Eukaryotic Microbiology: Parasites & pathogenesis

The bad eukaryotes

These critters cause considerable problems in many vertebrate & invertebrate animals

WHO indicates that of the 6 major unconquered human tropical diseases 4 are protozoan parasites:

General references on parasitology: TNTC!
"Handbook of Protoctista" Margulis, et al., 1990
"Human Parasitology", Bogitsh & Cheng, 1990
"Parasitic Protozoa", Kreier & Baker, 1987
"Molecular Approaches to Parasitology", 1995

Some general dirt
Parasite taxonomy is a quagmire, BEWARE!

For the hundreds of thousands of animal parasites, little more than morphologic work has been done

A dictionary of terms & concepts

Webster: a parasite is an organism that lives on or within animal of a other species, from which it derives nutrients

But the reality is that there is a continuum between a "parasite" & host in terms of the outcome of the interaction


Definitions, continued
Temporally, realtionship can be

Parasitism can be

By the location of their "home"

Host-Parasite relationships
Host defined as

  • definitive: if parasite attains sexual maturity therein
  • intermediate: serves as temporary but essential environment for the development /metamorphosis of the parasite
  • transfer/paratenic: temporary refuge and vehicle for reaching definitive host in life cycle

Types of parasites by major characteristics

infestation rather than infection

reinfection often occurs

  • Nematodes
  • Trematodes
  • flukes
  • etc.


viruses, bacteria, rickettsia, chlamydia, fungi

  • flagellates
  • microsporidia
  • myxosporidia
  • amebae
  • coccidia

    General organization of protozoan parasites

flagellates, binary fission (
Trypanosoma, Giardia)

pseudopodial locomotion (amebae)

apical end of spore is complex (

small, with polar tube & extrusion apparatus (
Nadelspora canceri)

spores of multicellular origin, polar capsules, spore valves (
Myxobolus cerebralis)

Ciliates, large (
Ichthyophthirius )

Examples of protozoan parasites

Sarcomastigophora: Giardia lamblia
Class: Zoomastigophora, Family Hexamitidae
Intestinal tract of all classes of vertebrates
trophozoite shaped like a cut pear

  • 2 oval nuclei dorsal to adhesive disc
  • central microtubular structures (median bodies)
  • 4 pairs of flagellae which originate between nuclei

lack both mitochondria and Golgi apparatus
feed by pinocytosis
16S RNA analysis indicates
G. lamblia quite distant from other eukaryotes: a new Kingdom?
antigenically variable (variable surface proteins)

Signs of disease are transient to severe diarrhea

  • starts 7 days after exposure, lasts to 1 month
  • chronic giardosis can lead to significant weight loss (steatorrhea)

*prevalence 2-5% in developed countries, up to 20-30% in underdeveloped countries
transmitted by cyst stage in feces
evidence points to water contamination by humans, domestic, and wild animals. DON'T DRINK THE WATER!
Epidemiology of
Giardia (slide)

Apicomplexans: (coccidians)
Two groups : those that are happy to be in the gut, and those that need to get some air occasionally

  • intestinal always:


  • Extraintestinal



1,600,000 at risk, 403,000 contracted disease from contaminated water (abbatoirs, cattle grazing, human sewage, wild animals)
1992 outbreak in Lane County in public pool 37% of children using pool were ill, median age = 5 years

A tough nut to crack
Disinfection/destruction of spores is difficult

  • heat: 60íC/6 min
  • uv : 15,000 mW/sec/2.5 h
  • air drying 4 h
  • Chlorine: 28,000 ppm/24 h
  • 5% ammonia/18 h

_ lysol: does not inactivate
_ 4% iodophor/18 h did not inactivate
_ 1% phenol/30 min did not inactivate
_ 3% H2O2/30 min did not inactivate

General diagram of apicomplexian (slide)

Epidemiology of Cryptosporidium (slide)


TEM of Pneumocystis carinii (apicomplexan?) in lung alveoli (slide)

Proposed life cycle for Pneumocystis

Life cycle of Toxoplasma gondii

A cat's eye view of Toxoplasma life cycle

Replication of Toxoplasma in cats experimentally fed 1 infected mouse


Microspoidian diseases

also recently E. salmonis causes a lymphoproliferative disease (neoplasm?) in Pacific salmon

How parasites harm their hosts: Pathogenesis of disease

Macroparasite damage is often mechanical caused by interference with normal function due to globs of parasites

Because microparasites will replicate to high numbers in (or on) the body, and the offenders are small, the damage can be caused by a number of other mechanisms

Pathogenic mechanisms:
cellular level

Paragonimus westermanni associated with bladder neoplasms
E. salmonis and lymphoproliferative disease in salmon

EXAMPLE: Lymphoproliferation caused by
Theileria sp. infection
Theileria is an intracellular parasite (infect blood cells) in the phylum Apicomplexa

mortality can be 90% in cattle in East Africa

not macrophages, granulocytes, or fibroblasts

Immortalization in vitro is probably mimiced in vivo
the process may be generated by an increase in IL-2, a lymphocyte growth factor and
IFN- , another stimulatory molecule

Pathogenesis of diarrhea in giardiosis (slide)

Chemotherapeutants for protozoans (slide)