Unit 2- Parasitism

Question 1

Definition of an ecological niche

Answer 1

A complex outline of tolerances and requirements of a species.

Question 2

Definition of the fundamental niche

Answer 2

The set of resources that an organism is capable of using in the absence of any competition with other species.

Question 3

Definition of the realised niche

Answer 3

The set of resources that an organism actually uses in response to the presence of interspecific competition.

Question 4

Define the competitive-exclusion principle

Answer 4

The observation that in some situations, where two species are competing the realised niche of the two species may be very similar. One species will lose out more in the competition and so its population will decline leading to local extinction.

Question 5

Define resource partitioning

Answer 5

When two species living in the same habitat have similar requirements but their realised niches are sufficiently different for them to co-exist. In this case, resource partitioning can occur whereby the two species exploit different components of the resource, thus reducing potential competition.

Question 6

Why do parasites tend to have a very narrow niche

Answer 6

They have very high host specificity.

Question 7

Why are many parasites said to be degenerate?

Answer 7

The host provides so many of the parasite’s needs that they are lacking in structures and organs found in other organisms.

Question 8

What is the niche for an ectoparasite?

Answer 8

On the surface of its host e.g. lice and leeches

Question 9

What is the niche for an endoparasite?

Answer 9

Inside the host. They can either infect body cavities or the tissue.

Question 10

What is the difference between the definitive and the intermediate host?

Answer 10

The definitive host is where the parasite reaches sexual maturity and the intermediate host, the parasite requires in order to complete its lifecycle.

Question 11

What are vectors?

Answer 11

Species that play an active part in the transmission of the parasites.

Question 12

Define transmission

Answer 12

The spread of a parasite to a new host.

Question 13

Define virulence

Answer 13

The harm that a parasite causes to a host species.

Question 14

What is the extended phenotype of the parasite?

Answer 14

Modifying host behaviour so that the parasite is passed on more easily.

Question 15

Name three factors that increase virulence?

Answer 15

1) Suppressing the host immune system- this means the parasites can survive and reproduce in the host without being attacked by the host’s defences.
2) Modify the size of the host- this allows the host to grow much larger so it can support the asexual reproduction of more parasites.
3) Reduce the host’s reproductive rate- this allows more of the host’s energy to be directed towards parasite reproduction.

Question 16

What are the stages of a basic viral attack?

Answer 16

Basic Viral Attack

1) Virus antigens attach to host cell surface.
2) Virus DNA is inserted into the host cell.
3) Virus DNA is replicated by host enzymes.
4) Virus genes are transcribed to RNA which is translated to make viral protein coats.
5) New virus particles are assembled and burst out of host cell.

Question 17

How do retroviruses work?

Answer 17

They use the enzyme reverse transcriptase to form DNA, which is then inserted into the genome of the host cell. This virus gene forms new viral particles when transcribed. There are various antiretroviral therapies that interrupt several stages of the lifecycle that can be used to manage the virus.

Question 18

What is zoonosis?

Answer 18

This is where a parasite moves from species to species.

Question 19

Name examples of non-specific first line immune responses

Answer 19

• The skin acts as a physical barrier against parasites and it also dry so many microbial parasites die on the surface.
• There are also chemical secretions such as mucus in the lungs which traps microbial parasites. Other chemical secretions include nasal mucus, earwax and tears which have the lysozyme enzyme to defend against microbial parasites. The skin and stomach also have acid secretions that kill most microorganisms.

Question 20

Name examples of non-specific second line immune responses

Answer 20

• There is an inflammatory response which happens when histamine is released from infected/ damaged cells. This causes a localised increase in blood flow creating a hot area which is basically a localised fever. This also increases the movement of cellular responses.
• There is also a cellular response by phagocytes and natural killer cells.
• Phagocytes are white blood cells which are able to migrate out from the blood into the tissue fluid which surrounds cells. The phagocytes can engulf bacteria, viruses and dust particles. The phagocytes check the surface of cells and particles for ‘self’ antigens. If these are not found, then the phagocyte engulfs and digests the foreign object.
• Natural killer cells are also white blood cells that can migrate into the tissue fluid. They can detect the abnormal cell-surface proteins found on virus-infected cells and on cancerous cells.
• The natural killer cells do not engulf the stricken cell but, instead attach to it and release chemicals into it. These chemicals induce apoptosis. Phagocytes then engulf and digest the resulting cell debris.

Question 21

Explain the process of a specific immune response

Answer 21

1. Immune surveillance is carried out by white blood cells.
2. T lymphocytes will recognise foreign antigens from pathogen that are displayed on the surface of infected cells.
3. T lymphocytes will destroy infected cells through inducing apoptosis and other mechanisms.
4. Phagocytes present antigens to lymphocytes.
5. B lymphocytes will recognise specific antigens and with T cell help will produce specific antibodies.
6. T and B lymphocyte response is amplified by selection and clonal expansion.
7. Different lymphocytes are expanded for each antigen.
8. Some B lymphocytes become antibody producing factories. These antibodies tag infected cells or free pathogen so they can be engulfed by macrophages.
9. Other B and T lymphocytes become immunological memory cells (this allows more rapid response if re-infection occurs).

Question 22

What are the three main ways parasites have evolved to evade the immune system?

Answer 22

1. Mimic host antigens- this helps the parasite to evade detection by the immune system.
2. Antigenic Variation- the parasite changes their surface antigens constantly so that the parasite is evolving quicker than the host immune system can respond.
3. Modify host’s immune response- the parasite may be able to suppress the activity of some T and B lymphocytes.

Question 23

What is the definition of epidemiology?

Answer 23

The study of disease, its outbreak and spread.

Question 24

Define endemic, epidemic and pandemic

Answer 24

If a disease is regularly found in a particular area or group of people it is endemic. If the disease has a widespread outbreak it is considered an epidemic. If the epidemic crosses major boundaries ie. across continents, then it is pandemic.

Question 25

Explain how herd immunity occurs

Answer 25

It occurs when any new infection in a population can be contained because susceptible hosts are too dispersed for the parasite to continue to spread.

Question 26

Define the herd immunity threshold

Answer 26

The density of resistant hosts required in a population to prevent an epidemic.

Question 27

How do vaccinations work?

Answer 27

Vaccinations expose the immune system to a non-infectious source of parasite antigens. This develops immunological memory to these antigens and so protects against infection by the real parasite.

Question 28

Why are parasites difficult to study in culture and find treatment for?

Answer 28

They require host conditions to thrive they are often difficult to study in culture. In addition to this as their tolerances and metabolism so closely match their host, it makes it even harder to find treatments that target the parasite and not the host.

Question 29

What are the benefits to improving parasite control?

Answer 29

Child mortality is reduced and the general health of children is greatly improved, since their bodily resources are not being redirected to support the parasite. The children, therefore, have more resources for growth and development and, since more children are healthy, this leads to population-wide improvements in child development and intelligence.

Question 30

Three methods to control parasites?

Answer 30

• Education- knowing how parasites are transmitted and educating people is one of the most effective methods to prevent spread.
• Sanitation- many parasites move through contaminated water supplies, but basic sanitation is not always available to areas with overcrowding. Large scale civil engineering projects are required in many areas of the world to prevent the increase of transmission with increasing population.
• Coordinated vector-control strategy can be highly effective. For example, measures to decrease the spread of malaria include spraying houses with insecticides and uses bed nets to prevent access of mosquitoes.

Question 31

Define lysosomes

Answer 31

Organelles that release digestive enzymes into the vacuole containing the parasite.

Question 32

What are the challenges in controlling parasites?

Answer 32

• Some parasites are difficult to culture in the laboratory (they need host conditions to thrive).
• Rapid antigen change has to be reflected in the design of vaccines.
• The similarities between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite.
• Often civil engineering projects to improve sanitation combined with coordinated vector control may often be the only practical control strategies but these are highly expensive for developing countries.
• Often there are challenges where parasites spread rapidly as a result of overcrowding or tropical climates.

Question 33

What are the six most common parasitic groups?

Answer 33

Protists, Anthropods, Platyhelminths, Nematodes, Viruses and Bacteria

Question 34

What must parasites with a higher virulence have?

Answer 34

A higher rate of transmission

Question 35

What are factors which can maximise parasite transmission

Answer 35

High density of hosts
Vectors (if hosts incapacitated)
Modification of host behaviour
Modification of host size and reproductive rate
Having an asexual phase which allows rapid build up of parasite
Having a rapid rate of evolution
Having water borne transmission (if hosts incapacitated)

Question 36

What are the steps in phagocytosis?

Answer 36

Phagocytes are attracted to the chemical signals of a pathogen.
A vacuole forms around the pathogen.
Lysosomes form and fuse with the vacuole.
Lysomes release digestive enzymes into the vacuole and the pathogen is broken down by the enzymes.
The vacuole disintegrates releasing digestive products into the cytoplasm of the phagocyte.

Question 37

What are examples of alterations of host behaviour?

Answer 37

Host foraging, movement, sexual behaviour, habitat choice or anti-predator behaviour

Question 38

Describe the general structure of viruses

Answer 38

Viruses contain genetic material in the form of DNA or RNA packaged in a protective protein coat. The protein coat of some viruses is surrounded by a lipid membrane envelope. This envelope is composed of host cell materials.