Parasitism

• Ecological niche


  multi-dimensional summary of tolerances and requirements of a species
• Fundamental niche


  a species has a fundamental niche that it occupies in the absence of any interspecific competition
• realised niche


  occupied in response to interspecific competition
• competitive exclusion


  can occur as a result of interspecific competition, where niches of two species are so similar that one declines to local extinction
• resource partitioning


  where realised niches are sufficiently different, potential competitors can co-exist by resource partitioning
• parasitism


  symbiotic interaction between parasite and host
• what does a parasite gain from parasitism?

  benefit in terms of nutrients at the expense of it's host
• reproductive potential


  greater reproductive potential for the parasite than the host, unlike predator-prey relationship
• niche of the parasite


  narrow specialised niche as they are very host-specific
• ectoparasite


  lives on the surface of it's host
• endoparasite


  lives within the tissues of the host
• degenerative


  as the host provides so many of the parasite's needs, many parasites are degenerate, lacking structures and organs found in other organsims
• parasitic life cycles


  many parasites require more than one host to complete their life cycle, however some parasites only require one host to complete their life cycle
• vector


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


  causes the human disease malaria
• Schistosomes


  cause the human disease schistomiasis
• Viruses


  parasites that can only replicate inside a host cell
• Virus composition


  contain genetic material in the form of DNA or RNA, packaged in a protective protein coat, some are surrounded by a phospholipid membrane derived from host cell materials.

  Outer surface 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 expressed to form new viral particles
• Transmission


  The spread of a parasite to a host
• Virulence


  harm caused to a host species by a parasite
• Ectoparasite transmission


  direct contact
• Endoparasite transmission


  transmitted by vectors or by consumption of immediate hosts
• Factors that increase transmission rates

  overcrowding of hosts when they are at a high density

  Mechanisms such as vectors and waterborne dispersal stages, that allow the parasite to spread even if infected hosts are incapacitated
• Host behaviour


  Often exploited and modified by parasites to maximise transmission

  becomes part of the extended phenotype of the parasite
• host immune system


  parasites often surpress the host immune system and modify host sixe and reproductive rate in ways that benefit the parasite growth, reproduction or transmission
• non-specific defences


  physical barriers, chemical secretions, inflammatory response, phagocytes, and NK cells destroying cells infected with viruses
• Specific cellular defences


  a range of white blood cells constantly circulates monitoring the tissues. If tissues become damaged or invaded, cytokines are released by the cells that increase blood flow resulting in non-specific and specific white blood cells accumulating at the site of infection or tissue damage
• parasite antigen recognition


  mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a parasite antigen. The binding of an antigen to a lymphocyte receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte.
• Lymphocyte


  some will produce antibodies and other can induce apoptosis in parasite-infected cells
• antibodies


  possess regions where the amino acid sequence varies greatly between different antibodies which give the antibody its specificity for binding antigen. When the antigen binds to this binding site an antibody-antigen complex forms which can result in inactivation of the parasite, rendering it susceptible to a phagocytes or can stimulate a response that results in cell lysis
• memory lymphocytes are also formed
• 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


  allows some parasites to change between different antigens during infection of a host and 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
• thevirus becomes active again when favourable conditions arise
• Epidemiology


  the study of the outbreak and spread of infectious disease