Biology 625 and 545
ANIMAL and HUMAN PARASITOLOGY
Lecture note outline (first class day)

Updated: 29 November 2004

The Swedish scientist Carolus von Linnaeus (1707-1778) developed the binomial system of nomenclature that is still in use today. As an example, lets use the species Homo sapiens ("humans" to you molecular biologists). Homo is the genus, sapiens is the trivial name (nomen triviale or specific epithet), and together they make up the species. The epithet sapiens is NOT the species, although some very basic texts erroneously say it is. Remember... a species is binomial ("two names").

Now, one would think that my simple explanation would be enough... but it won't be for some of you. You'll wish to test this on your first exam. I'll have some questions on the exam where (for instance) I ask for the genus and most students will correctly write "Homo". Some of you will, however, try to re-design the question and answer "Homo sapiens". This is not correct because you've given the species, NOT the genus. Then, on other questions, I'll ask students for the species and they'll write "sapiens" or "H. sapiens", which are both incorrect because what has been provided is the trivial name rather than the species (again, a species is both names... binomial).

So, to drill this exercise in a bit more, lets say that I ask you to write out the species for the human botfly. The correct answer is Dermatobia hominis. However, some of you will write "D. hominis", which is an incorrect abbreviation and could very well refer to the bacterium, Dermabacter hominis. More commonly, many of you will simply write in "hominis", which will be VERY incorrect and could refer to species such as the bacteria Actinobaccillus hominis, Cardiobacterium hominis, Dermabacter hominis, Facklamia hominis, or Staphlococcus hominis, the mycoplasm Mycoplasma hominis, the parasitic protists Blastocystis hominis, Enteromonas hominis, Pentatrichomonas hominis, Sarcocystis hominis, or Trachipleistophora hominis, or even the digene Gastrodiscoides hominis. Now, we could also use the epithet "coli" as an example (i.e. Escherichia coli, Balantidium coli, Entamoeba coli...). Need I say more?

1. Parasitology is a type of SYMBIOSIS (="living together") (Any plant, animal, or protist that is intimately associated with another organism of a different species; each member is termed a SYMBIONT). There are various type of symbiosis:
   1. PHORESIS ("traveling together" or "to carry") (A smaller organism, termed the PHORONT, is carried mechanically by a HOST) [For instance, bacteria, fungus, cysts, or eggs on insect legs or even passively within an arthropod gut]
   2. COMMENSALISM (when one symbiont, the COMMENSAL, benefits and the other animal is neither helped nor harmed) [True commensalism difficult to find, and may not even actually exist. Close inspections usually reveal either a mutualistic or parasitic association. Perhaps Entamoeba gingivalis in mouth to some degree; some pilotfish and remoras associated with sharks]
   3. MUTUALISM (each member, a MUTUALIST, depends upon the other; oblilgatory or facultative) [many examples in nature. For instance, flagellates produce cellulase in gut of termites; ciliates in ruminants; algae and fungus forming a lichen; crocodiles and Egyptian teeth cleaning plovers]
   4. PREDATION (where one member, the PREDATOR, benefits and a smaller organism, the PREY, is harmed; usually eaten) [This association is not usually considered a type of symbiosis, but it technically falls under the definition]. Examples include coyotes and rabbits, cats and mice.
   5. PARASITISM (where one member, the PARASITE, lives in or on another organism, the HOST, at the expense of that organism)

2. PARASITOLOGY [the study of the relationship between a parasite and its host] is the topic in this course. This method of existence is the single most successful way of making a living, and it has been estimated that no less than 80% of all species of organisms are parasites (1992, Sci. Am. 267(4): 42-48; 1998, Int J Parasitol 28(12): 1939-1941). It is likely that this is an under-estimation.

3. Parasitic relationships may be temporary, facultative, or obligatory

4. Basic types of parasitism and terminology
   1. ECTOPARASITE (lives on surface of the host. Appropriate terminology includes the terms "infected" and "infested") [i.e. ticks, lice, fleas]
   2. ENDOPARASITE (lives within the host; appropriate terminology is "infected;" infested is inappropriate terminology) [i.e. roundworms in gut; tapeworms in gut]
   3. HYPERPARASITE (parasite within a parasite) [i.e. malaria in mosquitos; tapeworm larvae in fleas]

5. VECTORS (transmits parasites from host to host)
   1. BIOLOGICAL VECTOR (essential in life-cycle of parasite)
   2. MECHANICAL VECTOR (unessential in life-cycle of parasite - phoretic)

6. Types of hosts (you'll need to know all of these for sure)
   1. DEFINITIVE OR FINAL HOST (host in which parasite reaches sexual maturity and reproduces)
   2. INTERMEDIATE HOST (some development in host, but does not reach sexual maturity; often asexual stages)
   3. PARATENIC OR TRANSPORT HOST (no parasite development; but parasite continues to live and is infective to next host; for instance, pseudophyllidean tapeworm larvae in fish)
   4. RESERVOIR HOST (non-human animals that serve as sources of infection to humans)

7. Some typical characterisitics of parasitism
   1. High reproductive potential (i.e. multiple fission in Apicomplexa; hermaphrodism of trematodes; partenogenesis in Strongyloides spp.; i.e. strobilation of tapeworms for high ova output; and overall high ova/larval output of many worms)
   2. Often unique morphological or physiological specializations, loss of structures, etc.
      1. loss of digestive tract of tapeworms
      2. loss of wings of fleas and lice
      3. loss of many sensory structures of nematodes
      4. development and refinement of a TEGUMENT; a living external layer of digenes, cestodes and acanthocephala that allows digestion and other functions across body surface
      5. development of special holdfast organs, including hooks, suckers, teeth, clamps, cutting plates, spines
      6. production of anti-coagulants in leeches and hookworms
   3. Often special site specificity
   4. Usually, but not always, non-lethal to host
   5. Generally more numerous than hosts
   6. Generally much smaller than host (if larger, then termed a predator)
   7. Often have evolved methods of evading host immune system
      1. Antigenic variation of trypanosomes
      2. Tough tegument of acanthocephalans
      3. Intracellular habitat of coccidia and Trichinella larvae
      4. Antigen acquisition of schistosomes
      5. Suppression eosinophil or neutrophil migration to the site of the parasite
      6. Encystment
      7. Ability to cleave antibodies or consume complement
      8. Ability to trigger certain arms of the immune response, which may in turn damage host tissue enough to facilitate parasite invasion
   8. Many parasites are now being shown to change host behavior (1994, Oikos 70(3): 479-484; 1995, Bioscience 45(2): 89-96). Growing number of scientists believe that many ecological studies need to include parasitology as component as much animal behavior can be explained by level of parasitism. Especially behavior of some insects harboring larval stages of parasites, and bird behavior in response to both ectoparasites and densities of some intestinal worms.

8. Level of pathology due to parasites is highly varied
   1. Physical trauma [cell-tissue destruction; i.e. migration of nematodes through tissues; ulceration of intestinal wall and liver by cysteine proteases of Entamoeba histolytica; displacement of tissue or structures by hydatids; protease digestion of epithelial cells by Trichomonas vaginalis; ulceration due to insertion of hooks, spines, etc. into intestinal wall]
   2. Nutritional diversion [i.e. giardiasis results in diarrhea and malabsorption; Diphyllobothrium absorbs vitamin B12]
   3. Toxins/Excretory products/Immune complexes [i.e. African trypanosomes slough antigen/Ab complexes that are absorbed by RBC's; complement activated; massive RBC lysis; i.e. excretory products of some trematodes and cestodes causing anaphylaxis; i.e. fibrosis and inflammation around schistosome eggs; i.e. granulomas, fibrosis, edema against adult filarids]
   4. Blood loss [i.e. hookworms and anaemia]

9. Typical ways parasites transmitted
   1. INGESTION from food or water / inhalation included here
   2. VECTORS
   3. DIRECT PENETRATION of skin from environment

10. Some additional terminology to be used in class
    1. Anthroponoses (human diseases that can be transmitted to animals)
    2. Epidemic (disease that affects a large number of humans and spreads rapidly)
    3. Epizootic (disease that affects a large number of non-human animals and spreads rapidly)
    4. Epizoic (living on the surface; a skin parasite)
    5. Euryxenous (broad host range)
    6. Heteroxenous (alternation of generations of a parasite)
    7. Incidence (the number of cases of an infection occurring during a given period of time in relation to the popultion unit in which they occur)
    8. Infection (parasitic invasion resulting in injury and reaction to injury)
    9. Latent (non-visible infection)
    10. Monoxenous (single host life cycle)
    11. Parasitemia (parasites in blood)
    12. Pathogenic (results in disease or morbid symptoms)
    13. Premunition (resistance to superinfection; depends upon survival of parasites in host and disappears with their elimination)
    14. Prevalence (number of organisms in a population infected with a parasite at any one time)
    15. Virulence (relative infectiousness of a parasite)
    16. Zoonosis (animal diseases that may be transmitted from animals to humans)