A single celled organism which has no rigid body structure.
about and take in food by extending pseudopods. Examples of parasitic amoebae include
Entamoeba histolytica (cause of amebic dysentery) and Naegleria sp. and
Acanthameba sp. (causes of eosinophilic meningitis).
A group of arthropods normally featuring 4 pairs of legs and two major
body segments (cephalothorax and abdomen). Parasitic arachnids include mites and ticks. The
group also includes the spiders and scorpions.
A group of organisms comprising a whole phylum to themselves (Phylum
Arthropoda). These organisms are characterized by having a number of jointed legs, numerous
body segments which may be fused or unfused and a hard outer covering or exoskeleton
made of chitin. Phylum Arthropoda contains the following Classes: Insecta (insects), Arachnida (spiders, mites, ticks, scorpions, etc),
Chilopoda (centipedes), Diplopoda (millipedes), and Crustacea (crabs, shrimp, lobsters, water
fleas, etc. Related groups include the Onychophora (Peripatus, etc), the Tardigrades
(water bears, etc) and the Pentastomids (tongue worms).
beating hairs on the outside of cells. In complex organisms like humans, these cilia may be found
on cells lining the respiratory passages, where they help the flow of mucus. In simpler organisms
they may aid in movement. Single-celled organisms which use cilia to move around are called
A commensal organism is one which lives within the body of another but
does not normally cause any harm. In times of stress, commensals may turn into pathogens (see
In parasitology, the term cyst may have two meanings. Firstly, a cyst may be the resistant
dormant stage of a single-celled organism which is passed out and encourages the propagation
of the species. Alternatively, cyst may refer to the intermediate stage of some tapeworms (e.g.,
hydatid cysts). This cyst must be eaten by the definitive host for it to be
The definitive host is the organism which houses the mature, or sexually
reproducing stage of the parasite. For example, the dog is the
definitive host of the hydatid tapeworm, while the mosquito is the definitive host of the malarial
Frequency of bowel movements or stool, often associated with a loose
Having two sexes (as opposed to hermaphroditic).
Diarrhea with associated blood and mucus discharge.
A parasite which lives principally on the outer
surface of an organism.
A parasite which lives principally with the tissues
of an organism.
A group of long, hair-like
nematodes in which the adults live in the blood or tissues of vertebrates. In some species,
the larvae may be found in the blood. Examples of diseases caused by
filarial worms include Elephantiasis and River Blindness.
A long beating hair found on a cell which normally aids in movement.
Human sperm cells have a flagellum. Single-celled organisms which move about using flagella
are called Flagellates.
A group of organisms comprising a whole phylum (Phylum
Platyhelminths). Flatworms have flat bodies (as the name suggests) and are normally hermaphroditic. Phylum Platyhelminths consists of three classes: Class Trematoda (the flukes), Class Cestoda (the tapeworms) and Class Tubellaria (the free-living flatworms
Planarians and ribbon worms).
group of organisms characterized by having a flat, unsegmented body and complex multi-stage
life-cycles. Flukes (comprising Class Trematoda) are members of the Phylum
or the flatworms, which also includes the Tapeworms and the non-parasitic Turbellarians (e.g., the Planarians).
Flukes are entirely parasitic, and are hermaphroditic, save for some groups
(e.g., the Schistosomes). Examples of flukes include the liver fluke
and the schistosomes.
A worm which spends a certain time during its
lifecycle living in the soil.
A species in which one organism contains both sets of sex
organism in which a parasite lives.
last stage of development of an insect, after the last ecdysis
(molt) of an incomplete metamorphosis, or after emergence from
pupation where the metamorphosis is complete. As this is the
only stage which is sexually mature, and has functional wings in
winged species, the imago is often referred to as the adult
stage. The Latin plural of imago is imagines,
and this is the term generally used by entomologists - however
imagos or imagoes are also acceptable spellings.
group of organisms comprising the Class Insecta of Phylum
Arthropoda. Insects are characterized by having 3 pairs of legs and three major body
segments (head, thorax and abdomen). Some species have wings. Parasitic insects include the
fleas and lice. Other groups, such as flies, mosquitoes and some beetles, are important vectors of parasitic disease or intermediate
The organism which houses the immature or non-sexually
reproducing stage of a parasite. For example, the sheep is the
normal intermediate host for the hydatid tapeworm, while humans are the intermediate host for
the malarial parasite.
immature stage of an organism which bears no structural resemblance to the mature stage. For
example, a maggot is the larva of a fly, a caterpillar is the larva of a moth or butterfly. Remember: A caterpillar is just a butterfly maggot.
A group of organisms also known as the Roundworms. Nematodes have
what can only be described as a typical "worm" shape - long, tapered at the ends and round in
cross-section (think of the shape of an earthworm, but earthworms are not
nematodes). They have an internal body cavity, with recognizable digestive and reproductive
tracts. Nematodes are generally dioecious. They reproduce by laying
eggs, or larvae which hatch from their eggs inside the body of the female worm. They are among
the most common multicellular parasite of humans in the world,
although the majority of nematodes are not parasitic, living in the soil. Examples of parasitic
roundworms include Human Roundworm (Ascaris), Pinworm/Threadworm, Whipworm,
Hookworm and Filarial Worms.
immature stage of an organism which largely resembles the adult stage, save for some minor
differences. For example, cockroach nymphs can be differentiated from the adults by the fact that
the nymphs do not have wings.
A parasite which cannot survive or reproduce
outside the body of its host organism.
An organism which is normally harmless (Commensal), but which may turn nasty if given the opportunity. For
example, one of the dangers for people in the last stages of HIV infection is infection by any
number of organisms which pose no threat to individuals with fully functioning immune
Believe it or not, parasitism can be a slippery term to define. The word
parasite can be liberally translated from the Greek to mean "eating at the same table". Therefore,
some people define a parasite as any animal which is wholly dependent upon another animal for
its food supply. While certainly broad (and most animals defined as parasites certainly fall under
this classification), it can also extend to any predator-scavenger relationship (we certainly don't
describe hyenas as parasitic on lions). Other definitions concentrate on where the parasite lives,
stating that a parasite must live on or in its host. This definition is even less
satisfactory, as there are many organisms which are transitory residents which we do not define
as parasites (e.g., normal microbial flora on the skin). The definition I favor is a combination of
the two above, which is best described in my ancient edition of the Pocket Oxford Dictionary
pa'rasite, n. Interested hanger-on, toady, sycophant; animal or plant living in or on
another & drawing nutriment from it.
Of course, this is not perfect, and I like to modify it to read "animal or plant living in or on
another animal or plant which is wholly dependent that other for food." There are exceptions to
every rule, and most organisms should be dealt with on an individual basis.
A host in which the parasite
does not undergo any development. For example, dogs and pigs may carry hookworm eggs from
one place to another, but the eggs do not hatch or pass through any development in these
A process which may occur in some sexually reproducing animals where
offspring are produced without fertilization.
Any organism which causes harm to its host.
A subgroup of the Kingdom Protista, or the single-celled organisms. The
name Protozoa is a carry-over from an old system of classification and is generally used
to described those single-celled organisms which show more animal than plant characteristics.
Naturally, such a distinction is meaningless, as animals and plants belong to completely different
kingdoms, but in general, Protozoa refers to those organisms which do not carry out
photosynthesis. Parasitic protozoa comprise a number of subgroups: The Sarcomastigophora (amebas and flagellates), The Ciliates (ciliated organisms), the Sporozoa
(malaria, Toxoplasma, Cryptosporidium and allies), and the
"dormant" stage in the life-cycle of some insects where the larva changes
into the adult (or imago).
A commonly mistaken term. Ringworm is the common name given to skin
infections by certain fungi. The correct term is Tinea. The condition is not caused by a
worm at all, and the name dates from a time where all ailments were
blamed on worms of some description.
A group of Flukes which live in the blood vessels
of their hosts. Unlike most other flukes, the
schistosomes are dioecious.
A group of single celled organisms which are characterized by having a
sexual and an asexual generation in their life-cycle. Examples of parasitic Sporozoans include
the malarial parasites, Toxoplasma and Cryptosporidium.
Name for the parasitic flatworms forming the class Cestoda.
All tapeworms spend the adult phase of their lives as parasites in the gut of a
vertebrate animal (called the primary host). Most tapeworms spend part of their
life cycle in the tissues of one or more other animals (called intermediate
hosts), which may be vertebrates or arthropods.
The active or feeding stage of a single-celled organism.
organism which transmits a parasitic organism from one host to another. Mechanical
Vectors merely carry the organism from one place to another (e.g., flies carrying
on their feet), while other vectors may form a necessary part of the life-cycle (e.g.,
multicellular organism which is generally longer than it is wide or deep. The scientific name for
worms is Helminth. In human parasitic terms there are three major groups of organisms
which are properly called worms: The Nematodes, the Flukes and the Tapeworms. These and other sorts
of worms may parasitize other organisms e.g., The Acanthacephalans (thorny headed worms) and
The Gordians (horsehair worms). Other sorts of worms are free living e.g., free-living nematodes,
The Annelids (e.g., earthworms, polychaetes, leeches, etc), Planarians (and other
An infection of a human by an organism which is usually parasitic in other
hosts. For example, since hydatid tapeworms are usually found in dogs and sheep, hydatid disease
is usually considered to be a zoonosis in humans.