Chapter 12-12.3- The transmission of communicable diseases

Question 1

What are the 2 main types of transmission (transmission of pathogens between animals)?

Answer 1

Direct transmission- The pathogen is transferred directly from one individual to another by:

• direct contact (contagious diseases)
• Inoculation
• Ingestion

Indirect transmission- Pathogen travels form one individual to another indirectly by:

• Fomites
• Droplet infection (inhalation)
• Vectors

Question 2

Explain the ways direct transmission can occur in animals

Answer 2

Direct contact (contagious diseases):

• Kissing or any contact with the body fluids of another person, for example a bacterial meningitis and many sexually transmitted diseases
• Direct skin-to-skin contact, for example, ring worm, athletes foot
• Microorganisms from faeces transmitted on the hands, for example, diarrhoeal diseases.

Inoculation:

• Through a break in the ski8n, for example, during sex (HIV/AIDS)
• from an animal bite, for example, rabies
• Through a puncture wound or through sharing needles e.g. septicaemia.

Ingestion:

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

Question 3

Explain the way indirect transmission can occurs in animals

Answer 3

Fomites:

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

Droplet infection (inhalation):

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

Vectors:

• A vector transmits communicable pathogens form 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 4

How does transmission between animals and humans occur?

Answer 4

Some communicable diseases can be passed from animals to people, for example the bird flu strain H1N1 and brucellosis, which is passed from sheep to people.

Minimising close 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 5

What are the factors affecting the transmission of communicable diseases in animals?

Answer 5

The probability of catching a communicable disease is increased by a number of factors:

• Overcrowded living and working conditions
• Poor nutrition
• A compromised immune system, including (in humans) having HIV/AIDS or needing immunosuppressant drugs after transplant surgery.
• (In humans) 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 6

How can the spread of communicable diseases in humans be prevented?

Answer 6

Hand washing- regular hand washing is the single most effective way of preventing the spread of many communicable diseases

Improvements in living and working conditions, for example, reducing overcrowding, ensuring good nutrition

Disposal of both bodily and household waste effectively.

Question 7

How does transmission of pathogens happen between plants ?

Answer 7

Plants do not move around, cough or sneeze, yet diseases spread rapidly through plant communities, plant pollen and seed, for example move widely.
Plants also have a less well developed immune system than humans.

Transmission occurs through direct and indirect transmission.

Question 8

How does direct and indirect transmission occur in plants?

Answer 8

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

Indirect transmission-

Soil contamination:
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 are 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.

Vectors:

Wind- bacteria, viruses and fungal or oomycete spores may be carried on the wind, e.g. Black sigatoka blown between Caribbean islands, P.infestans sporangia forms spores which are carried 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 are 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 s 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. For example, TMV survives for years in tobacco products, ring rot survives on farm machinery, potato sacks etc.

Question 9

What are the factors affecting the transmission of communicable diseases in plants?

Answer 9

Planting varieties of crops that are susceptible to disease

Over-crowding increases the likelihood of contact

Poor mineral nutrition reduces resistance of pants

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.

Question 10

What are the key factors in preventing the spread of communicable diseases in plants?

Answer 10

• 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- the 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.