Introduction To Parasitology

Parasitology (Dra. De Castro) Introduction to Parasitology January 21. 2008

What is a parasite?

Mutualism

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a pathogen that simultaneously injures and derives sustenance from its host

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a symbiosis in which 2 organisms mutually benefit from each other

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some parasites are commensals neither benefit nor harm their host

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e.g. termites and flagellates in their digestive system, which synthesize cellulose to aid in breakdown of ingested wood

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for example: Entamoeba coli

Parasitology – area of biology concerned with the phenomenon of dependence of one living organism on another Medical Parasitology – concerned primarily with the animal parasites of humans and their medical significance, as well as their importance in human communities

Biological Relationships

Parasitism -

a symbiotic relationship where one organism (the parasite), lives in or on another (the host), depending on the latter for its survival and usually at the expense of the host

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e.g. Entamoeba histolytica derives nutrition from the human host and causes amebic dysentery Parasites (According tot heir habitat or mode of development)

Symbiosis -

living together of unlike organisms

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also involve protection or other advantages to one or both partners

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Different form of symbiosis: distinguished on whether or not the association is detrimental to one of the 2 partners.

o

Commensalism

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Mutualism

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Parasitism

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Endoparasite: a parasite living inside the body of a host; presence of an endoparasite in a host is called an infection

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Ectoparasite: a parasite living outside the body of a host; presence of an ectoprasite inside the host is called an infestation

A parasite is considered erratic when it is found in an organ which is not its usual habitat

Parasites

Commensalism -

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symbiotic relationship: 2 species live together and one species benefits from relationship without harming or benefiting the other e.g. Entamoeba coli in the intestinal lumen is supplied nourishment and protected from harm, while it does not cause any damage to host

Eis*, Mel* and MR

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Obligate parasites need a host at some stage of their life cycle to complete third development and to propagate their species o

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E.g. tapeworms depend entirely upon their host for existence

Facultative parasite: may exist in a free living state or may become parasitic when the need arises

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Parasitology – Introduction to Parasitology by Dra de Castro

o

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•

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e.g. Diplydiasis in humans is accidental

Most roundworms are permanent parasites and live their entire adult lives in their hosts

e.g. ticks, flea and mites

Spurious parasite: a free-living organisms ha passes through the digestive tract without infecting the host o

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intermediate host: harbors the asexual or larval stage of the parasite

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humans are the definitive host in taeniasis

Pigs or cattle as intermediate hosts of Taenia spp. while snails are hosts of Schistomsoma spp.

Paratenic host: one in which the parasite does not develop further to later stages. However, the parasite remains alive and is able to infect another susceptible host

pigs are reservoirs of Balantidium coli

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field rats of pargonoimus westermani

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cats of Bruigia malayi

man may be the most important host in the spread of the disease or an incidental host of parasites prevalent in other animals

Vectors Vectors: responsible for transmitting the parasite from one host to another

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Biologic Vector: transmits the parasite only after the latter has completed its development within the host : aedes sucks blood form a patient with fliariasis, the parasite undergoes evereal stage of development from the fist stage larvae to 3rd stage larva before it is transmitted to another susceptible host

Host: classified based on their role in the life cycle of the parasite

o

o

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e.g. Isosopora

definitive or final host: one in which the parasite attains sexual maturity

Reservoir hosts: allow parasite’s life cycle to continue and become additional sources of human infection

humans are not always the final host

Hosts

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Paragonimus metacercariae in raw wild boar meat can pass through the intestinal wall of humans and complete its development

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Temporary parasite: lives on the host only for a short period o

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o

Permanent parasite: remains on or in the body of the host for its entire life cycle o

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e.g. leeches feed on blood on their hosts but when released in aquatic environment survive as free living organisms

Accidental or Incidental parasite: establishes itself in a host where it does not ordinarily live o

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•

Mechanical or phoretic vector: transports the parasite : flies and cockroaches that feed on fecal material may carry enteric organisms and transfer these to food.

Exposure and Infection Carrier: harbors a pathogen without manifesting any signs and symptoms Exposure: process of inoculating an infective agent Infection: connotes the establishment of the infecting agent in the host

Parasitology – Introduction to Parasitology by Dra de Castro

Incubation period (clinical incubation period): period between infection and evidence of symptoms Pre-patent period (biologic incubation period): period between acquisition of this parasite and evidence or demonstration of infection

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5. other sources -

another person , his beddings and clothing, the immediate environment he has contaminated

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e.g. asymptomatic carriers of Entmoeba histolytica working as food handlers

Autoinfection -

when an infected individual becomes his won direct source of infection

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e.g. enterobiasis: infective eggs end up in the hands by scratching the hperianal areas where the gravid females lay their eggs

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mouth is the most likely portal of entry

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where the self is the source of infection as seen in the life cycles of E. vermicularis, Hymenolipsis nana and Strongyloides stercoralis

most cesotdes, trematodes and protozoans are food born

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skin penetration e.g. hookworms and stronglyoides

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arthropods: as vectors transmit parasites through bites

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agents of malaria, filarial, leishmaniasis and trypanosomaiasis

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congenital transmission: toxoplasma gondii trophozoites can cross the placenta

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in transmammmary infection with Ancylostoma and Strongyloides the parasite maybe e transmitted through the mother’s milk

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Superinfection or Hyperinfection

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when the already infected individual is further infected with the same species leading to massive infection

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alteration in the normal life cycle of Strongyloides results in a large increase in worm burden  severe debilitation/death

Sources of infection

Modes of Transmission

1. contaminated soil and water: most common -

lack of sanitary toilets & use of human excreta as fertilizer  favor infection with ascaris, hookworm

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water contaminated with cysts of amoebae or flagellates as well as cercariae of Schistosoma

2. food: food may contain the infective stage of the parasite 3. arthropods: e.g. mosquitoes as vectors of malaria and fliaria parasites 4. other animals (wild or domesticated ) may harbor the parasite: e.g. cats are direct sources of Toxoplasma infection Other ways

Parasitology – Introduction to Parasitology by Dra de Castro

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inhalation of air-borne eggs of Enetorbius

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Sexual intercourse as in the case of richomonas vaginalis

Nomenclature •

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animal parasites are classified according to the International Code of Zoological Nomenclature each phylum is subdivided into classes which are further subdivided into orders, families, genera and species

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further subdivisions of suborder, superfamily and subspecies are employed at times

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family names are formed by adding – idae to the stem of the genus type

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names of the genera and species are italicized or underlined when written

Life Cycle

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clinical consequences of infections that affect an individual’s ell being

Diagnosis •

demonstration or parasites

- e.g. eggs, adults, larvae, cysts, oocysts and trophozoites •

detection of host immune response to the parasites

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e.g. antibodies and antigens

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in light infections when parasites are still immature, immunoassays may become useful

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Specimens for parasitic examination: stool, blood ,urine, sputum, cerebrospinal fluid, tissue aspirate, tissue biopsies and orifice swabs

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Microscopy – simple, informative, can establish rapid diagnosis, dependent on skill of microscopists

Life Cycle •

may be simple of complicated

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As the life cycle becomes more complicated, the lesser the chances are for eh individual parasite to survive perpetuation of a species of parasite depends upon its ability to ensure transmission from one host to the next

limited by low density of parasites in specimens: may be affected by stage of the disease, biology of the parasite

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Parasite concentration techniques: require additional equipment, supplies and reagents

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Parasite culture methods; require a clean bench and special culture media

Cumulative prevalence

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Special staining procedures of fluorescent microscopy available for better visualization

- Percentage of individuals in a population infected with at least one parasite

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Molecular diagnosis: molecular or DNA-Based methods

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address problems associated with microscopy and immunodiagnosis

Epidemiologic measure •

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Intensity of infection

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Number of worms per infected person

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Also refereed to as worm burden

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Measured directly by counting helminth eggs excreted in feces expressed as eggs per gram (epg)

DNA probes, detection of specific nucleic acid sequences

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Rapid diagnostic tests (RDTs)

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newer and more advance methods of immunodiagnosis (antigen-capture tests)

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Morbidity

Parasitology – Introduction to Parasitology by Dra de Castro

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detects fewer parasites, faster results

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easy to perform and require minimal training of personnel

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- group-level deworming where the (risk) group to be treated (without prior diagnosis) may be defined by age, sex, or social characteristics irrespective of infection status

Immunodiagnosis •

overcome problems related to microscopy examination

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detect antigen or antibodies in clinical specimens

- Immunofluorescent Assay ELISA, indirect Hemagglutination Assay, RIA, and dot blot •

antibody detection tests

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limited in distinguishing active from prior infections useful in detecting exposure and etiology of outbreaks an in mapping foci of infection

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Universal treatment - population-level deworming in which the community is treated irrespective of age, sex, infection status, or other social characteristics

Prevention and Control •

Morbiditty control - the avoidance of illness caused by infections

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Information-education-communication - a health education strategy that aims to encourage people adapt and maintain healthy life practices

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Environmental management - manipulation of environmental interaction with human beings

factors

or

their

- aim: prevent or minimize vector or intermediate host propagation and reduce contact between humans and infective agent

serve as alternatives when microscopic exam fails

Treatment • •

Deworming: use of antihelminthic drugs

Environmental sanitation: interventions to reduce environmental health risks - Includes safe disposal and hygienic management of human and animal excreta, refuse and waste water

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Cure rate - no. (expressed as percentage) of previously positive subjects found to be egg-negative on examination of a stool or urine sample at a set time after deworming

- Involves the control of vectors, immediate hosts, and reservoirs of disease - provision of safe drinking water and food safety

- Indicator used to measure reduction in intensity of infection Eradication versus Elimination •

Egg reduction rate (ERR) - percentage fall in egg counts after deworming

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- Indicator used to measure reduction in intensity of infection

Disease eradication - permanent reduction to zero of the worldwide incidence of infection caused by specific agent, as a result of deliberate efforts - continued measures no longer needed

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Selection treatment -involves individual-level deworming with selection for treatment based on a diagnosis of infection

Targeted treatment

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Disease elimination - reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts

Parasitology – Introduction to Parasitology by Dra de Castro

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- continued intervention/surveillance still required

Chemicalcomponents of body fluid - Lipase content of breastmilk: toxic to G. lamblia in vitro

Mechanisms by Which Parasites Cause Injury to Host Immunology of Parasitic infection •

Most common: interference with the vital processes of the host through the enzymes systems - Secretory and excretory products of parasites allow them to metabolize nutrients from the host - Ex. E histolytica trophozoites secrete cysteine proteinases digest cellular materials and degrade epithelial basement membrane facilitating tissue invasion

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Host’s cellular components on the skin and mucosal system of the GIT serves as a protective mechanism for the host - Langerhan’s cells, dendritic cells, mast cells and recirculating T lymphocytes on the skin (SALT) - Gut associated lymphoid tissues (GALT)

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Invasion and destruction of host tissue - Plasmodium invades rbc, and causes its rupture

Mounting of acquired specific responses against parasite antigen - Response: recognize self from non self - Destroys the parasite

Host immune response to parasitic infection •

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Deprivation of the host of essential nutrients and substances - Heavily hookworm infestation causes iron deficiency anemia

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Genetic make-up the host Effects of the ofHost on the Parasite

Well defined genetic control Parasite antigens are processed and presented to T helper lymphocytes (Th1 or Th2)→ produce lymphokines→ bring about CMI response or antibody production Antibodies involved: IgE, IgG, IgM and IgA Most common response: - eosinophilia and elevated serum IgE

- influence the interaction with the parasite - sickle-cell trait Falciparum malaria

confers

some

protection

- Ex. marked immunologic response in Angiostrongylus cantonesis and T. canis

from

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Nutritional status of the host - high protein diet not suitable for the development of intestinal protozoan

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Immune processes Acquired immunity important in modifying severity of disease in endemic areas

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Bound IgE antibodies mediate explusion of adult GI helminthes IgG1 and IgE - act as mast cells and basophils→degranulation and eventual release pharmacologically active substances →Type 1 hypersensitivity reaction called anaphylaxis Ex. rupture of larval infection with E. granulosus

• Immunology and Parasitic Infection Host-Parasite Interactions •

Non specific defenses - intact skin: hookworms synthesize a protein that aids in the penetration process

- acidic pH of vaginal secretions and gastric acid juice: T. vaginalis trophozoites: unable to survive the acidic environment of the vagina •

Mucus secretion - envelope parasites (G. lamblia) affecting its motility and reducing pathology on the host

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IgG and IgM prevent penetration of rbcs by Plasmodia spp and Babesia spp Secretory IgA: protect against metacestode and GI infxns IgM and IgA: mediate antibody dependent cell mediated cytotoxicity in G. lamblia infection

Parasite Evasion mechanism •

Immune suppression - reduce immune function of macrophages leading to defective antigen processing in Plasmodium infection

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Antigenic variation

Parasitology – Introduction to Parasitology by Dra de Castro

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- surface protein variation seen in T. gambiense and G. lamblia infection •

Host mimicry - tegument of Schistosoma spp adult can acquire antigenic molecules from the host

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Intracellular sequestration - Trypanosoma and Leishmanic proliferate in macrophages and escape the host immune response

Protozoa All protozoa fall under Kingdom Protista

Adverse effects of Immune Response to the host •

Intense or abnormal immune responses may result in pathological manifestation - T cruzi→ massive damage not only to infected cells but also to surrounding nerve cells and myofibers - recurrent Plasmodium infection, immune complexes are associated with Hyperactive Malarious Splenomegaly (HMS)→ patient suffer from persistent splenomegaly and anemia 

In P. malariae infection, these immune complexes are deposited in glomerular basement membrane →kidney failure and nephritic syndrome

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Nucleus/nuclei Cyroplasm Outer limiting membrane Cellular elaborations: organelles * Locomotory apparatus

☻May require a wet environment for feeding, locomotion, osmoregulation and reproduction ☻Form infective stages called cysts which are resistant to environmental changes compared to the vegetative stages ccalled trophozoite

Groups of Parasites with Medical and Public Health Importance

Unicellular protozoan Parasitic infections Multicellular metazoan

Kingdom Protista

Phylum

Organelle of locomotion

Phylum Sarcomastigophora Sarcomastig ohora

Ciliophora

Microspora

Apicomplexa

Subphylum Mostigophora

Flagella(whip-like structure)

Subphylum Sarcodina Pseudophodia extrusions)

Metazoan parasites Phylum Ciliophora Helminths

Annelids Ex. Leeches

Arthropods

Nematodes

Flatworms Trematodes Cestodes

(foot-like

Cilia (hair-like projection)

Parasitology – Introduction to Parasitology by Dra de Castro Phylum Apicomplexia

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Apical complex at the anterior end consisting of polar rings, subpellicular tubules, conoid, rhoptries, and micronemes

Phylum Microspora

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Extrusion apparatus to insert infective material to the host cell

Provided with separate sexes; some may be parthenogenetic With sensory organs in the anterior (amphids) and posterior ends (phasmids) -those with phasmids are called phasmids nematodes -without phasmids: aphasmids nematodes

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3 aphadsmids worms of medical importance: - Trichuris, Trichinella and Capillaria

Classification of Protozoan Parasites Phylum

Subphylum

Sarcomastigophora Mastigoph ora

Species

Cestodes: Tapeworms

Chilomastix mesnili, Dientamoeba fragilis, Giardia lamblia, Trichomas hominis, T. tenax, T. vaginalis, Leishmania brazilensis, L. donovani, L. tropica, Trypanosoma brucei complex, T. cruzi

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Belong to Platyhelminthes Dorsoventrally flattened with bilateral symmetry

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Segmented, ribbon-like appearance Adult tapeworms are hermaphroditic - provided with scolex (main attachment to the host)→ neck region, from it will start segmentation or strobilization

• Acanthamoeba castellani, Endolimax nana, Entamoeba coli, E. dispar, E. histiolytica lodamoeba butschlii, Naegleria fowleri

2 orders with medical importance: Pseudophyllidea and Cyclophyllidea Sarcodina Ciliophora

Balantidium coli

Apicomplexa

Babesia spp., Cryptosporidium hominis, Isospora belli, Cyclospora cayatenesis, Plasmodium spp., Toxoplasma gondii

Microspora

Enterocytozoon bineusi, Encephalitozoon spp., Vittaforma cornea, Pleistophora spp., Trachipleistophora hominis, Microsporidium spp., Brachiola vesicularum

Classification of Metazoan Parasites

Ascaris, scolex Capillaria, Enterobius, Spatulate with Globular with 4 Hookworm, muscular Strongyloides, sucking groves Trichuris called suckers bothria Angiostrongylus, Filaria, Trichenella Extraintestinal Genital pores Present but has uterine Present but without uterine Cestoidea

Cyclophyllide

pore, instead undergo the process of apolysis whereby gravid segments Diphylidium, Echinococcus, Hymenolepis, are detached from the main Raillientina, Taenia body and eggs are eventually released Diphyllobothrium, Spirometra

Recovery of Segments are not Both eggs and segments Artylechinostomum, Clonorchis, Echinostoma, bothPseudophyllid eggs and generally found are recovered from patients ea Fascioloa, Fasciolopsis, Heterophyids, Opisthorchis, segments Paragonimus, Schistosoma

Chilopoda

Elongated, cylindrical, with bilateral symmetry and a complex digestive tract

pore which allows release of eggs from gravid uterus

a

Arachnida

Nematodes

Cyclophylidea tapeworms

Intestinal scolex

Eggs Arthrophods

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Pseudophyllidean tapeworms

Nematoda

Eggs are operculated and Eggs are nonoperculated immature and require and are passed out readily aquatic development of containing the hexacaanth Mites, Scorpion, Spider, Ticks the embryo called embryo coracidium Centipedes

Crustacea Intermediate host Diplopodia Insecta

Copepods, Crabs Require 2 IH; In the 1st IH, eggsMillipedes encyst as procercoid

Only 1 IH, but diff species prod. diff types of encysted larvae in the IH Flies, the Flea, 2nd Beetle, IH: Lice, as Wasps, Bugs, Mosquitoes

In pleocercoid larvae Tongue worms

Pentastomida

Ex.

Taenia

produce

Parasitology – Introduction to Parasitology by Dra de Castro This is best represented by Diphyllobothrium which causes adult infection and Spirometra which causes larval infection in man

cysticerus type Hymenolepis, Dipylidium and raillietina produce the cysticercoid type Echinococcus produce hydatid

Infection with adult tapeworms is through the consumption of infected intermediate hosts *Some are infected with the larval stage of L. solium called cysticercosis and of Echinococcus spp. Called hydatid cyst

Trematodes or Flukes • •

Leaf-like and unsegmented Adults have an oral sucker and a ventral sucker called acetabulum Require 2 intermediate hosts

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While the 1st IH is always a snail, the 2nd IH vary from fish, to crustacean, to another snail, or to fresh water plants Generally grouped based on their habitat o Adult shistosomes sre found in the mesenteric veins→ blood flukes o Adult Paragonimus found in the lung parenchyma o Flukes that inhabit the livea and bile passages: Fasciola, Clonorchis and Opisthrchis o Flukes that inhabit the intestines: Fasciolopsis, Echinostoma and Heterophyids

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Arthropods • • •

Billaterally symmetrical organisms with segmented and jointed appendages Chitinous exoskeleton covers the body Includes: insects, mites, ticks, spiders, scorpions, centipedes, millipiedes and crustaceans

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