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Parasitology Glossary

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Ameba:     A single celled organism which has no rigid body structure. Amebas move 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).

Arachnid:     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.

Arthropod:     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).




Cestode:     (See Tapeworm)

Cilia:     Small 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 ciliates.

Commensal:     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 opportunistic pathogen).

Cyst:     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 infected.



Definitive Host:     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 parasite.

Diarrhea:     Frequency of bowel movements or stool, often associated with a loose consistency.

Dioecious:     Having two sexes (as opposed to hermaphroditic).

Dysentery:     Diarrhea with associated blood and mucus discharge.



Ectoparasite:     A parasite which lives principally on the outer surface of an organism.

Endoparasite:     A parasite which lives principally with the tissues of an organism.



Filarial Worm:     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.

Flagellum:     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.

Flatworms:     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 e.g., Planarians and ribbon worms).

Flukes:     A 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 Platyhelminths, 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.



Geohelminth:     A worm which spends a certain time during its lifecycle living in the soil.


Helminth:     (see Worm)

Hermaphrodite:     A species in which one organism contains both sets of sex organs.

Host:     The organism in which a parasite lives.



Imago:    The 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.

Insect:     A 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 hosts.

Intermediate Host:     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.





Larva:     An 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.




Nematode:     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.

Nymph:     An 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.



Obligate Parasite:     A parasite which cannot survive or reproduce outside the body of its host organism.

Opportunistic Pathogen:     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 systems.



Parasite:  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.

Paratenic Host:     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 animals.

Parthenogenesis:     A process which may occur in some sexually reproducing animals where offspring are produced without fertilization.

Pathogen:     Any organism which causes harm to its host.

Platyhelminths:     (see Flatworms)

Protozoa:     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 Microsporidia.

Pupa:     The "dormant" stage in the life-cycle of some insects where the larva changes into the adult (or imago).




Roundworm:     (see Nematode)

Ringworm:     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.



Schistosomes:     A group of Flukes which live in the blood vessels of their hosts. Unlike most other flukes, the schistosomes are dioecious.

Sporozoan:     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.



Tapeworm:    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.

Trophozoite:     The active or feeding stage of a single-celled organism.




Vector:     An 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 feces on their feet), while other vectors may form a necessary part of the life-cycle (e.g., mosquitoes in malaria).


Worm:     A 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 Turbellarians).





Zoonosis: 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.