2.5 Parasitism

• 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
• 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
• Parasitism is a symbiotic interaction between a parasite and its host (+/-)
• 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
• Some parasites require only one host to complete their life cycle. However, many parasites require more than one host to complete their life cycle
• Definitive host - The organism on or in which a parasite reaches sexual maturity. 
• Intermediate host - The host that the parasite might require in order to complete its life cycle.
• Viruses contain genetic material in the form of DNA or RNA, packaged in a protective protein coat
• 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
• Transmission is the spread of a parasite to a host
• Ectoparasites are generally transmitted through direct contact
• Parasites often suppress the host immune system and modify host size and reproductive rate in ways that benefit the parasite growth, reproduction or transmission
• Examples of non-specific defences include:​

  • Physical barriers and chemical secretions - 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. 
• Examples of non-specific defences include:​

  • Inflammatory response - injured cells release signalling molecules. This results in enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes. 
• Examples of non-specific defences include:​

  • Phagocytes - Killing of parasites using powerful enzymes contained in lysosomes, by engulfing them and storing them inside a vacuole in the process of phagocytosis .
• Examples of non-specific defences include:​

  • Natural killer cells destroying cells infected with viruses - These 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
• Specific cellular defences:

  Mammals contain many different lymphocytes , each possessing a receptor on its surface, which can potentially recognise a parasite antigen . Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte .
• Antibodies possess regions where the amino acid sequence varies greatly between different antibodies. This variable  region gives the antibody its specificity for binding to antigens. 
• Antigenic  variation in some parasites allows them to change between different antigens during the course of infection of a host.

  It may also allow reinfection of the same host with the new variant . This is why the flu virus is so successful, there are many strains each with their own antigens and you can get the flu many times in your lifetime.
• 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.​

  An example of this is the ​herpes virus.
• Challenges arise where parasites spread most rapidly as a result of overcrowding or tropical  climates . Overcrowding can occur in refugee  camps that result from war or natural  disaster or rapidly growing cities in LEDCs (Less economically developed countries). 
• These conditions make co-ordinated  treatment  and control programs difficult to achieve.​

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  Civil engineering projects to improve sanitation combined with co-ordinated vector control may often be the only practical control strategies.