Download Principles of Parasitology

Chapter 11: Eukaryotic Microorganisms and Parasites
Principles of Parasitology
•Parasite: an organism that lives at the expense of another organism (host)
•Parasites that cause disease are called pathogens
•Parasitology is the study of parasites
•Historically, in the development of the science of biology, parasitology came to refer to the study of
protozoa, helminths, and arthropods that live at the expense of other organisms
Parasites in Relation to Their Hosts
•Ectoparasites live on the surface of other organisms (e.g. ticks and lice)
•Endoparasites live within the bodies of other organisms (e.g. protozoa and worms)
•Most parasites are either
obligate parasites: must spend at least some of their life cycle in or on a host
facultative parasites: normally are free-living, but can obtain nutrients from a host
Parasites are also categorized acording to the duration of their association with their hosts:
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Permanent parasites (tapeworms): remain in or on a host once they have invaded it
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Temporary parasites (biting insects): feed on and then leave their hosts
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Accidental parasites (ticks): invade an organism other than their normal host
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Hyperparasitism (malaria): refers to a parasite itself having parasites
•Vector: agents of transmission, of many human parasitic diseases
An organism that transfers a parasite to a new host is a vector
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Biological vector: A vector in which the parasite goes through part of its life cycle (malaria
mosquito is both a host and a biological vector)
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Mechanical vector: A vector in which the parasite does not go through any part of its life cycle
during transit (flies that carry parasite eggs, bacteria, or viruses from feces to food)
Hosts
•Definitive hosts: harbor a parasite while it reproduces sexually
•Intermediate hosts: harbor the parasite during some other developmental stages
•Examples:
Mosquito is the definitive host for the malaria parasite because that parasite reproduces sexually in
the mosquito; the human is an intermediate host
•Reservoir hosts are infected organisms that make parasites available for transmission to other hosts
Many parasites have one or more of the following mechanisms for evading host defense mechanisms:
Many parasites have one or more of the following mechanisms for evading host defense mechanisms:
•Encystment
•Changing the parasite’s surface antigens
•Causing the host’s immune system to make antibodies that cannot react with the parasite’s antigens
•Invading host cells, where the parasites are out of reach of host defense mechanisms
Characteristics of Protists
•Members of the kingdom Protista
Diverse assortment of organisms
Unicellular, eukaryotic organisms
True nuclei and membrane-enclosed organelles
Microscopic and vary in diameter from (5um – 5mm)
Examples:
The Funguslike Protists
•Also called water molds (Oomycota) and slime molds (saprophytes)
Have some characteristics of fungi and some of animals

Water molds, mildews and plant blights produce flagellated spores, called zoospores
Slime molds are commonly found as glistening, viscous masses of slime on rotting logs; they also
live in other decaying matter or in soil
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Plasmodial slime molds form a multinucleate, amoeboid mass called a plasmodium
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Cellular slime molds produce pseudoplasmodia, fruiting bodies, and spores with characteristics
different from plasmodial slime molds
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Pseudoplasmodium is a slightly motile aggregation of cells that produces fruiting bodies, which in
turn produce spores
Animal-Like Protists (protozoa)
•Heterotrophic, mostly unicellular organisms
•Most are free living and some are commensals
(live in or on other organisms without harming them)
•Mastigophorans have flagella, few are free-living, but most live in symbiotic relationships
•Sarcodines are usually amoeboid and move by means of pseupodia. A few have flagella at some stage.
•Apicomplexans are parasitic and immobile. Sporozoites, merozoites, trophozoites, and gametocytes.
Examples:
Ciliates
Largest group of protozoans
Have cilia over most of their surfaces for movement and assist in food gathering
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Balantidium coli is the only ciliate that parasitizes humans and causes dysentery
•Have several highly specialized structures
•Contractile vacuole which regulates fluids
•Trichocysts tentacles used to capture prey
•Conjugation structure for genetic exchange
Characteristics of Fungi
•Fungi, studied in the specialized field of mycology, are a diverse group of heterotrophs
Many are saprophytes that digest dead organic matter and wastes
Some are parasites that obtain nutrients from the tissues of other organisms
•Most fungi, such as molds and mushrooms, are multicellular, but yeasts are unicellular
Structural Components of Fungi
•Thallus: The body of a fungus
The thallus of most multicellular fungi consists of a mycelium
Mycelium: a loosely organized mass of threadlike structures called hyphae
Mycelial cells release enzymes that digest substratum
Cell walls of a few fungi contain cellulose, but most contain chitin
Chitin: a polysaccharide also found in the exoskeletons of arthropods
Reproduction
•Many fungi reproduce both sexually and asexually and only a few have only asexual reproduction
•Asexual reproduction always involves mitotic cell division, which in yeast occurs by “budding”
•Sexual reproduction occurs in several ways:
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plasmogamy: haploid gametes unite, and their cytoplasm mingles
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Dikaryotic: if the nuclei fail to unite, a “two-nucleus” cell forms
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Karyogamy: Eventually the nuclei fuse in this process to produce a diploid cell
Parasitic Fungi
•These fungi have three requirements for invasion:
Proximity to the host
Ability to penetrate the host
Ability to digest and absorb nutrients from host cells
Classification of Fungi
•Fungi are classified according to the nature of the sexual stage in their life cycles
Such classification is complicated by two problems:
No sexual cycle has been observed for some fungi
It is often difficult to match the sexual and asexual stages of some fungi
•Dimorphism: the ability of an organism to alter its structure when it changes habitats
Zygomycota
•Includes black bread mold, Rhizopus stolonifer
Ascomycota (Sac Fungi)
•Includes some yeasts, as well as ergot of rye
Club Fungi (Basidiomycota)
•Include mushrooms, toadstools, rusts, and smuts
•Rusts and smuts parasitize plants and cause significant crop damage
•Have hyphae to form mycelia and club-shaped sexual structures called basidia (Basidiomycota)
•Basidiospores: sexual spores in a typical basidiomycete life cycle
Fungi Imperfecti (Deuteromycota)
•No sexual stage has been observed in their life cycles
•Without information on the sexual cycle, taxonomist cannot assign them to a taxonomic group
•By their vegetative characteristics and production of asexual spores, most of these fungi seem to belong
with the sac fungi
Characteristics of Helminths
•Helminths, or worms, are bilaterally symmetrical
They have left and right halves that are mirror images
Also has a head and tail end
•Tissues are differentiated into three distinct layers:
•Ectoderm
•Mesoderm
•Endoderm
•Helminths that parasitize humans include flatworms and roundworms
Helminths
•Flatworms (Platyhelminthes) are primitive worms which lack a coelom and have a simple digestive tract
with a single opening. Most flatworms are hermaphroditic.
•Roundworms (nematodes) share many characteristics with flatworms, but they have a pseudocoelom.
They have cylindrical bodies with tapered ends and are covered with a thick, protective cuticle
Parasitic Helminths
•Flukes
•Tapeworms
•Adult roundworms of the intestine
•Roundworm larvae
Flukes
•Two types of fluke infections occur in humans:
Involves tissue flukes, which attach to the bile ducts, lungs, or other tissues
Involves blood flukes, which are found in blood in some stages of their life cycle
•Parasitic flukes have a complex life cycle often involving several hosts
Miracidia: free-swimming forms
Sporocysts: life form after penetration of molluskan or snail hosts and divide to form rediae
Rediae give rise to free-swimming cercariae which penetrate another arthropod host to encyst as
metacercariae
Tapeworms
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Consist of a scolex, or head end with suckers that attach to the intestinal wall and a long chain of
hermaphroditic proglottids
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Proglottids: body components that contain mainly reproductive organs of both sexes
•The life cycle of
tapeworms usually includes the following stages:
Embryos develop inside eggs and are released from proglottids
Proglottids and eggs leave the host’s body with the feces
Another animal ingests vegetation or water contaminated with eggs and eggs hatch into larvae,
which invade the intestinal wall
A larvae can develop into a cysticercus (bladder worm), or it can form a cyst
A cyst can enlarge and develop many tapeworm heads within it (hydatid cyst) and if an animal
eats flesh containing this, each scolex can develop into a new tapeworm
Adult Roundworms
•Most roundworms that parasitize humans live much of their life cycle in the digestive tract
•Usually enter the body by ingestion with food or water, but some penetrate the skin (e.g. hookworm)
•The life cycles of intestinal roundworms show considerable variation
Examples:
Characteristics of Arthropods
•Constitute the largest group of living organisms
•80% of all animal species belong to phylum Arthropoda
•Characterized by:
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Jointed chitinous exoskeletons
Segmented bodies
Jointed appendages
Have a true coelom
Small brain and an extensive network of nerves
Sexes are distinct and females lay many eggs
Classification of Arthropods
•Certain members of three subgroups (classes) of arthropods are important either as parasites or as disease
vectors
•Arachnids
•Insects
•Crustaceans
•Properties (Table 11.4)
Insects
•Have three body regions
•Head
•Thorax
•Abdomen
•Three pairs of legs
•Highly specialized mouth parts
Crustaceans
•Generally aquatic arthropods
•Typically have a pair of appendages associated with each segment
•Appendages include:
•Mouthparts
•Claws
•Walking legs
•Appendages that aid in swimming or copulation