Introduction To Parasitology

1. Parasitology
•Parasites include things that live internal
(endoparasites) or external (ectoparasites) to the
infected (or infested) host
•Broadly speaking, all pathogens may be classified as
parasites
•Narrowly speaking, Parasitology is the science that
studies the relatively large parasites including
parasitic Protists, Worms, and, to a lesser degree,
pathogenic Fungi and Arthropod ectoparasites
•Today we’ll consider Protists, Helminths (worms),
Fungi, and the Arthropod Vectors of infectious
disease

2. Vectors and Hosts
•A Vector is a living organism that carries a disease-
causing organism to new Hosts
•This distinction can be arbitrary, however, depending
on which organism’s infection we are most
concerned with
•Thus, the Anopheles mosquito is a vector for the
parasitic disease malaria because we care more
about the health of the human host than that of the
mosquito
•However, in addition, the malaria parasite has a
much greater impact on the health of the human host
than it does on the health of the mosquito vector

3. Biological vs. Mechanical Vector
•The real confusion between host and vector comes
from the concept of Biological Vector
•Within (or upon) both a host and a Biological Vector
the parasite undergoes some aspect of its life cycle
•Thus, in one sense, all hosts that can pass a parasite
on, particularly to another species of host, is also a
Biological Vector
•However, if the parasite does not undergo some
aspect of its life cycle as it is transported by one
organisms to a second, then the first organism is
described as a Mechanical Vector
•E.g., flies can be mechanical vectos of feces-born
pathogens such as Salmonella

4. Types of Hosts
•Definitive Host: The host in which
the parasite goes through its
sexual cycle (I.e., fertilization &
meiosis)
•E.g., mosquitoes serve as
definitive hosts of the malaria
parasite (Plasmodium spp.)
•E.g., snakes (~30 species) serve
as definitive hosts of Sarcocystis
singaporensis, a disease of
mammals

5. Types of Hosts
•Intermediate Host: Host in which
the parasite replicates but does
not go through its sexual cycle
•E.g., mammals, including
humans, serve as intermediate
hosts of the malaria parasite
(Plasmodium spp.)
•E.g., mammals (~30 species,
including humans) serve as
intermediate hosts of Sarcocystis
singaporensis
S. singaporensis
has been
proposed as
biological control
of rodents pests—
apparently it can
selectively kill
them!

6. Types of Hosts
•Reservior Hosts: The reservoir host is the population
in which a parasite resides when it isn’t affecting a
population that we care more about
•E.g., the rabies virus normally is passed back and
forth among wild mammals (these serve as the
reservoir hosts—the populations that we don’t much
care about)
•Occasionally, however, rabies can infect pets, and
thereby us (the population we do care about)
•To prevent the latter (i) we vaccinate pets, (ii) avoid
handling wild animals, and (iii) otherwise attempt to
reduced the incidence of rabies in reservoir pops.

7. Types of Hosts

8. Parasitic Protists

9. Amoeba proteus

10. Balantidium coli

11. Entamoeba histolytica
Trophozoites engulfing detached epithelial cells
ingested
epithelial cell
trophozoite
second
epithelial cell
Parasitology Today
Vol. 3, 117

12. Giardia lamblia

13. Leishmania
spp.

14. Micorsporidia

15. Plasmodium
spp.

16. Sarcocystis
spp.

17. Tetrahymena
spp.

18. Toxoplasma gondii

19. Trichomonas vaginalis

20. Trypanosoma brucei

21. Trypanosoma cruzi

22. Kingdom Fungi

23. Fungal Anatomy
•Heterotrophic, exoenzyme-producing
absorbers
•Chitinaceous cells walls
•Yeasts or, more typically, Hyphae
•Mycelia (tangled masses of hyphae,
typically that has grown into a substrate)
•Thallus, the word used to describe the
“body” of a fungus (same word as used to
describe the body of an algae
•Fungi live by growing mycelia into (or onto)
substrate (e.g., a dead plant), secreting
exoenzymes from these cells, and then
absorbing digested nutrient into the mycelia

24. Fungus Life Cycle

25. Germination from Spore

26. Septa (sing. Septum)
aseptate
septate

27. Mycelia (sing.
Mycelium)

28. Fungal Ecology
•Fungi and bacteria are principle decomposers in biosphere
(they and bacteria)
•The secrete exoenzymes to digest nutrients extracellularly,
which are then brought into cells directly across plasma
membranes
•Fungi are virtually the only organism capable of breaking
down lignin (the stuff that makes wood woody)
•Fungi act as disease-causing organisms of animals and,
especially, of plants
•Fungi produce antibiotics (e.g., Penicillin) which they use to
limit ecological competition from bacteria
•We take advantage of fungus-mediated decomposition in the
production of bread, beer, wine, cheeses, and soy sauce!

29. Mycoses
•Mycoses are diseases caused by fungi
•Mycoses are particularly a problem given
weakened immunity or habitual exposure to
moist conditions (e.g., wet socks)
•We can distinguish mycoses into:
•Superficial (meaning affecting only surface,
keratinized tissues)
•Subcutaneous (meaning invading below the
skin or into the lymph)
•Systemic (meaning invasion throughout the
the body)
•Greater penetration generally results in
greater body damage

30. Superficial Mycoses

31. Subcutaneous Mycoses

32. Systemic Mycoses
Examples
•Coccidioidomycosis
•Histoplasmosis (disease of Ohio Valley)
•Blastomycosis
•Paracoccidioidomycosis
These are thermally dimorphic fungi
that exist in nature, soil
Inhalation pulmonary inf. 
dissemination
No evidence of transmission among
humans or animals

33. Fungal Classification
We can also distinguish fungi into:
•Yeasts (single-celled fungi)
•Molds (filamentous, asexual fungi)
•Macrofungi (macroscopic fruiting bodies
supported by mycelia and hyphae)

34. Yeasts
Candida albicans
Budding yeast
Schizosaccharomyces pombe
Fission yeast
•Generic name given to unicellular fungi
•About 250 named species
•Most yeast reproduction is asexual and
takes place by cell fission or budding

35. Molds
•Molds are rapidly, asexually reproducing
filamentous fungi
•Note that many molds can go on to
reproduce sexually, producing fruiting
bodies, though at that point they are no
longer considered “molds”

36. Macrofungi

37. Dimorphism
•Some fungi can grow as yeasts or as
hyphae, depending on growth
conditions
•This can lead to a single fungal
species being classified as more than
one species
•Fungi also can display sexual versus
asexual growth, further confusing
fungal classification

38. Fungal Spores
•Spores are a means of fungal reproduction
•(fungi also can reproduce by fragmentation)
•All fungal spores are haploid and non-motile
•Asexual spores (produced by mitosis)
•Sexual spores (produced by meiosis)
•Zygospores, Ascospores, Basidiospores

39. Zygomycete Life Cycle

40. Ascomycete Life Cycle

41. Basidiomycete Life Cycle

42. Saccharomyces cerevisiae

43. Sordaria macrospora

44. Parasitic Helminths
•Flat worms (Phylum Platyhelminthes)
•Flukes (Schistosoma—schistosomiasis)
•Tape worms (Taenia—beef tape worm)
•Round worms (Phylum Nematoda)
•Ascaris, Dracunculus (Guinea
Worm),Trichinella, Wuchereria
•Hookworms (Ancylostoma)
•Pinworms (Enterobius)

45. Ancylostoma duodenale
•Flat worms (Phylum Platyhelminthes)
•Flukes (Schistosoma—schistosomiasis)
•Tape worms (Taenia—beef tape worm)
•Round worms (Phylum Nematoda)
•Ascaris, Dracunculus (Guinea
Worm),Trichinella, Wuchereria
•Hookworms (Ancylostoma)
•Pinworms (Enterobius)

46. Ascaris lumbricoides

47. Dracunculus medinensis

48. Enterobios vermicularis

49. Schistosoma japonicum
•Flat worms (Phylum Platyhelminthes)
•Flukes (Schistosoma—schistosomiasis)
•Tape worms (Taenia—beef tape worm)
•Round worms (Phylum Nematoda)
•Ascaris, Dracunculus (Guinea
Worm),Trichinella, Wuchereria
•Hookworms (Ancylostoma)
•Pinworms (Enterobius)

50. Taenia saginata
•Flat worms (Phylum Platyhelminthes)
•Flukes (Schistosoma—schistosomiasis)
•Tape worms (Taenia—beef tape worm)
•Round worms (Phylum Nematoda)
•Ascaris, Dracunculus (Guinea
Worm),Trichinella, Wuchereria
•Hookworms (Ancylostoma)
•Pinworms (Enterobius)

51. Trichinella spiralis
•Flat worms (Phylum Platyhelminthes)
•Flukes (Schistosoma—schistosomiasis)
•Tape worms (Taenia—beef tape worm)
•Round worms (Phylum Nematoda)
•Ascaris, Dracunculus (Guinea
Worm),Trichinella, Wuchereria
•Hookworms (Ancylostoma)
•Pinworms (Enterobius)

52. Wuchereria bancrofti