(a) (i) Niche

• An ecological niche is a multi-dimensional summary of tolerances and requirements of a species
• A species has a fundamental niche that it occupies in the absence of any interspecific competition
• A realised niche is occupied in response to interspecific competition
• 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
• Where the realised niches are sufficiently different, potential competitors can co-exist by resource partitioning

(ii) The parasite niche

• Parasitism is a symbiotic interaction between a parasite and its host (+/-), where 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
• 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
• An ectoparasite lives on the surface of its host, whereas an endoparasite lives within the tissues of its host

(b) 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
• The definitive host is the organism on or in which the parasite reaches sexual maturity.
• Intermediate hosts may also be required for the parasite to complete its life cycle.
• A vector plays an active role in the transmission of the parasite and may also be a host

Malaria

• The human disease malaria is caused by Plasmodium
• 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.

Schistosomes

• Schistosomes cause the human disease 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.

Viruses

• Viruses are parasites that can only replicate inside a host cell
• Viruses contain genetic material in the form of DNA or RNA, packaged in a protective protein coat
• Some viruses are surrounded by a phospholipid membrane derived from host cell materials
• The outer surface of a virus contains antigens that a host cell may or may not be able to detect as foreign
• 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 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

(c) Transmission and virulence

• Transmission is the spread of a parasite to a host
• Virulence is the harm caused to a host species by a parasite
• Ectoparasites are generally transmitted through direct contact or by consumption of intermediate hosts
• Endoparasites of the body tissues are often transmitted by vectors
• Factors that 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
• 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
• The host behaviour becomes part of the extended phenotype of the parasite
• Parasites often suppress the host immune system and modify host size and reproductive rate in ways that benefit the parasite growth, reproduction or transmission

(d) Defence against parasitic attack

• Immune response in mammals has both non-specific and specific aspects

Non-specific defences

• Physical barriers, chemical secretions, inflammatory response, phagocytes, and natural killer cells destroying cells infected with viruses are examples of non-specific defences
• 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.
• Injured cells release signalling molecules.
• This results in enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes.
• Killing of parasites using powerful enzymes contained in lysosomes, by engulfing them and storing them inside a vacuole in the process of phagocytosis.
• Natural killer cells can identify and attach to cells infected with viruses, releasing chemicals that lead to cell death by inducing apoptosis.

Specific cellular defences

• A range of white blood cells constantly circulate, monitoring the tissues
• 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
• Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen
• Specific lymphocyte names are not required – even though you know them from Higher.
• Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte
• Some selected lymphocytes will produce antibodies, others can induce apoptosis in parasite-infected cells
• Antibodies possess regions where the amino acid sequence varies greatly between different antibodies
• This variable region gives the antibody its specificity for binding antigen
• 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
• Memory lymphocyte cells are also formed
• 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.

(e) Immune evasion

• Parasites have evolved ways of evading the immune system
• Endoparasites mimic host antigens to evade detection and modify host immune response to reduce their chances of destruction
• 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
• 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

(f) Challenges in treatment and control

• Epidemiology is the study of the outbreak and spread of infectious disease
• The herd immunity threshold is the density of resistant hosts in the population required to prevent an epidemic
• Vaccines contain antigens that will elicit an immune response
• The similarities between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite
• Antigenic variation has to be reflected in the design of vaccines
• Some parasites are difficult to culture in the laboratory making it difficult to design vaccines
• 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
• Overcrowding can occur in refugee camps that result from war or natural disaster or rapidly growing cities in LEDCs.
• Civil engineering projects to improve sanitation combined with co-ordinated vector control may often be the only practical control strategies
• 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