Lecture 1.                 Clinical Parasitology                 J. D. MacLean
McGill Centre for Tropical Disease

This set of lecture note and images is presented as a draft. Please feel free to submit feedback, images that you think might benefit future viewers to JD MacLean  dick.maclean@mcgill.ca . Also available are the lecture slides in .pdf format. It is recommended that you view the images found in the .pdf format.

 References
The best reference text available in clinical parasitology is: Beaver, Jung and Cupp. Clinical Parasitology, 9th ed., Lea and Febiger, Philadelphia, 1984.

The best Tropical Disease texts are;
1) Manson's Tropical Tropical Diseases ed GC Cook et al. 21st ed ,2003 WB Saunders Co., London

2) Hunter's Tropical Medicine and Emerging Infectious Diseases ed Strictland GT 8th ed, 2000 WB Saunders Co., Philadelphia

3) Tropical Infectious Diseases, Principles,Pathogens & Practice. ed RL Guerrant et al. Churchill Livingstone, Philadelphia  2006

Other texts or reference sources of a more condensed nature are:

1) Markell and Voge. Medical Parasitology. 6th ed. 1991. Saunders Co.
2) Brown and Neva. Basic Clinical Parasitology. Appleton-Century-Crofts. Norfolk Connecticut.

There are a number of WEB sites with large collection of parasite images. As of April 2004 I include:

http://www.dpd.cdc.gov/dpdx         Centres for Disease Control, Atlanta

Introduction
These notes are a guide to the undergraduate medical student lecture series at McGill University. They are not meant to replace a textbook or information that can only be presented well in lecture or tutorial (problem solving) format. Hyperlinks to an image library are indicated in the text. The images are of high definition and are best viewed with a fast computer.

Emphasis is placed on Canadian parasites and parasites which present major problems to world health.

Definitions

Medical parasitology: the study of the parasites of man and their medical consequences.

Parasite: living organism requiring intimate prolonged contact with another living organism to meet some of its basic nutritional needs. In a more restricted definition, it refers to organisms that are not viruses, bacteria, fungi, rickettsia, or chlamydia and obviously include organisms of varying complexity from a unicellular protozoa to a complex multicellular helminths.

Host: organism harbouring a parasite.

Definitive host: animal harbouring the adult or sexually mature stage of the parasite.

Intermediate host: animal in which development occurs but in which adulthood is not reached.

Life cycle: for survival and reproduction reasons many parasites evolve through a number of morphologic stages and several environments or different hosts. The sequence of morphologic and environmental stages is referred to as the life cycle.

Parasitic infection: invasion by endoparasites (protozoa and helminths).

Parasitic disease: invasion and pathology produced by endoparasites.

Parasitic infestation: external parasitism by ectoparasites (arthropods).

Commensalism: the association of two different species or organism in which one is benefited and the other is neither benefited nor harmed. (e.g. non pathogenic intestinal protozoa)

Reservoir host: an animal that harbours a species of parasite that can be transmitted to and infect man.

Vector: an arthropod or other living carrier that transports a pathogenic organism from an infected to a non-infected host. This can be passive transport or as an essential host in the life cycle of the pathogenic organism (i.e. a biologic vector).

Carrier: a host that harbours a parasite but exhibits no clinical signs or symptoms.

Zoonosis: a disease involving a parasite for which the normal host is an animal, and wherein man can also be infected.

Protozoa: a subkingdom consisting of unicellular eukaryotic (Greek-karyon=nut=nucleus) animals.

Eukaryote: a cell with a well-defined chromosome in a membrane bound nucleus. (versus prokaryotic bacteria with nucleic acid material bound in a nuclear membrane).

World annual rates of morbiditv and mortality

Taxonomy

.

Intestinal Protozoa

OVERVIEW

The following areas of knowledge are suggested as especially important for the beginner.

- biology: systematics, structural and motility features

- pathogenesis: in small intestine of Giardia, Microsporidia, Cryptosporidia, Cyclospora
                               in large intestine of : Entamoeba histolytica

- epidemiology: zoonoses, carrier states, fecal-oral transmission

- clinical features: acute, chronic, asymptomatic carrier states, opportunism in the                                                immunocompromised

- diagnosis: stool examination, stains, transport preservatives

- treatment: metronidazole, diodoquine, trimethoprim/sulfa

- problems: drug resistance; pathogenesis; laboratory identification

Taxonomy
The classification of the medically important parasites is as follows (ref. Beaver-Clinical Parasitology 1984) within the kingdom "Animalia".

Subkingdom Protozoa: 45,000 unicellular species, each defined
                       in the phylum according to organelles, locomotion, life cycle
                       and type of reproduction.

   Phylum Sarcomastigophora.

             Subphylum - Mastigophora: movement with flagella - e.g. Trichomonas, Giardia

             Subphylum - Sarcodina: pseudopodia, e.g. amoeba

   Phylum Apicomplexa: apical complex, no locomotor apparatus; sexual
                                                        reproduction, e.g. malaria, Isospora, Toxoplasma

   Phylum Ciliophora: movement with cilia, e.g. Balantidium.

   Phylum Microspora: e.g.   Enterocytozoon

trophozoites
cysts
	Sarcodina eg E. histolytica	Ciliophora eg. Balantidium	Mastigophora eg. Giardia	Apicomplexa eg Cyclospora	Microspora eg Enterocytozoon

Subkingdom - Metazoa multicellular organisms.

       Phylum - Nematoda: round worms, round in cross section; separate
                                       sexes; complete digestive tract; 500,000 species only a
                                        few parasitic to man; e.g. hookworm., filaria

      Phylum - Platyhelminthes: flat worms; incomplete or absent digestive
                                        tract; no body cavity; mostly hermaphroditic.

                     Class Trematoda: flukes; leaf shaped unsegmented body,
                                          often complex life cycle; e.g. lung fluke.

                     Class Cestoidea: tapeworms; segmented bodies each segment
                                          containing complete set of male and female reproductive
                                          organs; no alimentary tract, nutrition by absorption through
                                          body wall. e.g. beef tape worm.

PROTOZOA

There are 45,000 species of protozoa. A small number are parasites of man, some pathogenic and others non-pathogenic (commensals).

The life cycles of the protozoa vary from simple binary fission (e.g. Entamoeba histolytica) in one host to a complicated sequence of morphologic transformations through several hosts (intermediate and definitive) e.g. malaria.

The biology of the organism, pathogenesis of disease and epidemiology will be discussed with emphasis on the common or representative organisms.

A taxonomic approach to classification is of biological importance but a clinical classification is useful for the physician.

Taxonomic or Clinical

Intestinal protozoa
The important intestinal protozoa that infect man fall within the following 5 catagories:

  Only eight have been shown to be pathogenic and therein lies a major problem. Some significant expertise is required to diagnose the genus or species of the protozoan in a laboratory specimen; lacking this, diagnoses are very difficult.

The characteristics important to the clinical parasitology microscopist include nuclear shape and size, chromatin distribution, the micrometer measured size of the protozoan, intracellular organelles and locomotion.           

Entamoeba histolytica    Subphylum: Sarcodina

E. histolytica trophozoite           with ingested RBCs		E. histolytica trophozoite

Biology: Two morphological stages occur;                                                                                                  

Trophozoite - metabolically active invasive stage, moves with pseudopodia, ingests RBC, lives in  colon and is found in fresh diarrheal stool; divides by binary fission.
                       - trophozoite 10-60 µm
                       - cogwheel distribution of nuclear chromatin
                        - hematophagous
                        - unidirectional movement with pseudopodia

Cyst - "vegetative" inactive form resistant to unfavourable environmental conditions outside human host;
                         - 4 nuclei
                         - this is the infective form resistant to stomach acid if swallowed
                         - survives up to 30 days; excyst to trophozoite on passing through stomach
                         - cyst 10-20 µm
                         - chromotoidal body        

Pathogenesis: - Digests (liquifies) human host cells (colon wall, neutrophils, liver cells)

Disease states: - asymptomatic carrier- - symptomatic infection - amoebic dysentery - mucoid bloody     - amoebic - liver or lung abscess

Diagnosis:
- stool examination - for trophozoites and cysts
- amoebic serology
- abscess aspirate
 - Entamoeba dispar  a non-pathogen is indistinguishable by microscopy and is a much more common intestinal protozoan than Entamoeba histolytica. Antigen capture and PCR tests can distinguish E. dispar from E. histolytica in heavier infections.                      

Treatment:
Invasive states (Dysentery, Liver abscess): metronidazole
Carrier states: diiodoquine, diloxanide furoate, or paromomycin

Giardia lamblia

Subphylum: Mastigophora

Biology:
- occurs as both a flagellated trophozoite and a non-flagellated cyst form

- trophozoite (9-21 µm long), motile, with 8 long flagella, ventral sucker which attaches to duodenal mucosa; lives only in small intestine; non invasive.

- cyst (8-12 µm); resistant to external environment, to municipal chlorination; intermittently expelled in stool.

Giardia trophozoites		Giardia cyst

CDC Atlanta
http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Giardiasis_il.htm

  Epidemiology:
- faecal oral spread
- prevalence about 3-5% in Canada; increased in some travelers, backpackers, institutions, day cares and any groups with increased fecal-oral spread.
- zoonosis - found in most mammals; esp. beaver, cattle,  cats, dogs, etc.

Pathogenesis: - mechanism unknown; toxin? host immunity?   

Giardia trophozoites in small intestine     - more common in IGA deficient - some immunity occurs post infection - pathology: villus atrophy and crypt  hyperplasia

 Clinical:
- 90% of infected are asymptomatic carriers
- acute giardiasis includes diarrhea, gas, anorexia for 1-2 weeks
- chronic giardiasis - diarrhea, malabsorption

Diagnosis:
- stool examination
- duodenal fluid (aspirate or string test)
- giardia antigen detection in stool

Treatment: - metronidazole, atabrine.

Cryptosporidum              Phylum: Apicomplexa

Biology: - very small unicellular (2-4 µm) bodies perched  on upper intestinal epithelium; enveloped in a  membrane of host origin (therefore intracellular).  - undergoes metamorphosis through asexual binary fission and then sexual stages similar to other Apicomplexa    (see malaria and toxoplasmosis). - the oocyst (2-4 µm) passed in stool is resistant to environment and is the infecting stage.

CDC Atlanta http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Cryptosporidiosis_il.htm

  Epidemiology:
- zoonosis - found in calves, sheep, goats, and many other animals
- faecal oral spread
- 2-3% prevalence in humans
- increased prevalence in day cares, travellers, and in immunocompromised e.g. AIDS, cancer.

Pathogenesis:
- not yet clear how intestinal damage is caused
- ?toxins, ? host immune reaction

Clinical:
- asymptomatic - maybe the majority
- symptomatic - acute diarrhea of 1-2 weeks
- chronic diarrhea and malabsorption most commonly in immunocompromized

Diagnosis:
- stool examination - special stains (modified Ziehl-Neelsen, fluorescent)
- small intestinal biopsy
- Cryptosporidium antigen detection in stool

Treatment:
- no effective treatment, Spiramycin experimental

Cyclospora cayetanensis            Phylum: Apicomplexa

Biology: - small unicellular, live within cytoplasm of epithelial cells of duodenum - similar life cycle stages as cryptosporidium - the oocyst (diagnostic stage) found in stool is ~9 µm

CDC Atlanta

http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Cyclosporiasis_il.htm

 

Epidemiology:
- uncertain what is the reservoir
- increased incidence in travellers and occurs in epidemics in North Americans eating Central American berries (raspberries), pesto

Pathogenesis: - damage to small intestinal mucosa

Clinical: - assymptomatic
                  - symptomatic: acute diarrhea of several weeks

Diagnosis: - stool examination and concentration; modified Ziehl-Neelsen stains

Treatment: - Septra (trimethoprin - sulfa).

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                                  Vaginal protozoa
Trichomonas vaginalis           Subphylum: Mastigophora
Biology:
- A unicellular (av. 13 µm) flagellate which moves with 3-4 terminal flagella and a flagella in an undulating membrane - no cystic form.
- multiples by binary fision
- lives in close association with vaginal, urethral and prostatic tissue

Trichomonas vaginalis	Trichomonas vaginalis	Trichomonas vaginalis

Pathophysiology:
- causes degeneration and desquamation of local tissues; mechanism unclear.

Epidemiology
- found in 3-5% of female population of N.A.
- increased in STD clinics (50%), prostitutes 50-75%
- only in humans; no animal reservoir
- sexual transmission, but also on fomites
- can persist for 2 years in host

Clinical:
- asymptomatic - vast majority
- symptomatic - vaginitis, prostatitis, urethritis

 

 

http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Trichomoniasis_il.htm
Diagnosis:
- "wet mount" of vaginal secretions, urine sediment or prostatic massage; characteristic fast (darting) movement.

Treatment: - Metronidazole.

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OTHER intestinal protozoa

pathogens:
Balantidium coli (large intestine, Ciliata)
Isospora belli (small intestine, Apicomplexa)
Dientamoeba fragilis (small intestine, Mastigophora)
Enterocytozoon bienusi (small intestine, Microspora)

non-pathogens:
Entamoeba dispar
Entamoeba coli
Endolimax nana
Iodomoeba butschlii
Blastocystis hominis
Trichomonas hominis
Enteromonas hominis

 

Since ept 29, 2005