11. Symbionts, Zoonoses And The Origin Of New Epidemics

Question 1

What are the two types of mites?

Answer 1

1. Demodex which is a harmless commensal.

2. Sarcoptes which is an ectoparasite, an itch mite that causes scabies and burrows under the skin.

Question 2

What are symbionts?

Answer 2

Organisms living together

Question 3

What are mutualists?

Answer 3

Symbionts that both benefit each other

Question 4

Why have parasites evolved to be less harmful?

Answer 4

• A parasite is not transferred if the host is killed rapidly
• The parasite benefits if the host is abundant
• Hosts react most strongly to harmful parasites

Question 5

How have hosts evolved to be resistant?

Answer 5

Resistant hosts reproduce more

Question 6

What is the role of common gut bacteria in the body?

Answer 6

• They are either virtually harmless parasite that use some of the food in our gut or commensals that eat what we find indigestible
• Some are mutualists

Question 7

What do the mutualists common gut bacteria do?

Answer 7

• Partly digest food which helps us absorbs it
• Some manufacture B and K group vitamins
• Some produce other useful compounds (stimulate immune system)
• Some feed on harmful excretions of other bacteria

Question 8

What is the role of Bifidus bacteria for the baby immune system?

Answer 8

Bifidus bacteria on the vagina and nipples of lactating women ensures the baby gets Bifidus
- Bifidus protects by decreasing other bacteria

Question 9

What symptoms did the yersina pestis bacterium cause in humans?

Answer 9

• Fever, buboes, prostration, delirium, blackened skin, death

Question 10

What was the mode of spread for the yersina pestis bacteria?

Answer 10

• The bacterium was passed from the rat fleas
• A ball of bacteria grows in the flea’s gut which creates a blockage
• The flea seeks a new host as it is hungry
• The flea sucks harder but fails, instead recoiling the bacteria right into the bite wound

Question 11

How did the bubonic plague demonstrate zoonosis?

Answer 11

The fleas of wild rodents (gerbils) went to black rats and their fleas when to humans

Question 12

What causes the rabies disease?

Answer 12

Large RNA virus that is specific to the nerves

Question 13

What animals does the rabies virus come from?

Answer 13

There are reservoirs in resistant foxes, raccoons, bats

- It passes to dogs then humans

Question 14

What three things affect the spread of disease?

Answer 14

1. Density and movement of hosts
2. Size of host population
3. Nutritional and knowledge status of the host population

Question 15

How does density and movement of hosts affect the spread of disease?

Answer 15

• High density means greater contact between individuals

- Increased movement gives more contact between individuals

Question 16

How does the size of the host population affect the spread of disease?

Answer 16

• A disease needs an ongoing supply of susceptible individuals
  (New births or immigrants)

Question 17

How does the nutritional and knowledge status of the host population affect the spread of disease?

Answer 17

• Lack of protein can impair immune function
• Micronutrients (vit C, zinc) are essential for immune function
• Affects public understanding of how to avoid infection

Question 18

What is horizontal transmission?

Answer 18

Between individuals, or via vector or intermediate host

Question 19

What is vertical transmission?

Answer 19

Between generations, mother to child

Question 20

What do we graph on each axis to map the rate of spread of the disease?

Answer 20

No infected vs time

Question 21

For the graph of the rate of the spread of a disease, what is the rate of spread at a given time determined by?

Answer 21

The slope

Slope = r x N

Question 22

What is r in the graph of the rate of spread of a disease?

Answer 22

R is the intrinsic rate of infection
R = p x C x D
Where P = probability of passing on the disease at each contact, C= effective average number of contacts per year and D= duration of infectiousness

Question 23

How do we know there will be an epidemic?

Answer 23

If R > 1

Question 24

What is the role of ecosystem disturbance in increasing zoonoses?

Answer 24

• Brings humans into contact with new hosts and parasites
• Contact between domestic and wild animals
• Increases nutritional stress in wild populations
• Greater chance of spreading native pathogens

Question 25

What are some ways we can achieve bio awareness?

Answer 25

• Reduce biopollution by keeping exotics out with quarantine
• Increase immunisations and public health measures of ourselves and poor countries
• Address pest and disease problem relating to food production for a growing population
• Minimise the evolution of resistance
• Anticipate climate change effects