Introduction to parasitologyIntroduction to parasitology

Parasitology is a type of Parasitology is a type of SYMBIOSISSYMBIOSIS ( (living living togethertogether) (Any plant, animal, or protist that is ) (Any plant, animal, or protist that is intimately associated with another organism of a intimately associated with another organism of a different species; different species; each member is termed aeach member is termed a SYMBIONTSYMBIONT). ).

There are various type of symbiosis:There are various type of symbiosis:

PHORESISPHORESIS:: ( (traveling together" or "to traveling together" or "to carrycarry") (A smaller organism, termed the ") (A smaller organism, termed the PHORONTPHORONT, is carried mechanically by a , is carried mechanically by a HOSTHOST) [For instance, bacteria, fungus, ) [For instance, bacteria, fungus, cysts, or eggs on insect legs or even cysts, or eggs on insect legs or even passively within an arthropod gut] passively within an arthropod gut]

COMMENSALISMCOMMENSALISM:: ((when one symbiont, the when one symbiont, the

COMMENSAL, benefits and the other COMMENSAL, benefits and the other animal is neither helped nor harmedanimal is neither helped nor harmed..

True commensalism difficult to find, and True commensalism difficult to find, and may not even actually exist.may not even actually exist.

MUTUALISMMUTUALISM ( (each member, aeach member, a MUTUALIST, depends upon the MUTUALIST, depends upon the other; oblilgatory or facultativeother; oblilgatory or facultative))

For instance, flagellates produce For instance, flagellates produce cellulase in gut of termitescellulase in gut of termites

ciliates in ruminantsciliates in ruminants algae and fungus forming a lichenalgae and fungus forming a lichen . .

PREDATIONPREDATION ((where one member, the where one member, the PREDATOR, benefits and a smaller PREDATOR, benefits and a smaller organism, the PREY, is harmed; organism, the PREY, is harmed; usually eatenusually eaten))

Examples include Examples include coyotes and rabbits, coyotes and rabbits,

cats and mice. cats and mice.

PARASITISMPARASITISM ((where one member, where one member, the PARASITE, lives in or on another the PARASITE, lives in or on another organism, the HOST, at the expense organism, the HOST, at the expense of that organismof that organism) )

PARASITOLOGYPARASITOLOGY [ [the study of the the study of the relationship between a parasite and its hostrelationship between a parasite and its host]]

Parasitic relationships may beParasitic relationships may be:: temporary temporary facultative facultative obligatory obligatory

Basic types of parasitism and Basic types of parasitism and terminologyterminology::

ECTOPARASITE ECTOPARASITE (lives on surface of the host. (lives on surface of the host. Appropriate terminology includes the terms Appropriate terminology includes the terms ""infectedinfected" and "" and "infestedinfested") [i.e. ticks, lice, ") [i.e. ticks, lice, fleasfleas] ]

ENDOPARASITEENDOPARASITE (lives within the host; (lives within the host; appropriate terminology is "infected;" infested is appropriate terminology is "infected;" infested is inappropriate terminology) inappropriate terminology) [[i.e. roundworms in i.e. roundworms in gut; tapeworms in gutgut; tapeworms in gut] ]

HYPERPARASITEHYPERPARASITE ((parasite within a parasite within a parasite)parasite)

Malaria in mosquitosMalaria in mosquitos..Tapeworm larvae in fleasTapeworm larvae in fleas..

VECTORSVECTORS (transmits parasites from (transmits parasites from host to host) host to host)

BIOLOGICAL VECTORBIOLOGICAL VECTOR (essential in (essential in life-cycle of parasite) life-cycle of parasite)

MECHANICAL VECTORMECHANICAL VECTOR (unessential in (unessential in life-cycle of parasitelife-cycle of parasite((

Types of hosts Types of hosts ::DEFINITIVE OR FINAL HOSTDEFINITIVE OR FINAL HOST (host in which parasite reaches sexual (host in which parasite reaches sexual

maturity and reproduces) maturity and reproduces)

INTERMEDIATE HOSTINTERMEDIATE HOST (some development in host, but does (some development in host, but does

not reach sexual maturity; often not reach sexual maturity; often asexual stages)asexual stages)

PARATENIC OR TRANSPORT HOSTPARATENIC OR TRANSPORT HOST (no parasite development; but parasite (no parasite development; but parasite continues to live and is infective to next continues to live and is infective to next host; for instancehost; for instance, , pseudophyllidean pseudophyllidean tapeworm larvae in fish) tapeworm larvae in fish)

RESERVOIR HOSTRESERVOIR HOST (non-human animals that serve as sources (non-human animals that serve as sources

of infection to humans) of infection to humans)

Typical ways parasites transmittedTypical ways parasites transmitted:: INGESTIONINGESTION from food or water / from food or water /

inhalation inhalation

VECTORSVECTORS

DIRECT PENETRATIONDIRECT PENETRATION of skin from of skin from environment environment

ProtozoaProtozoa

The Protozoa are considered to be a The Protozoa are considered to be a subkingdomsubkingdom of the of the kingdomkingdom ProtistaProtista, , although in the classical system they were although in the classical system they were placed in the placed in the kingdom Animaliakingdom Animalia..

More than 50,000 species have been More than 50,000 species have been

described, most of which are free-living described, most of which are free-living organismsorganisms

StructureStructure

The smallest (mainly intracellular forms) The smallest (mainly intracellular forms) are are 1 to 10 µm long, but 1 to 10 µm long, but Balantidium coliBalantidium coli may measure 150 µm.may measure 150 µm.

Protozoa are unicellular eukaryotes.Protozoa are unicellular eukaryotes.

As in all eukaryotes, the nucleus is As in all eukaryotes, the nucleus is enclosed in a membrane. enclosed in a membrane.

In protozoa other than ciliates, the In protozoa other than ciliates, the nucleus is vesicularnucleus is vesicular..

with scattered chromatin giving a with scattered chromatin giving a diffuse diffuse appearance to the nucleusappearance to the nucleus, all nuclei in , all nuclei in the individual organism appear alike.the individual organism appear alike.

One type of vesicular nucleusOne type of vesicular nucleus contains a contains a more or less central body, called an more or less central body, called an endosome or karyosomeendosome or karyosome. .

The endosome lacks DNA in the parasitic The endosome lacks DNA in the parasitic amebas and trypanosomes.amebas and trypanosomes.

In the phylum Apicomplexa, on the other In the phylum Apicomplexa, on the other

hand, the vesicular nucleus has one or hand, the vesicular nucleus has one or more nucleoli that contain DNA.more nucleoli that contain DNA.

The ciliates have both a micronucleus The ciliates have both a micronucleus

and macronucleus, which appear quite and macronucleus, which appear quite homogeneous in composition. homogeneous in composition.

Some protozoa have a Some protozoa have a cytostomecytostome or cell or cell "mouth" for ingesting fluids or solid "mouth" for ingesting fluids or solid particles.particles.

Contractile vacuoles for osmoregulation Contractile vacuoles for osmoregulation occur in some, such as occur in some, such as NaegleriaNaegleria and and BalantidiumBalantidium. .

Many protozoa have subpellicular Many protozoa have subpellicular microtubules; in the Apicomplexa, which microtubules; in the Apicomplexa, which have no external organelles forhave no external organelles for locomotionlocomotion, , these provide a means for these provide a means for slow movement. slow movement.

ClassificationClassification

In 1985 the Society of Protozoologists In 1985 the Society of Protozoologists published a taxonomic scheme that published a taxonomic scheme that distributed the Protozoa into distributed the Protozoa into six phylasix phyla. . Two of these phylatheTwo of these phylathe SarcomastigophoraSarcomastigophora and the and the ApicomplexaApicomplexa contain the most contain the most important species causing human disease. important species causing human disease.

This scheme is based on morphology as This scheme is based on morphology as revealed by light, electron, and scanning revealed by light, electron, and scanning microscopy. microscopy.

Life Cycle Stages Life Cycle Stages

During its life cycle, a protozoan During its life cycle, a protozoan generally passes through several stages generally passes through several stages that differ in structure and activity. that differ in structure and activity.

TrophozoiteTrophozoite:: (Greek for "animal that (Greek for "animal that feeds")feeds") is a general term for the active, is a general term for the active, feeding, multiplying stage of most feeding, multiplying stage of most protozoa.protozoa.

In parasitic species this is the stage In parasitic species this is the stage usually associated with pathogenesis. usually associated with pathogenesis.

In the hemoflagellates the terms In the hemoflagellates the terms amastigoteamastigote, , promastigotepromastigote, , epimastigoteepimastigote, , and and trypomastigotetrypomastigote designate trophozoite designate trophozoite stages that stages that differ in the absence or differ in the absence or presence of a flagellum and in the position presence of a flagellum and in the position of the kinetoplast associated with the of the kinetoplast associated with the flagellum.flagellum.

A variety of terms are employed for A variety of terms are employed for stages in the Apicomplexa, such as stages in the Apicomplexa, such as tachyzoitetachyzoite and and bradyzoitebradyzoite for for Toxoplasma Toxoplasma gondiigondii. .

Other stages in the complex asexual and Other stages in the complex asexual and sexual life cycles seen in this phylum are sexual life cycles seen in this phylum are the the merozoitemerozoite (the form resulting from (the form resulting from fission of a multinucleate schizont) and fission of a multinucleate schizont) and sexual stages such as sexual stages such as gametocytesgametocytes and and gametesgametes..

Some protozoa form cysts that contain Some protozoa form cysts that contain one or more infective forms.one or more infective forms.

when the trophozoite of when the trophozoite of Entamoeba Entamoeba histolyticahistolytica first forms a cyst, it has a single first forms a cyst, it has a single nucleus.nucleus.

As the cyst matures nuclear division As the cyst matures nuclear division produces four nuclei and during produces four nuclei and during excystation four uninucleate metacystic excystation four uninucleate metacystic amebas appear amebas appear

Similarly, a freshly encysted Similarly, a freshly encysted Giardia Giardia lamblialamblia has the same number of internal has the same number of internal structures (organelles) as the trophozoitestructures (organelles) as the trophozoite..

However, as the cyst matures the However, as the cyst matures the organelles double and two trophozoites organelles double and two trophozoites are formed. are formed.

Cysts passed in stools have a protective Cysts passed in stools have a protective wall, enabling the parasite to survive in wall, enabling the parasite to survive in the outside environment for a period the outside environment for a period ranging from days to a year, depending on ranging from days to a year, depending on the species and environmental conditions. the species and environmental conditions.

ReproductionReproduction

Reproduction in the Protozoa may be Reproduction in the Protozoa may be asexualasexual, as in the , as in the amebasamebas and and flagellatesflagellates that infect humans, or that infect humans, or

both asexual and sexual, as in the both asexual and sexual, as in the ApicomplexaApicomplexa of medical importance. of medical importance.

The most common type of The most common type of asexualasexual multiplication is multiplication is binary fissionbinary fission, in which , in which the organelles are duplicated and the the organelles are duplicated and the protozoan then divides into two complete protozoan then divides into two complete organisms organisms

Division is longitudinal in the flagellates Division is longitudinal in the flagellates and transverse in the ciliatesand transverse in the ciliates..

Endodyogeny is a form of asexual Endodyogeny is a form of asexual division seen in division seen in ToxoplasmaToxoplasma and some and some related organisms.related organisms.

Two daughter cells form within the Two daughter cells form within the parent cellparent cell..

In In schizogonyschizogony, a common form of , a common form of asexualasexual division in the Apicomplexa, division in the Apicomplexa,

the nucleus divides a number of times, the nucleus divides a number of times, and then the cytoplasm divides into and then the cytoplasm divides into smaller uninucleate merozoites. smaller uninucleate merozoites.

In In Plasmodium, ToxoplasmaPlasmodium, Toxoplasma, , and other and other apicomplexans, the sexual cycle involves the apicomplexans, the sexual cycle involves the production of gametes (production of gametes (gamogonygamogony),),

fertilization to form the fertilization to form the zygotezygote, ,

encystation of the zygote to form an encystation of the zygote to form an oocystoocyst,,

and the formation of infective sporozoites and the formation of infective sporozoites ((sporogonysporogony) within the oocyst. ) within the oocyst.

NutritionNutrition

The nutrition of all protozoa is The nutrition of all protozoa is holozoicholozoic..

they require organic materials, which they require organic materials, which may be particulate or in solution.may be particulate or in solution.

Amebas engulf particulate food or Amebas engulf particulate food or droplets through a sort of temporary droplets through a sort of temporary mouth, perform digestion and absorption mouth, perform digestion and absorption in a food vacuole, and eject the waste in a food vacuole, and eject the waste substances.substances.

Many protozoa have a permanent mouth, Many protozoa have a permanent mouth,

the cytosome or micropore,the cytosome or micropore, through through which ingested food passes to become which ingested food passes to become enclosed in enclosed in food vacuoles.food vacuoles.

PinocytosisPinocytosis is a method of ingesting is a method of ingesting nutrient materials whereby fluid is drawn nutrient materials whereby fluid is drawn through small, through small, temporary openings in the temporary openings in the body wall.body wall.

The ingested material becomes enclosed The ingested material becomes enclosed

within a membrane to form a food within a membrane to form a food vacuole. vacuole.