Introduction to Parasitology

Topic: Parasites - Introduction

• Parasitology is the study of the relationship between a parasite and its host.
• Parasites infect other organisms, relying on the host for shelter, nourishment, and development.
• Parasites live on or in hosts, benefiting at the host's expense.
• Hosts are organisms that harbor parasites.
• Clinical parasitology focuses on parasites in humans and their medical relevance.
• Parasitism is a symbiotic relationship where one party (the parasite) benefits at the expense of the other (the host).
• Parasites can cause disease, though some do not.
• Parasitic relationships are generally considered detrimental, as parasites deprive hosts of nutrients.

Classification of Parasites

• Ectoparasites reside outside the host’s body; examples include arthropods like mites, ticks, and lice.
• Endoparasites reside inside the host's body; examples include parasitic protozoa and helminths.
• Pathogenic parasites cause disease.
• Non-pathogenic parasites colonize the host.
• Definitive hosts harbor the adult or sexual stage of a parasite.
• Intermediate hosts harbor the larval or asexual stage of a parasite.
• Reservoirs harbor the parasite and serve as a source of infection.

Sources & Portals of Parasitic Infection/Exit

• Parasitic infections can stem from contaminated soil, water, and food.
• Vectors can transmit parasites.
• Domestic or wild animals can be sources.
• Infection can occur through contact with another person.
• Parasites enter the body through the mouth, skin penetration (larval or vector-mediated), sexual contact, inhalation, transplacental, and transmammary routes.
• Parasites exit through the anus, urine, sputum (flatworms), and other discharges.

Factors Determining Intensity of Parasitic Infection:

• Location includes endemic areas and forests.
• Socio-economic conditions are poor or unhygienic.
• Age and sex factors include very young and very old individuals with weak immune systems.
• Water and food supply plays a role.

General Classification of Parasites:

• Protozoans are unicellular, including Sarcodina (amoebas causing amebiasis), Sporozoa (apicomplexans like Plasmodium and Toxoplasma), Mastigophora (flagellates affecting blood and intestines), and Ciliata (ciliates like Balantidium coli).
• Metazoans are multicellular, including Nemathelminthes (roundworms), Platyhelminthes (flatworms, further divided into cestodes/tapeworms and trematodes/flukes), and Arthropods (arachnids such as ticks and mites, and insects such as fleas and lice).

Medically Important Protozoans: Amoeba

• Trophozoites are mobile forms with pseudopods, non-flagellated, and uninucleated.
• Cysts can have 1-4 nuclei; the mature cyst (4 nuclei) is infective.
• Entamoeba histolytica is the only pathogenic intestinal amoeba, causing amebiasis via contaminated food and water.
• They contain a single nucleus in the trophozoite.
• Nuclear membranes have chromatin granules with central or peripheral chromatin.
• They form cysts typically with two or four nuclei.
• E.Dispar is morphologically similar but not hematophagous
• Pseudopod movement is essential for diarrhea.
• Ectoplasm is an extension of the cytoplasm
• Endoplasm has inclusions.
• Active feeding and growing describes trophozoites
• A trophozoite will contain 5 peripheral chromatin granules
• Cysts are dormant, non-motile, infective, non-feeding, and spherical, typically found in formed or semi-formed stool and can survive outside the host.
• Infection with results from ingesting cysts not trophozoites
• Fecal-oral or use of contaminated equipment are other sources
• Typically cysts form in feces but trophozoites in stool

Excystation & Encystation

• Excystation happens in the small intestines, transforming cysts into trophozoites.
• Ingested cysts reach the stomach where acids react, leading to excystation in the small intestine and production of metacysts.
• Metacysts divide and liberate trophozoites.
• Trophozoites are more toxic then cysts and cause disease
• Trophozoites feed on bacteria and bacteria remains
• Encystation occurs in the large intestine due to dehydration, transforming trophozoites into cysts.
• Pseudopods condenses from irregular to spherical
• Mature cysts will undergo mitosis

Free-Living Amoeba - Acanthamoeba spp.

• Acanthamoeba spp is ubiquitous, free-living ameba and the etiologic agent of Acanthamoeba keratitis (AK) and granulomatous amebic encephalitis (GAE).
• The trophozoite stage has prominent "thorn-like" appendages (acanthopodia).
• The cyst stage is highly resilient.
• It is an aquatic organism and feeds on bacteria or tissues.
• Acanthamoeba only two stages, cysts and trophozoites.
• The trophozoite replicates by mitosis, and is the infective stage
• Both cysts and trophozoites enter the body through the eye, nasal passages to the lower respiratory tract, or broken skin.
• It can cause keratitis or granulomatous amebic encephalitis.
• Exhibits a single large nucleus with a centrally-located nucleolus, a large endosome, finely-granulated cytoplasm, and a contractile vacuole.

Free-Living Amoeba - Naegleria Fowleri

• Naegleria fowleri is a "brain-eating amoeba" which is the etiologic agent of Primary Amoebic Encephalitis (PAM).
• It thrives in hot springs and warm aquatic environments.
• Acquired through inhalation (oral/nasal route), it causes degenerative or cytopathic effects.
• This amoeba causes an acute form of the disease in healthy patients and is the only pathogenic Naegleria specie.
• The two vegetative forms are ameba (trophozoite) and flagellate (swimming form).
• Measures 10 to 35 μm but when rounded are usually 10 to 15 μm.
• The cytoplasm is granular and contains vacuoles
• The single nucleus is large and has a dense karyosome and a lack in peripheral chromatin
• Enters by penetrating thenasal tissu and causes primary amebic meningoencephalitis (PAM).

Protozoans: Flagellates (Mastigophora)

• Giardia lamblia/Giardia intestinalis: reservoirs- beavers, have an intestinal flagellate, and is indicated with drinking unpurified waters
• The trophozoite has a pear-shape, 14x7 micrometer = size of 2 RBC, 2 nuclei (centrally located = old man’s eyeglasses), 4 pairs of flagella.
• The Blepharoplasts give rise to flagella
• They have no mouth, no cytostome
• Parabasel body is modified
• Has an axoneme
• Cysts contain 2-4 nuclei with granular cytoplam
• Contamination is water/food related
• Infections is excysted in the duodenum
• Trichomonas vaginalis is pathogenic with vaginitis
• Has 4 anterior flagellated
• Axostyle adheres to epithelial cells

Roundworms (Nematodes)

• Nematoda is the class
• Is unsegmented
• Has mouth and anus structures
• Has seperate sexes with eggs
• Reproduction is by oviparous or small worm
• Infections are acquired by ingestion, surface infection and vectors

Common Roundworm infections

• Pinworm: vermicularis
• Eggs and worms infect
• Can be directly transmitted
• Human are the only host and no cats
• Eggs leave in stool and may itch
• Lay eggs in the anal-reactal region
• Eggs produce in hours
• Vagina affected and are
• Ascariasis is from ingestion where eggs are exposed is type of worm
• Can be acquired with water supply and no DH
• Type of worms are able migrate in the heart in tissues

Medically Important Tapeworms & Flukes

• Tapeworms are segmented, possess scolex and neck, are hermaphroditic,
• Most times multiplication is due to encysted larvae.
• Taenia Solium infect pig sand results in humans
• Larvae from the pig is digested in the body and matures to adults
• Cysticercus celluosa e forms after digestion
• Flukes infect 2-3 hosts and contain no pigments.
• Has 2 intermediate hosts (snail & grass) which produces metacercariae
• Adult worms contains large
• Intestines connects for highly branches caeca
• Ceca infect snail intermediate
• Liver flukes can affect the chinese & oriental
• Body is covered on spines and is hermaphrodite -Miracidium hatches
• Can be free swimming and ciciated