Key Area 5 - Parasitism

Question 1

Niche

Question 2

What is an ecological niche?

Answer 2

An ecological niche is a multi-dimensional
summary of tolerances and requirements of a species.

Question 3

What does a species occupy in the absence of any interspecific
competition?

Answer 3

A species has a fundamental niche that it
occupies in the absence of any interspecific
competition.

Question 4

How is a realised niche occupied?

Answer 4

A realised niche is occupied in response to
interspecific competition.

Question 5

What occurs as a result of interspecific competition?

Answer 5

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.

Question 6

What can occur when the realised niches are sufficiently different?

Answer 6

Where the realised niches are sufficiently
different, potential competitors can co-exist
by resource partitioning.

Question 7

The Parasite Niche

Question 8

What is parasitism?

Answer 8

Parasitism is a symbiotic interaction between a parasite and its host (+/-).

Question 9

What does a parasite gain from the expense of its host?

Answer 9

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.

Question 10

Why do most parasites have a narrow niche?

Answer 10

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.

Question 11

What is an ectoparasite and endoparasite?

Answer 11

An ectoparasite lives on the surface of its
host, whereas an endoparasite lives within
the tissues of its host.

Question 12

Parasitic Life Cycles

Question 13

How many hosts are required for some parasites to complete their life cycle?

Answer 13

Some parasites require only one host to
complete their life cycle.

Many parasites require more than one host to complete their life cycle.

Question 14

What is the definitive host?

Answer 14

The definitive host is the organism on or in
which the parasite reaches sexual maturity.

Question 15

What are intermediate hosts required for the parasite to complete?

Answer 15

Intermediate hosts may also be required for the parasite to complete its life cycle.

Question 16

Explain the role of the vector

Answer 16

A vector plays an active role in the
transmission of the parasite and may also be a host.

Question 17

Explain the human disease malaria is caused by Plasmodium

Answer 17

An infected mosquito, acting as a vector,
bites a human. Plasmodium enters the
human bloodstream. Asexual reproduction
occurs in the liver and then in the red blood
cells. When the red blood cells burst
gametocytes are released into the
bloodstream. Another mosquito bites an
infected human and the gametocytes enter
the mosquito, maturing into male and female gametes, allowing sexual reproduction to now occur. The mosquito can then infect another human host.

Question 17

Explain how schistosomes cause the human disease schistosomiasis

Answer 17

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

Question 18

What are viruses?

Answer 18

Viruses are parasites that can only replicate
inside a host cell.

Question 19

What do viruses contain?

Answer 19

Viruses contain genetic material in the form
of DNA or RNA, packaged in a protective
protein coat.

Question 20

What are some viruses surrounded by?

Answer 20

Some viruses are surrounded by a
phospholipid membrane derived from host
cell materials.

Question 21

What does the outer surface of a virus contain?

Answer 21

The outer surface of a virus contains
antigens that a host cell may or may not be
able to detect as foreign

Question 22

Explain the viral life cycle stages:

Answer 22

Viral life cycle stages: infection of host cell
with genetic material, host cell enzymes
replicate viral genome, transcription of viral
genes and translation of viral proteins,
assembly and release of new viral particles.

Question 22

What do RNA retroviruses use to form DNA?

Answer 22

RNA retroviruses 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 22

Transmission & Virulence

Question 23

What is transmission?

Answer 23

Transmission is the spread of a parasite to a host.

Question 24

What is virulence?

Answer 24

Virulence is the harm caused to a host species by a parasite.

Question 25

How are ectoparasites generally transmitted?

Answer 25

Ectoparasites are generally transmitted through direct contact. Endoparasites of the body tissues are often transmitted by vectors or by consumption of intermediate hosts.

Question 26

List factors that increase transmission rates:

Answer 26

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 27

How is host behaviour often exploited & modified?

Answer 27

Host behaviour is often exploited and modified by parasites to maximise transmission. Alteration of host foraging, movement, sexual behaviour, habitat choice or anti-predator behaviour.

Question 28

What does the host behaviour become part of?

Answer 28

The host behaviour becomes part of the extended phenotype of the parasite.

Question 29

What do parasites often suppress and modify?

Answer 29

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 30

Defence Against Parasitic Attack

Question 31

Immune response in mammals has what two aspects?

Answer 31

Immune response in mammals has both nonspecific and specific aspects.

Question 32

Non-Specific Defences

Question 33

Give examples of non-specific defences

Answer 33

Physical barriers, chemical secretions, inflammatory response, phagocytes, and natural killer cells destroying cells infected with viruses are examples of non-specific defences.

Question 34

Explain the action of physical barriers & chemical secretions

Answer 34

Epithelial tissue blocks the entry of parasites; hydrolytic enzymes in mucus, saliva and tears destroy bacterial cell walls; low pH environments of the secretions of stomach, vagina and sweat glands denatures cellular proteins of pathogens.

Question 35

Explain the action of the inflammatory response

Answer 35

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

Question 36

Explain the action of natural killer cells

Answer 36

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

Question 36

Explain the action of phagocytes

Answer 36

Killing of parasites using powerful enzymes contained in lysosomes, by engulfing them and storing them inside a vacuole in the process of phagocytosis.

Question 37

Specific Cellular Defences

Question 38

What cells constantly circulate & monitor the tissues?

Answer 38

A range of white blood cells constantly circulate, monitoring the tissues.

Question 39

What happens if tissues become damaged or invaded?

Answer 39

If tissues become damaged or invaded, cells release cytokines that increase blood flow resulting in non-specific and specific white blood cells accumulating at the site of infection or tissue damage.

Question 40

What is the action of different lymphocytes in mammals?

Answer 40

Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen.

Question 41

What occurs after an antigen binds to a lymphocyte’s receptor?

Answer 41

Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte.

Question 41

What can some selected lymphocytes produce or induce?

Answer 41

Some selected lymphocytes will produce antibodies, others can induce apoptosis in parasite-infected cells.

Question 42

What do antibodies possess?

Answer 42

Antibodies possess regions where the amino acid sequence varies greatly between different antibodies. This variable region gives the antibody its specificity for binding antigen.

Question 43

What is formed when an antigen binds to a binding site and what does this result in?

Answer 43

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 44

What does initial antigen exposure produce? When this occurs, what becomes enhanced?

Answer 44

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 44

What cells are also formed?

Answer 44

Memory lymphocyte cells are also formed.

Question 45

Immune Evasion

Question 46

What have parasites evolved to evade?

Answer 46

Parasites have evolved ways of evading the immune system.

Question 47

What do endoparasites mimic?

Answer 47

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

Question 48

What does antigenic variation in some parasites allow?

Answer 48

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 49

How might some viruses escape immune surveillance?

Answer 49

Some viruses escape immune surveillance by integrating their genome into host genomes, existing in an inactive state known as latency.

Question 50

When will a virus become active again?

Answer 50

The virus becomes active again when favourable conditions arise.

Question 51

Challenges In Treatment & Control

Question 52

What is Epidemiology the study of?

Answer 52

Epidemiology is the study of the outbreak and spread of infectious disease.

Question 53

What is the herd immunity threshold?

Answer 53

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

Question 53

What can make it difficult to find drug compounds that only target the parasite?

Answer 53

The similarities between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite.

Question 54

What do vaccines contain that will elicit an immune response?

Answer 54

Vaccines contain antigens that will elicit an immune response.

Question 55

What has to be reflected in the design of vaccines?

Answer 55

Antigenic variation has to be reflected in the design of vaccines.

Question 56

Why may it be difficult to design vaccines?

Answer 56

Some parasites are difficult to culture in the laboratory making it difficult to design vaccines.

Question 57

Where may challenges arise?

Answer 57

Challenges arise where parasites spread most rapidly as a result of overcrowding or tropical climates. These conditions make co-ordinated treatment and control programs difficult to achieve.

Question 58

How may overcrowding occur?

Answer 58

Overcrowding can occur in refugee camps that result from war or natural disaster or rapidly growing cities in LEDCs.

Question 59

What may be the only practical control strategies to improve sanitation?

Answer 59

Civil engineering projects to improve sanitation combined with co-ordinated vector control may often be the only practical control strategies.

Question 60

What can improvements in parasite control reduce?

Answer 60

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.