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:

• shistasomiasis
  
• malaria
  
• FILIARIASIS
  
• trypanasomiasis
  
• leishmaniasis
  
• (6) LEPROSY
  

REFERENCES
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!

• flexibility in pigeonholing these creatures is an absolute necessity

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

• life cycles are determined for the more important pathogens
• molecular work has been done primarily on the most "important" (read human) parasites
• some efforts are being made at understanding the parasites at the epidemiologic level

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

• thus, one basic characteristic of parasites is they're genetically or enzymatically "deficient", or lazy

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

Definitions

• symbiont: An organism which spends a portion or all of its life intimately associated with another living organism of a different pecies
  

  • a neutral term, not implying good or bad
    

• commensalism: does not involve a physiological interaction or dependency (hermit crabs & anemones)
  
• phoresis: mechanical carriage of a smaller animal by a larger (algae on aquatic arthropods
  
• parasitism: parasite is physiologically dependent on host
  

Definitions, continued
Temporally, realtionship can be

• permanent
  
• temporary (mosquitoes) - Dan Quayle, correct this)
  

Parasitism can be

• obligatory: where to complete life cycle, must clamber onto, or into, a host
  
• facultative: free-living organism that can take advantage of a host if the opportunity arises
  

By the location of their "home"

• ectoparasites: exterior of the body
  
• endoparasites: interior of the body
  

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
Macroparasites

• Pathogen does not increase to high numbers within the host during infection

infestation rather than infection

• Long generation time relative to host life span
  
• Multicellular
  
• Immunity tends to be of short duration and ineffective

reinfection often occurs

• Nematodes
  
• Trematodes
  
• flukes
  
• etc.

Microparasites

• Short reproductive rate compared to host life span
  
• Produce large numbers of progeny in host
  
• Small size
  
• Induce effective host immunity

viruses, bacteria, rickettsia, chlamydia, fungi
protozoa

• flagellates
  
• microsporidia
  
• myxosporidia
  
• amebae
  
• coccidia

  
  General organization of protozoan parasites

Phylum SARCOMASTIGOPHERA
Subphylum MASTIGOPHORA
flagellates, binary fission (Trypanosoma, Giardia)

Phylum SARCODINA
pseudopodial locomotion (amebae)

Phylum APICOMPLEXA
apical end of spore is complex (Cryptosporidium)

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

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

Phylum CILIOPHORA
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)

Giardiosis
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:

Eimeria
Cyclospora
Cryptosporidium

• Extraintestinal

Caryospora
Neospora
Sarcocystis
Toxoplasma

Cryptosporidium

• Disease reported in 79 mammalian species
  
• first human case reported in 1976
  
• 1982 started seeing Cryptosporidium in AIDS patients (mortality rate up to 50%)
  
• Recent surveys have indicated up to 20% of population infected
  
• major outbreak in Milwaukee, Wisconsin in 1993

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: 60¡C/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

Microspora

• Teeny, tiny (usually 1 µm) in size
  
• obligate intracellular parasites of all invertebrate (including ciliates & apicomplexans!) and all vertebrate classes
  
• usually divide by both asexual (schizogony) and sexual (sporogony) means
  
• ribosomes & rRNA sizes are typical of prokaryotes (70S, 23S & 16S) rather than eukaryotes
  
• lack mitichondria and lysosomes
  
• infect cells by injecting their internal components into the cell via polar tube
  

Microspoidian diseases

• pebrine or silkworm disease (Nosema bombycis) led to demise of silk industry in late 1800's
  
• likewise threat to bee industry (N. apis)
  
• Enterocytozoon is a mild parasite of man, but with AIDS it becomes deadly

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

• low infestation rate will not result in disease

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

• cell lysis: intracellular parasites multiply to high numbers within the cell and burst the cell
  
• hyperplasia: increase in host cell number as a result of parasite presence, e.g., inflammation
  
• hypertrophy: increase in cell size, e.g., E's in plasmodial infections
  
• metaplasia: conversion of one cell type to another, e.g., fibrocytes & flukes
  
• neoplasia: unrestricted multiplication of cells, e.g.,

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

• transmitted to cattle by tick bites causing East Coast Fever

mortality can be 90% in cattle in East Africa

• Sporozoites introduced into blood by tick bite
  
• Passively, but specifically, attach to lymphocytes (T)

not macrophages, granulocytes, or fibroblasts

• active internalization of sporzoite which leaves host membrane around parasite
  
• host membrane dissolved & sporozoite free in cytoplasm
  
• binds to host microtubule array
  
• replicates to form a 20 nuclei cell
  
• host cell undergoes clonal expansion
  
• parasite keeps up and replicates simultaneously
  
• host cell ruptures & releases pirolasms which infect erythrocytes
  
• infected lymphocytes grown in vitro are immortalized

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)