Organisms and Evolution - Parasitism Flashcards
What is an ecological niche?
A multi-dimensional summary of tolerances and requirements of a species
What is the fundamental niche of a species?
A niche that is occupied in the absence of any interspecific competition
What is the realised niche of a species?
A niche that is occupied in response to interspecific competition
What can occur 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
How can potential competitors co-exist where the realised niches are sufficiently different?
By resource partitioning
What is parasitism?
A symbiotic interaction between a parasite and its host (+/-)
How does a parasite gain benefit?
In terms of nutrients at the expense of its host
Compare the reproductive potential of a parasite and that of it’s host?
Unlike in a predator–prey relationship, the reproductive potential of the parasite is greater than that of the host
Describe the niche of most parasites?
They have a narrow (specialised) niche as they are very host-specific
Why are many parasites degenerate, lacking structures and organs found in other organisms?
As the host provides so many of the parasite’s needs
What is an ectoparasite?
A parasite that lives on the surface of it’s host
What is an endoparasite?
A parasite that lives within the tissues of it’s host
Describe parasitic life cycles?
Some parasites require only one host to complete their life cycle
Many parasites require more than one host to complete their life cycle
What is the definitive host?
The organism on or in which the parasite reaches sexual maturity
What other type of host may parasites need to complete their life cycle?
Intermediate host
What is the role of a vector?
It plays an active role in the transmission of the parasite and may also be a host
What parasite causes the human disease malaria?
Plasmodium
Describe how humans become infected with malaria?
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
What parasite causes the human disease schistosomiasis?
Schistosomes
Describe how humans become infected with schistosomiasis?
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
What are viruses?
Parasites that can only replicate inside a host cell
What do viruses contain?
Genetic material in the form of 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 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
RNA retroviruses use which enzyme to form DNA, which is then inserted into the genome of the host cell?
Reverse transcriptase
After RNA retroviruses use the enzyme reverse transcriptase to form DNA, which is then inserted into the genome of the host cell, what happens next?
Viral genes can then be expressed to form new viral particles
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 are ectoparasites generally transmitted?
Through direct contact or by consumption of intermediate hosts
How are endoparasites of the body tissues generally transmitted?
By vectors
What factors increase transmission rates?
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.
What do parasites often do to maximise transmission?
Exploit and modify host behaviour
What host behaviours do parasites exploit and modify?
Host foraging,
Movement,
Sexual behaviour,
Habitat choice or anti-predator behaviour
The host behaviour modified by the parasite becomes part of what?
The extended phenotype of the parasite
What do parasites often do in ways that benefit the
parasite growth, reproduction or transmission?
Suppress the host immune system and modify host size and reproductive rate
What are the two types of immune response in mammals?
Specific and non-specific
What are the non-specific defences?
Physical barriers, Chemical secretions, Inflammatory response, Phagocytes, Natural killer cells destroying cells infected with viruses
What is the role of epithelial tissue in non-specific defence?
It blocks the entry of parasites
What is the role of hydrolytic enzymes in mucus, saliva and tears in non-specific defence?
To destroy bacterial cell walls
What is the role of low pH environments of the secretions of stomach, vagina and sweat glands in non-specific defence?
To denature cellular proteins of pathogens
Injured cells release signalling molecules. What does this result in?
Enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes
How are parasites killed using powerful enzymes
contained in lysosomes?
By engulfing them and storing them inside a vacuole in the process of phagocytosis
What is the role of natural killer cells?
They can identify and attach to cells infected with viruses, releasing chemicals that lead to cell death by inducing apoptosis
How are the tissues monitored in specific cellular defences?
A range of white blood cells constantly circulate, monitoring the tissues
What happens of tissues become damaged or invaded in specific cellular defences?
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
What do mammals contain that can potentially recognise a parasite antigen?
Many different lymphocytes, each possessing a receptor on its surface
What does binding of an antigen to a lymphocyte’s
receptor do?
Selects that lymphocyte to then divide and produce a clonal population of this lymphocyte
What are the roles of some lymphocytes selected to divide and produce a clonal population?
Some will produce antibodies,
Others can induce apoptosis in parasite-infected cells
What do antibodies possess?
Regions where the amino acid sequence varies greatly between different antibodies
What gives the antibody its specificity for binding antigen?
The variable region
When the antigen binds to the variable binding site, what can the antigen-antibody complex formed result in?
Inactivation of the parasite, rendering it susceptible to a phagocyte, or can stimulate a response that results in cell lysis
What does initial antigen exposure produce?
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
Parasites have evolved ways to do what?
Evade the immune system
How do endoparasites evade detection by the immune system?
By mimicking host antigens and modifying host immune response
What does antigenic variation in some parasites allow
them to do?
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
How do 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
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
What do vaccines contain?
Antigens that will elicit an immune response
What makes it difficult to find drug compounds that only target the parasite?
The similarities between host and parasite metabolism
What has to be reflected in the design of the vaccine?
Antigenic variation
What makes it difficult to design vaccines?
Some parasites are difficult to culture in the laboratory
Challenges involving parasites arise under what conditions?
Where parasites spread most rapidly as a result of overcrowding or tropical climates
What can occur in refugee camps that result from war or natural disaster or rapidly growing cities in LEDCs?
Overcrowding
Unsuitable conditions (e.g. overcrowding, tropical climates) can make what hard to achieve?
Co-ordinated treatment and control programs
What may often be the only practical control strategy for difficult situations with parasites?
Civil engineering projects to improve sanitation combined with co-ordinated vector control
What effects do improvements in parasite control have?
Reduce child mortality and result in population-wide
improvements in child development and intelligence, as individuals have more resources for growth and development
