1. Parasitism

2. Niche
The niche for an organism is often defined as the role an organism plays in its environment
The environmental niche encompasses the requirements of the species along with its tolerances to the environment in which it exists.
The environmental niche is multidimensional as organisms interact with the biotic and abiotic factors in their environment

3. Niche A species has a fundamental niche.
This is the niche it occupies if there is no interspecific competition influencing it.
A species is more likely to occupy its realised niche.
This is the reality when interspecific competition influences organisms

4. Niche
Interspecific competition can result in competitive exclusion if the niches of two species are so similar that one species declines locally to extinction
If realised niches are different enough between competitors they can co-exist by resource partitioning

5. The parasite niche
A parasite is a symbiont gaining benefit at the expense of its host (usually nutrients)

6. The parasite niche
The reproductive potential of the parasite is greater than the host
Parasitic wasp larvae

7. The parasite niche
Parasites tend to have a narrow niche as they are very host specific.
The host provides many of the parasite’s needs.
Parasites often lack structures and organs (or they are degenerate) found in other organisms as they are no longer needed.

8. The parasite niche
Ectoparasites live on the surface of its host. Endoparasites live within the host

9. The parasite niche
The organism on or in which the parasite reaches sexual maturity is the definitive host
Intermediate hosts may also be needed 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

10. Parasite 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

11. Parasite transmission and virulence
Factors that increase transmission rates are:
Overcrowding of hosts in high density populations
Spreading mechanisms used by the parasite even when the host is incapacitated including vectors and water borne dispersal

12. Parasite transmission and virulence
Host behaviour can be exploited and modified by parasites to maximise transmission
This can include changes in host foraging, movement, sexual behaviour, habitat choice and anti-predator behaviour
These modified behaviours essential become an extended part of the parasites phenotype

13. Parasite transmission and virulence
Parasites often suppress the host immune system and modify host size and reproductive rate in such a way that the parasites growth, reproduction and transmission may be benefitted
Parasite distribution will not be uniform across hosts in a population
Sexual and asexual phases in parasite life cycles allow rapid evolution and rapid build-up of parasite populations

14. Immune response to parasites
Epidemiology is the study of the outbreak and spread of infectious disease.
Herd immunity is the protection given indirectly to the non-immune minority by the immune majority.
The herd immunity threshold is the density of resistant hosts in the population required to prevent an epidemic.
(Depends on pathogen virulence, vaccine efficacy and population contact parameters)

15. Non specific defences of mammals
Physical barriers (epithelial cells)
Chemical secretions (sweat, sebum, tears, enzymes, saliva, mucus, acid)
Inflammatory response (histamine, cytokines)
Phagocytes
Natural killer cells destroying abnormal cells (virus infected cells triggered to self destruct by signal molecule)

16. Specific cellular defence in mammals
Immune surveillance by white blood cells
Damage or invasion stimulates cytokine production
Phagocytes and T cells are produced and attracted to the site
Clonal selection of T lymphocytes
An antigen will be recognised by a T lymphocyte
The T lymphocyte will divide repeatedly to form a clonal population

17. Specific cellular defence in mammals
T lymphocytes targeting immune response and destroying infected cells by inducing apoptosis
Helper T cells (TH cells) secrete cytokines activating phagocytes, TC cells and B cells)
Cytotoxic T cells (TC cells) destroy infected cells including by apoptosis
Phagocytes presenting antigens to lymphocytes
Phagocytes will present pathogen’s antigens on its surface
Helper T cells activated to produce activated TH cells or memory TH cells

18. Specific cellular defence in mammals
The clonal selection of B lymphocytes
Specific B cells stimulated by cytokines from activated TH cells
Many B cells made as clones
Some activated to make specific antibody others
Long term survival of some members of T and B lymphocytes clones to act as immunological memory cells

19. Immune response to parasites
Endoparasites mimic host antigens to evade detection by the immune system
Endoparasites modify host immune response to reduce their chances of destruction
Antigenic variation in some parasites allows them to evolve faster than the host immune system can respond to the new antigen

20. Parasitic life cycles Common parasites include: Protists
Platyhelminths
Nematodes,
Arthropods
Bacteria
Viruses

21. Parasitic life cycles - protists

22. Parasitic life cycles - platyhelminthes

23. Parasitic life cycles - nematodes

24. Parasitic life cycles - arthropods

25. Parasitic life cycles - bacteria

26. Parasitic life cycles - viruses

27. Parasitic life cycles
Many parasites require more than one host to complete their life cycle
Plasmodium spp. which cause malaria in humans
Schistosoma spp. (platyhelminth) which cause schistosomiasis (bilharzia) in humans

28. Parasitic life cycles
Ectoparasites and endoparasites of the main body cavities, such as the gut, are generally transmitted through direct contact or by consumption in secondary host
Endoparasites of the body tissues are often transmitted by vectors

29. Parasitic life cycles
Parasites can complete their life cycle within one host such as some endoparasitic amoeba and ectoparasitic arthropods, bacteria and viruses.
Human diseases include tuberculosis, caused by bacteria and influenza and HIV caused by viruses

30. Parasitic life cycles - viruses
Viruses are infectious agents 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.
The outer surface of a virus contains antigens that a host cell may or may not be able to detect as foreign.
RNA retroviruses use the enzyme reverse transcriptase to form DNA, which is then inserted into the genome of the host cell.
This virus gene forms new viral particles when transcribed.

31. Challenges in treatment and control
Some parasites are difficult to culture in the laboratory.
Rapid antigen change has to be reflected in the design of vaccines
The similarity between host and parasite metabolism makes it difficult to find drug compounds that only target the parasite

32. Challenges in treatment and control
Civil engineering projects to improve sanitation combined with coordinated vector control may often be the only practical control strategies

33. Challenges in treatment and control
Challenges arise where parasites spread most rapidly as a result of overcrowding or tropical climates.
Improvements in parasite control reduces child mortality and results in population-wide improvements in child development and intelligence as individuals have more resources for growth and development