U2)KA5) parasites : transmission and virulence

Question 1

what is transmission

Answer 1

• the spread of a parasite to a new host is called transmission

Question 2

what is virulence - what does it cause

Answer 2

the harm that a parasite causes to a host species
- this reduces the hosts evolutionary fitness because the parasite is redirecting the host’s resources to the parasite reproduction

Question 3

how are ectoparasites and endoparasites transmitted

Answer 3

• ectoparasites are generally transmitted through direct contact or by consumption of intermediate hosts
• Endoaparasites of the body tissues are often transmitted by vectors

Question 4

factors which increase transmission rates

Answer 4

• where population of hosts is overcrowded (pass easily to the host eg ( head lice where there are many humans close together )
• presence of abundant vectors
• waterborn dispersal stages

Question 5

how do vectors increase transmission rate

Answer 5

• allows parasite to spread , even when infected host is incapacitated
  eg the mosquito spread plasmodium parasite from people who are immobilised due to malarial fever

Question 6

waterborne dispersal stages in a parasite life cycle are an advantage. Why?

Answer 6

• waterborne dispersal stages allow the parasite to spread even when the host is incapacitated
• they take advantage of the liquid medium to spread to new hosts
  eg - cholera victims are quickly immobilised by the disease , but their waste transmits the bacterium back into the water supply where it can be taken in by new hosts

Question 7

host behaviour is often _____ and ______ by parasites to maximise transmission

Answer 7

exploited

modified

Question 8

example of parasite exploiting(taking benefit of) the natural behaviours of hosts in order to maximise transmission

Answer 8

eg - Schistosoma exploits the behaviour of mammals that wade into lakes. The parasite is released from an aquatic snail(intermediate host) so it has a waterborne dispersal stage which can burrow into the skin of the legs of new hosts in the water

Question 9

the host’s behaviour can be modified - explain

Answer 9

• hosts behaviour can be modified by infection so that the parasite is passed on more easily. in this case the hosts behaviour becomes part of the extended phenotype of the parasite

Question 10

the 5 categories of behaviour modification , and their extended phenotype

Answer 10

-

Question 11

a parasite with a high transmission rate will experience selection pressure for increased virulence , increasing ____ ____ and _____ in the host

Answer 11

parasite growth

reproduction

Question 12

there are three major ways that that endoparasites achieve this: (parasite growth and reproduction)

Answer 12

1) supress the host immune system : so that parasites survive and reproduce in the host without being attacked by the hosts defences
2) modify the size of the host : so that it grows much larger and can support the asexual reproduction of more parasites eg infected mud snails in Asia grow abot 25%
3) reduce the hosts reproductive rate so more of the hosts energy can be directed to parasite reproduction eg Saculinna is a parasitic barnacle that destroys the gonads of its crab host