and brucellosis, which is passed from sheep to people
• Minimising close contact with animals and washing animals and washing hand thoroughly following any such contact can reduce infection rates.
• People can also act as vectors of some animal diseases, sometimes with fatal results, for example, foot-and-mouth disease.

Question 8

Factors affecting the transmission of communicable diseases in animals (A)

Answer 8

• overcrowded living and working conditions
• poor nutrition
• A compromised immune system, including having HIV/AIDS or needing immunosuppressant drugs after transplant surgery
• Poor disposal of waste, providing breeding sites for vectors
• Climate change - this can introduce new vectors and new diseases, for example increased temperatures promote the spread of malaria as the vector mosquito species is able to survive over a wider area
• Culture and infrastructure - in many countries traditional medical practises can increase transmission
• Socioeconomic factors - for example, a lack of trained health workers and insufficient public warning when there is an outbreak of disease can also affect transmission rates

Question 9

Direct transmission (P)

Answer 9

• This involves direct contact of a healthy plant with any part of a diseased plant
• Examples: ring rot, tobacco mosaic virus (TMV), tomato and potato blight, and black sigatoka

Question 10

Indirect transmission: Soil contamination (P)

Answer 10

• Infected plants often leave pathogens (bacteria or viruses) or reproductive spores from protoctista or fungi in the soil
• These can infect the next crop
• Examples: black sigatoka spores, ring rot, bacteria, spores of P.infestans and TMV
• Some pathogens (often as spores) can survive the composting process so the infection cycle can be completed when contaminated compost is used.

Question 11

Indirect transmission: vectors (P)

Answer 11

• Wind : Bacteria, vruses and fungal or oomycete spores may e carries on the wind e.g. Black sigatoka blown between Caribbean islands, P.infestans sporangia form spores which are carries by the wind to other potato crops/tomato plants
• Water: Spores swim in the surface film of water on leaves, raindrop splashes carry pathogens and spores, etc. Examples - spores of P.infestans (potato blight) which swim over films of water on the leaves
• Animals: Insects and birds carry pathogens and spores from one plant to another as they feed. Insects such as aphids inoculate pathogens directly into plant tissues
• Humans: Pathogens and spores are transmitted by hands, clothing, fomites, farming practices and by transporting plants and crops around the world. Example - TMV survives for years in tobacco products, ring rot survives on farm machinery, potato sacks, etc.

Question 12

Factors affecting the transmission of communicable diseases in plants (P)

Answer 12

• Planting varieties of crops that are susceptible to disease
• over-crowding increases the likelihood of contact
• poor mineral nutrition reduces resistance of plants -damp, warm conditions increase the survival and spread of pathogens and spores
• Climate change; increased rainfall and wind promote the spread of diseases, changing conditions allow animal vectors to spread to new areas; drier conditions may reduce the spread of disease