12.3 TRANSMISSION OF COMMUNICABLE DISEASES

Question 1

what are two types of transmission of communicable diseases?

Answer 1

• direct
• indirect

Question 2

how can diseases be transmitted directly in animals?

Answer 2

• direct contact (contagious diseases)
• inoculation
• ingestion

Question 3

what is direct contact?

Answer 3

• kissing or any contact with the body fluids of another person e.g. bacterial meningitis and many sexually transmitted diseases
• direct skin-to-skin contact e.g. ring worm, athletes foot
• microorganisms from faeces transmitted on the hands e.g. diarrhoeal diseases

Question 4

what is inoculation?

Answer 4

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

Question 5

what is ingestion?

Answer 5

• taking in contaminated food or drink or transferring pathogens to the mouth from the hands e.g. amoebic dysentery, diarrhoeal diseases

Question 6

how can diseases be transmitted indirectly in animals?

Answer 6

• fomites
• droplet infection (inhalation)
• vectors

Question 7

what are fomites?

Answer 7

• inanimate objects such as bedding, socks or cosmetics can transfer pathogens e.g. athletes foot

Question 8

what is droplet infection?

Answer 8

• 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 e.g. influeza, TB

Question 9

what are vectors?

Answer 9

• a vector transmits communicable pathogens from one host to another
• vectors are often but not always animals e.g. mosquitoes transmit malaria
• water can act as a vector of disease e.g. diarrhoeal diseases

Question 10

how is the probability of catching a communicable disease increased in animals?

Answer 10

• overcrowded living and working conditions
• poor nutrition
• compromised immune system
• poor disposal of waste, providing breeding sites for vectors
• climate change (introduce new vectors and new diseases
• culture and infrastructure
• socioeconomic factors

Question 11

what is direct transmission in plants?

Answer 11

• involves the direct contact of a healthy plant with any part of a diseased plant
• e.g. ring rot, tobacco mosaic virus, tomato and potato blight, and black sigatoka

Question 12

what is involved in indirect transmission in plants?

Answer 12

• soil contamination
• vectors

Question 13

what is soil contamination?

Answer 13

• infected plants often leave pathogens or reproductive spores from protoctista or fungi in the soil
• these infect the next crop
• some pathogens (often as spores) can survive the contamination process so the infection cycle can be completed when contaminated compost is used
• e.g. black sigatoka spores and ring rot bacteria

Question 14

in plants, what are the common vectors of disease?

Answer 14

• wind
• water
• animals
• humans

Question 15

how does wind act as a vector of disease in plants?

Answer 15

• bacteria, viruses and fungal or oomycete spores may be carried on the wind
• e.g. black sigatoka blown between Caribbean islands

Question 16

how does water act as a vector of disease in plants?

Answer 16

• spores swim in the surface film of water on leaves; raindrop splashes carry pathogens and spores, etc.
  -e.g. spores of potato blight which swim over films of water on leaves

Question 17

how do animals act as a vector of disease in plants?

Answer 17

• 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

Question 18

how do humans act as a vector of disease in plants?

Answer 18

• pathogens and spores are transmitted by hands, clothing, fomites, farming practices and by transporting plants and crops around the world
• e.g. ring rot survives on farm machinery

Question 19

how is the probability of catching a communicable disease increased in plants?

Answer 19

• 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 new diseases)

Question 20

how can spread of diseases in plants be reduced?

Answer 20

• leave plenty of room between plants to minimise the spread of pathogens
• clear fields as thoroughly as possible (remove all traces of plants from the soil at harvesting)
• rotate crops (spores or bacteria will eventually die if they do not have access to the host plant)
• follow strict hygiene practices (measures such as washing hands, washing boots, sterilising storage sacks, washing down machinery, etc)
• control insect vectors