Unit 2 - Parasitism

Question 1

Ecological niche

Answer 1

All the factors that influence the distribution of an individual species.

Includes abiotic factors (eg. temperature, pH, salinity) and biotic interactions (eg. parasites and predator-prey).

It summarises the tolerances and requirements of a species.

Question 2

Fundamental niche

Answer 2

The set of resources that a species is capable of using in the absence of interspecific competition.

Question 3

Realised niche

Answer 3

The set of resources that a species actually uses in response to the presence of interspecific competition.

Question 4

Competitive exclusion principle

Answer 4

Occurs as a result of interspecific competition when the realised niches of 2 species are almost identical.

One species will decline resulting in local extinction. eg. grey/red squirrels

Question 5

Resource partitioning

Answer 5

2 species living in a habitat have similar requirements, but can co-exist due to different realised niches.

2 species exploit different components of the resource, reducing competition. eg different beak lengths in wading birds.

Question 6

Parasitism

Answer 6

A type of symbiosis where the parasite gains nutrients at the expense of the host.

These resources are used for growth and reproduction, and the host also uses resources to defend itself against parasite attack.

The reproductive potential of the parasite is much greater that that of the host.

Question 7

Ectoparasite

Answer 7

Live and feed on the surface of their host eg. ticks, lice

Question 8

Endoparasite

Answer 8

Live within their host eg. tapeworm, Plasmodium (malaria)

Question 9

Parasite niche

Answer 9

As a result of co-evolution, parasite adaptations are selected in response to adaptations that have evolved in the host. (Red Queen hypothesis).

Parasites tend to have a narrow, specialised niche as a result.

They are host specific and show resource partitioning - different species inhabiting different parts of the host’s body.

Parasites are often degenerate - they lack structures found in other organisms, eg. a digestive system, sense organs.

Question 10

Symbiosis

Answer 10

An intimate ecological relationship between 2 species.

Parasitism = +/- relationship
Mutualism = +/+
Commensalism = +/o

Question 11

Direct lifecycle

Answer 11

Lifecycle is completed within one species.

Often use direct contact as a transmission mechanism.

Often ectoparasites eg. headlice

Question 12

Indirect lifecycle

Answer 12

The parasite requires more than one host to complete its lifecycle.

Definitive host - the organism in which the parasite carries out sexual reproduction.

Intermediate host - the organism in which developmental/asexual stages occur.

Vectors - play an active part in the transmission of the parasite and increase efficiency of transmission. May also (but not always) be a host eg. mosquito

Question 13

Plasmodium

Answer 13

Protists that cause malaria in humans.

An infected mosquito acting as a vector, bites a human and transmits the parasite.

The parasites reproduce asexually in the liver and red blood cells, which burst releasing gametocytes.

These are collected by another mosquito, mature into gametes and sexual reproduction occurs in the mosquito (definitive host).

The parasites migrate to the salivary glands of the mosquito, ready to infect the next host.

Question 14

Schistosoma

Answer 14

Platyhelminthes that cause Schistosomiasis in humans.

Larvae burrow through the skin of humans, as they wade in fresh water.

They migrate to the small intestine and carry out sexual reproduction.

Fertilised eggs pass out in the faeces into the water.

The eggs hatch and form larvae that infect water snails.

Asexual reproduction in the snail produces a motile larva that can swim and burrow into human skin.

Question 15

Virus

Answer 15

An infectious parasitic agent that can only replicate inside a host cell.

They are not really ‘living’ as they don’t carry out any normal functions of living organisms except reproduction, which requires a host cell.

Question 16

Virus structure

Answer 16

Viruses are made of nucleic acid (DNA or RNA) packaged in a protective protein coat (capsid).

The outer surface has antigens (proteins) coded for by viral genes.

Antigens may trigger an immune response in the host organism, if they are recognised as ‘foreign’.

Question 17

DNA viruses

Answer 17

Their genetic material is in the form of DNA. eg. smallpox, herpes.

Virus antigens ‘dock’ with receptors on the surface of the host cell, and the host cell is infected with viral DNA.

The virus genome is replicated by the host cell, and viral genes are transcribed to RNA, which is translated into viral proteins.

New virus particles are assembled and are released from the host cell, ready to infect other cells.

Question 18

RNA viruses

Answer 18

Have an RNA genome, eg. influenza, hepatitis C

Reproduce as DNA viruses, except the viral genome is replicated directly using an enzyme from the virus.

Viral DNA is never made.

Question 19

RNA retroviruses

Answer 19

Use the enzyme reverse transcriptase to make a DNA copy of the viral genome. eg. HIV

The DNA copy is inserted into the genome of the host cell, and is replicated as part of the normal cell cycle in the host.

Viral genes are then expressed to form new virus components, which are assembled and burst out of the host cell.

Question 20

Transmission

Answer 20

The spread of a parasite to a new host.

Question 21

Virulence

Answer 21

The harm that a parasite causes a host species, which reduces its evolutionary fitness.

Question 22

Increasing parasite transmission rates

Answer 22

• Overcrowding of hosts, living at high population densities. eg. head lice. spread by direct contact.
• Vectors allow the parasite to spread even if the host is incapacitated, eg. by malaria
• Waterborne dispersal allows parasites to enter the water and spread to new hosts, if the original host is immobilised and improves transmission efficiency.

Question 23

Factors that increase virulence

Answer 23

High transmission rates exert a selection pressure for increased virulence, to increase parasite growth and reproduction.

• suppress the host’s immune system so parasites are not attacked.
• modify the size of the host - make it grow bigger so it can support more parasites.
• reduce the host’s reproduction rate so resources are directed to parasite reproduction.

Question 24

Modification/exploitation of host behaviour

Answer 24

Used to maximise chances of transmission.

Schistosoma exploits the wading behaviour of mammals.

If the host’s behaviour is modified it becomes part of the extended phenotype of the parasite. eg. habitat choice - an ant infested with parasitic flatworms will climb up a blade of grass to rest, and is more likely to be eaten by a sheep.

Question 25

First line defences

Answer 25

Non- specific physical and chemical barriers that prevent parasites from entering the body fluids.

Physical - epithelial tissue, nasal hairs.

Chemical - mucus (moved by cilia), stomach acid, sweat, hydrolytic enzymes in tears.

Question 26

Second line defences

Answer 26

Non- specific responses that happen after the parasite enters the body.

The 3 types are:

• inflammatory response
• phagocytes
• natural killer cells

Question 27

Inflammatory response

Answer 27

Injured or infected cells release histamine (a signalling molecule).

Causes blood vessels to dilate, increasing blood flow so the area becomes red and warm.

Blood vessels become more permeable and fluid leaks into the tissues causing swelling and pain.

Anti-microbial proteins and phagocytes migrate to the area.

Question 28

Phagocytes

Answer 28

White blood cells that migrate from blood to tissue fluid.

They engulf and digest foreign objects that lack ‘self’ antigens using powerful enzymes found in lysosomes.

Dead phagocytes leave infected wounds as pus.

Question 29

Natural killer cells

Answer 29

Another type of white blood cell that can migrate from the blood into the tissues.

Detect abnormal cell surface proteins on virus infected and cancerous cells.

Killer cells attach to the infected cell and release chemicals into it, triggering apoptosis.

Phagocytes engulf and digest remaining debris.

Question 30

Third line defences

Answer 30

Specific responses triggered by antigens on the surface of the parasite.

Involve specific lymphocytes.

Question 31

Cytokines

Answer 31

Chemicals released by damaged or invaded tissues.

Increase blood flow so that non-specific and specific white blood cells, which are constantly circulating, gather at the site of infection.

Question 32

Lymphocytes

Answer 32

White blood cells found mainly in lymph glands.

Have one type of antigen-receptor protein on their surface, which can bind to one specific antigen.

Question 33

Immune surveillance

Answer 33

Carried out by lymphocytes in the lymph glands as lymph (tissue fluid drained from the cells) passes through.

The lymphocytes check for specific antigens, using the receptor proteins on their surface.

Phagocytes are also checked for presented antigen fragments.

Question 34

Clonal selection

Answer 34

A specific lymphocyte binds to a target antigen and is activated.

It divides rapidly to make many identical clones, with the same specific antigen receptor.

Question 35

Antibodies

Answer 35

Globular proteins released by a certain type of lymphocyte, which are specific for a target antigen on a parasite.

The antibodies bind to the parasite, forming an antibody-antigen complex, which immobilises the parasite or triggers its cells to undergo lysis.

The complex can also be engulfed by a phagocyte.

Question 36

Killer lymphocytes

Answer 36

Specific lymphocytes that bind to cells infected with parasites and induce apoptosis by injecting chemicals.

Question 37

Immunological memory

Answer 37

Some lymphocyte clones remain as memory cells, once the infection has been defeated.

If a specific antigen is detected again, the response is larger and faster.

Produced as a result of vaccination - a harmless version of the parasite is presented to the immune system, to trigger an immune response and to produce memory cells.

Question 38

Immune evasion

Answer 38

Parasites have evolved ways to evade the host’s immune system, as a result of the ongoing ‘Red Queen’ race between host and parasite.

• mimic host antigens to hide from the immune system, eg. Schistosoma
• antigenic variation resulting in constantly changing cell surface antigens eg. Plasmodium
• integrate parasite genome into host genome, remaining in an inactive (latent) state eg. HIV

Question 39

Epidemiology

Answer 39

The study of the outbreak and spread of infectious disease.

Used to plan and evaluate strategies to prevent the spread of the disease in the future.

Question 40

Herd immunity

Answer 40

Vaccinations are used to increase the number of resistant hosts in a population, restricting the spread of a disease.

New infections are contained, as susceptible hosts are too dispersed amongst the population for transmission to occur.

Herd immunity threshold = the density of resistant hosts required to prevent an epidemic.

Protects individuals who are unable to be vaccinated.

Question 41

Problems with treating parasitic diseases

Answer 41

Difficult to develop vaccines for parasites, as they are hard to culture in a lab, and parasites undergo rapid antigen change, making vaccine design very complex.

Host and parasite metabolism are often very similar. Drugs that target parasites may harm the patient, and may lead to resistance.

Most parasitic diseases are found in poorer parts of the world, so pharmaceutical companies don’t invest in them, as they can’t recoup their investment and make a profit.

Question 42

Controlling parasites

Answer 42

Civil engineering projects, such as improving sanitation, reduces transmission of parasites with waterborne stages eg. Schistosoma

A co-ordinated vector control strategy will reduce transmission eg. mosquito nets, spraying houses with insecticide to control malaria.

Difficult in overcrowded unsanitary conditions (eg. refugee camps) and tropical climates which encourage rapid spread of parasites.

Question 43

Benefits of improving parasite control

Answer 43

Reduction in child mortality

Better general health of children.

Children have more resources for growth and development, as they are not diverted to the parasites.

The population as a whole is healthier and intelligence increases.