9. Parasitism

5ai. niche

what is an ecological niche

a multi-dimensional summary of tolerances and requirements of a species

when does a species occupy a fundemental niche

in the absence of any interspecific competition

when does a species occupy a realised niche

in response to interspecific competition

what can happen due to interspecific competition

competitive exclusion

what is competitive exclusion

where the niches of 2 species are so similar that one declines to local extinction

when can potential competitors co-exist

when the realised niches are sufficiently different and they can exist by resource partitioning

5aii. the parasite niche

what is a parasite

parasites gains benefit in terms of nutrients at the expense of its host

what is the difference between predator-prey relationships and parasitism

the repoductive potential of the parasite is greater than that of the host

what kind of niche do most parasites have

narrow

why do most parasites have a narrow niche

they are very host specific

what term is used to describe parasites due to how the host provides so many of the parasites needs

degenerate

what happens due to parasites being degenerate

they lack structures and organs found in other organisms

what types of parasite are there

ectoparasite

endoparasite

where do ectoparasites live

on the surface of the host

where do endoparasites live

within the tissues of its host

5b. parasitic life cycles

how many hosts do some parasites need to complete their life cycle

1

how many hosts do most parasites need to complete their life cycle

more than 1

what types of host are there

definitive

intermediate

what role do definitive hosts have

they are the organism on or in which the parasite reaches sexual maturity

what role do intermediate hosts have

they may also be required for the parasite to complete its life cycle

what is the role of a vector

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

what is malaria caused by

plasmodium

what is the process of how malaria spreads

an infected mosquito, acting as a vector, bites a human so plasmodium enters the bloodstream

asexual reproduction occurs in the liver and then in the red blood cells that burst releasing gametocytes 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

what causes schistosomiasis

schistosomes reproduce sexually in the human intestine

the fertilised eggs pass out via faeces into water where they develop into larvae that infect water snails, where asexual reproduction occurs producing another type of motile larvae, which escape the snail and penetrate the skin of a human, entering the bloodstream

what are viruses

parasites that can only replicate inside a host cell

what do viruses contain

DNA or RNA, packaged in a protective protein coat

what are some viruses surrounded by

a phospholipid membrane derived from host cell materials

what does the outer surface of a virus contain

antigens that a host cell may or may not be able to detect as foreign

what are the stages of the viral life cycle

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

how do retroviruses work

uses the enzyme reverse transcriptase to form DNA from RNA which is inserted into the genome of the host cell

what can viral genes be expressed to form

new viral particles

5c. transmission and virulence

what is transmission

the spread of a parasite to a host

what is virulence

the harm caused to a host species by a parasite

how do ectoparasites typically transmit parasites

direct contact

how do endoparasites of body tissues typically transmit parasites

by vectors or by consumption of intermediate hosts

what factors increase transmission rates

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

how do parasites maximise transmission

host behaviour is often exploited and modified

what are examples of host behaviour that can be exploited

host foraging

movement

sexual behaviour

habitat choice

anti-predator behaviour

what happens to the host behaviour

it becomes part of the extended phenotype of the parasite

what do parasites often do to the host

suppress the host immune system

modify host size

reproductive rate in ways that benefit the parasite growth, reproduction or transmission

5d. defence against parasitic attack

what kinds of immune response do mammals have

non-specific

specific

what are examples of non-specific defences

physical barriers

chemical secretions

inflammatory response

phagocytes

natural killer cells destroying cells infected with viruses

what does epithelial tissue do

blocks the entry of parasites

what are examples of epithelial tissue

hydrolytic enzymes in mucus, saliva and tears

low pH environments of the secretions of stomach, vagina and sweat glands

how do hydrolytic enzymes block entry to parasites

destroy bacterial cell walls

how do low pH environments block entry to parasites

denatures cellular proteins of pathogens

what do injured cells release

signalling molecules

what happens due to the release of signalling molecules

enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes

how are parasites killed in phagocytosis

using powerful enzymes contained in lysosomes, by engulfing them and storing them inside a vacuole

how do natural killer cells kill parasites

they identify and attach to cells infected with viruses, releasing chemicals that lead to cell death by inducing apoptosis

what do white blood cells do around the body

a range of white blood cells constantly circulate, monitoring the tissues

what happens if a tissue becomes 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

why do mammals contain many different lymphocytes

each of them possess a receptor on its surface which can potentially recognise a parasite antigen

what happens when an antigen binds to the receptor of a lymphocyte

the lymphocyte divides and produces a clonal population of the lymphocyte

what processes can lymphocytes induce

produce antibodies

induce apoptosis

how are antibodies different

there are regions where the amino acid sequence varies greatly between different antibodies

what does the variable region allow

gives the antibody its specificity for binding antigen

what can happen when the antigen binds to the binding site

antigen-antibody complex formed resulting in the inactivation of the parasite

what happens to the inactivated parasite

its susceptible to a phagocyte, or can stimulate a response that results in cell lysis

form memory lymphocytes

what causes memory lymphocytes to form

initial antigen exposure produces memory lymphocytes specific for that antigen that can produce a secondary response when the same antigen enters the body in the future

what is the benefit of having a secondary response

antibody production is enhanced in terms of speed of production, concentration in blood and duration

5e. immune evasion

how have parasites evolved

evolved ways of evading the immune system

how do endoparasites evade the immune system

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

how has antigenic variation helped parasites infect hosts

allows them to change between different antigens during the course of infection of a host

it could also allow re-infection of the same host with a new variant

how does integrating genomes work

escape immune surveillance by integrating their genome into host genomes by existing in latency

what does latency mean in terms of parasites

existing in an inactive state

when does the parasite become active again

when favourable conditions arise

5f. challenges in treatment and control

what is epidemiology

the study of the outbreak and spread of infectious disease

what is the herd immunity threshold

the density of resistant hosts in the population required to prevent an epidemic

how do vaccines work

they contain antigens that will elicit an immune response

why is it difficult to find drug compound that only target the parasite

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

what has to be reflected in the design of vaccine

antigenic variation

why can it be hard to design vaccines

some parasites are difficult to culture in labs

what conditions cause parasites to spread most rapidly

overcrowding

tropical climates

where can overcrowding occur

in refugee camps that result from war or natural disasters

rapidly growing cities in less economically developed countries

what is made more difficult due to the conditions that cause parasites to spread rapidly

co-ordinated treatment

control programs

what sector helps to control the spread of parasites

civil engineering

how can parasite spread be controlled

improve sanitation

co-ordinated vector control

what does improvements in parasite control lead to

reduce child mortality

population-wide improvements in child development and intelligence

why does improvement in parasite control lead to imrpovements in child development and intelligence

individuals have more resources for growth and development