Parasitism

Question 1

Describe niche.

Answer 1

• An ecological niche is a multi-dimensional summary of tolerances and requirements of a species.
• A species occupies fundamental niche in the absence of any interspecific competition.
• A species occupies realised niche in response to interspecific competition.
• As a result of interspecific competition, competitive exclusion can occur, where the niches of two species are so similar that one declines to local extinction.
• Where the realised niches are sufficiently different, potential competitors can co-exist by
  resource partitioning.

Question 2

Describe the parasitic niche.

Answer 2

• Parasitism is a symbiotic interaction between a parasite and its host (+/-)
• A parasite gains benefit in terms of nutrients at the expense of its host
• Unlike in a predator–prey relationship, the reproductive potential of the parasite is greater than that of the host
• Most parasites have a narrow (specialised) niche as they are very host-specific
• As the host provides so many of the parasite’s needs, many parasites are degenerate, lacking structures and organs found in other organisms
• An ectoparasite lives on the surface of its host, whereas an endoparasite lives within the tissues of its host

Question 3

Describe parasitic life cycles.

Answer 3

• Some parasites require only one host to complete their life cycle
• Many parasites require more than one host to complete their life cycle
• A vector plays an active role in the transmission of the parasite and may also be a host

Question 4

Describe plasmodium life cycle.

Answer 4

1. An infected mosquito (vector) bites a human.
2. Plasmodium enters the human bloodstream.
3. Asexual reproduction occurs in the liver and then in RBCs.
4. When the RBCs burst, gametocytes are released into the bloodstream.
5. Another mosquito bites an infected human and the gametocytes enter the mosquito, maturing into male and female gametes, allowing sexual reproduction to occur.
6. The mosquito can then infect another human host.

Question 5

Describe the schistosome life cycle.

Answer 5

1. Schistosomes reproduce sexually in the human intestine.
2. The fertilised eggs pass out via faeces into water where they develop into larvae.
3. The larvae infect water snails, where asexual reproduction occurs which produces another type of motile larvae.
4. Larvae escape the snail and penetrate the skin of a human, entering the bloodstream.

Question 6

What are viruses?

Answer 6

• Viruses are parasites that can only replicate inside a host cell.
• Viruses contain genetic material (DNA or RNA), packaged in a protective protein coat (capsid).
• Some viruses are surrounded by a phospholipid membrane derived from host cell materials.
• The outer surface contains antigens that a host cell may or may not be able to detect as foreign.

Question 7

Describe the virus life cycle.

Answer 7

1. Infection of host cell with genetic material
2. Host cell enzymes replicate viral genome
3. Transcription of viral genes
4. Translation of viral proteins
5. Assembly
6. Release of new viral particles

Question 8

What are RNA retroviruses?

Answer 8

• Use the enzyme reverse transcriptase to form DNA, which is then inserted into the genome of the host cell.
• Viral genes can then be expressed to form new viral particles.

Question 9

What is transmission?

Answer 9

• The spread of a parasite to a host
• Ectoparasites are generally transmitted through direct contact.
• Endoparasites are often transmitted by vectors or by consumption of intermediate hosts.

Question 10

What is virulence

Answer 10

The harm caused to a host species by a parasite

Question 11

What factors increase transmission rate?

Answer 11

• the overcrowding of hosts when they are at high density
• mechanisms, such as vectors and waterborne dispersal stages, that allow the parasite to spread even if infected hosts are incapacitated.

Question 12

Describe influence of parasites on host behaviour.

Answer 12

• Host behaviour is often exploited and modified by parasites to maximise transmission.
• This includes alteration of host foraging, movement, sexual behaviour, habitat choice or anti-predator behaviour.
• The host behaviour becomes part of the extended phenotype of the parasite.
• Parasites often suppress the host immune system and modify host size and reproductive rate in ways that benefit the parasite growth, reproduction or transmission

Question 13

State non-specific defences against parasites.

Answer 13

• physical barriers
• chemical secretions
• inflammatory response
• phagocytes
• natural killer cells destroying cells infected with viruses

Question 14

What is inflammatory response?

Answer 14

• Injured cells release signalling molecules.
• This results in enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes.

Question 15

What are natural killer cells?

Answer 15

Natural killer cells can identify and attach to cells infected with viruses then release chemicals that lead to cell death by inducing apoptosis.

Question 16

State specific defences against parasites.

Answer 16

• immune surveillance
• lymphocytes
• antibodies
• memory lymphocytes

Question 17

What is immune surveillance?

Answer 17

• A range of white blood cells constantly circulate, monitoring the tissues.
• If tissues become damaged or invaded, cells release cytokines that increase blood flow resulting in non-specific (e.g. phagocytes) and specific white blood cells (e.g. lymphocytes) accumulating at the site of infection or tissue damage.

Question 18

What are lymphocytes?

Answer 18

• Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen
• Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte
• Some selected (B) lymphocytes will produce antibodies, others (T) can induce apoptosis in parasite-infected cells

Question 19

What are antibodies?

Answer 19

• Antibodies possess regions where amino acid sequence varies greatly between different antibodies.
• The variable region gives the antibody its specificity for binding to an antigen.
• When the antigen binds to this binding site the antigen-antibody complex formed can result in inactivation of the parasite, rendering it susceptible to a phagocyte, or can stimulate a response that results in cell lysis.

Question 20

What are memory lymphocytes?

Answer 20

• Initial antigen exposure produces memory lymphocyte cells specific for that antigen that can produce a secondary response when the same antigen enters the body in the future.
• When this occurs, antibody production is enhanced in terms of
  speed of production, concentration in blood and duration.

Question 21

Describe immune evasion.

Answer 21

Endoparasites mimic host antigens to evade detection and modify host immune response to reduce their chances of destruction

Question 22

What is antigenic variation?

Answer 22

• Antigenic variation in some parasites allows them to change between different antigens during the course of infection of a host.
• It may also allow re-infection of the same host with the new variant.

Question 23

Describe latency.

Answer 23

• Some viruses escape immune surveillance by integrating their genome into host genomes, existing in an inactive state known as latency
• The virus becomes active again when favourable conditions arise

Question 24

What is epidemiology?

Answer 24

The study of the outbreak and spread of infectious disease

Question 25

What is herd immunity?

Answer 25

The herd immunity threshold is the density of resistant hosts in the population required to prevent an epidemic.

Question 26

Describe vaccines.

Answer 26

Vaccines contain antigens that will elicit an immune response.

Question 27

Describe challenges in vaccination and treatment.

Answer 27

• The similarities between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite.
• Antigenic variation has to be reflected in the design of vaccines.
• Some parasites are difficult to culture in the laboratory making it difficult to design vaccines.

Question 28

Describe challenges in transmission.

Answer 28

1. Overcrowding
2. Tropical climates - promote large populations of insect vectors which results in parasites being more abundant.

Question 29

Describe improvements in parasite control.

Answer 29

• Civil engineering projects to improve sanitation combined with coordinated vector control may often be the only practical control strategies.
• Improvements in parasite control reduce child mortality and result in population-wide improvements in child development and intelligence, as individuals have more resources for growth and development.