Parasitology 101 – Words That Describe Parasites
Parasite Terms and Their Meanings for the General Reader
Jun 26, 2009
Parasites are living things (organisms) that live on, or in, and get all their nutrients from other living things. (For the sake of simplicity, this article will refer to parasites living in other things but it's important to remember that some, like lice, live on the outside.) Many parasites live in other organisms only some of the time, and many switch from one living thing to another at some point during their lives. A common perception of parasites is that they always do serious harm to the organism they’re living in, but we’re learning that parasites don’t always do harm, and they may even be beneficial under certain circumstances.
What is a Parasite Host?
The host is the organism that the parasite lives in:
- A definitive host is an organism that the adult, or sexually reproductive stage of the parasite lives in. When a human has an adult beef tapeworm in the intestine producing eggs, he or she is a definitive host.
- An intermediate host is an organism that hosts an immature parasitic stage, or a stage that reproduces asexually. Humans are intermediate hosts to malaria parasites, which multiply in human liver and blood cells without having male and female cells.
- A paratenic host is an organism that is infected with a parasite and can pass it on to another host, but in which the parasite does not develop further.
- Reservoir hosts, with respect to human parasites, are hosts that are infected with a parasite and keep it alive even if the parasite is wiped out in humans. Reservoir hosts can spread the parasite and reintroduce it to human populations.
Groups of Parasite Hosts
Parasite hosts are often referred to by general terms such as arthropod, or invertebrate:
- Annelids are segmented worms. They include earthworms, leeches, and sea worms, but not intestinal worms.
- Arthropods are creatures with jointed legs and usually many of them: insects, spiders, mites, centipedes, crabs, scorpions etc.
- Crustaceans are arthropods. They include crabs, lobsters, shrimps, scorpions, wood lice etc.
- Invertebrates are animals without backbones. Annelids and arthropods are invertebrates. So are mollusks (see below).
- Mollusks usually have hard outer shells: clams, scallops, mussels, and snails. Some mollusks, such as slugs and octopuses, don’t have hard shells.
- Vertebrates have backbones: people and other mammals, birds, reptiles, amphibians, and even fish.
Groups of Parasites
Many different types of animals have developed a parasitic way of life — even some plants:
- Arthropods, when parasitic, usually live on the outside of the host. They include mites and insects.
- Cestodes are tapeworms, worms with flat segmented bodies that look a bit like strings of rice grains or cucumber seeds.
- Helminths are worms, usually parasitic ones.
- Nematodes are roundworms, worms with cylindrical, tube-like bodies.
- Protozoa are parasites that consist of just one cell. An amoeba is a protozoan. Flagellates with their slender whip-like flagellae, and ciliates, covered with tiny bristle-like cilia are protozoa as well.
- Trematodes are flat worms or flukes, usually flat and shaped like a long thin leaf.
Other Parasitology Definitions
There are other terms that are frequently found in articles about parasites:
- A cyst is a stage of many protozoan parasites that survives well in the environment. Cysts of human parasites are infective to humans.
- Hermaphrodites are organisms with both male and female organs. Many tapeworms and flukes are hermaphrodites.
- Larvae are an immature stage of many organisms including insects and worms. Fly larvae are often referred to as maggots.
- Night soil is human sewage used as fertilizer for food crops.
- An oocyst is a stage of some protozoan parasites that results from sexual reproduction. Oocysts are often infective to humans.
- An operculum is a lid-like structure in a parasite egg that opens to release the larva.
- Parthenogenesis is the production of offspring by a female without first mating with a male.
- Proglottids are the individual segments of tapeworms.
- A scolex is the head of a tapeworm, typically armed with attachment structures such as hooks or suckers.
- A trophozoite is a moving, feeding, and multiplying stage of protozoan life cycles. There are not male and female trophozoites: they multiply asexually.
- A zoonosis is a disease that has been transmitted to humans from an animal.
Sources
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
Medical Parasitology 5th ed. Leventhal, Ruth and Russell F. Cheadle. Philadelphia: F.A. Davis, 2002.
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