Terms in Parasitology: Parasite – Host Relationship

Parasite:

• An organism that lives on or in a host organism and gets its food at the expense of its host.
• Example: Plasmodium, which causes malaria in humans.

Parasitism:

• A symbiotic relationship where one organism (the parasite) benefits at the expense of the other (the host).
• Example: The relationship between ticks and mammals.

Host:

• An organism that harbours a parasite, typically providing nourishment and shelter.
• Example: Humans hosting the malaria parasite.

Definitive Host:

• The host in which a parasite reaches maturity and, if applicable, reproduces sexually.
• Example: Humans for the tapeworm Taenia saginata.

Intermediate Host:

• A host that harbors the parasite only for a short transition period, during which (usually) some developmental stage is completed.
• Example: Snails for the larval stage of Schistosoma.

Reservoir Host:

• A host that serves as a source of infection and potential reinfection of humans and other animals.
• Example: Rodents for Leishmania.

Vector:

• An organism that transmits a parasite from one host to another.
• Example: Mosquitoes for the malaria parasite.

Endoparasite:

• A parasite that lives inside the body of its host.
• Example: Tapeworms in the intestines of humans.

Ectoparasite:

• A parasite that lives on the external surface of its host.
• Example: Lice on human scalp.

Obligate Parasite:

• A parasite that cannot complete its life cycle without exploiting a suitable host.
• Example: Plasmodium sp.

Facultative Parasite:

• A parasite that can live both in a host and independently.
• Example: Naegleria fowleri.

Intracellular Parasitism:

• The parasite lives within the cells of the host.
• Example: Plasmodium sp., the malaria-causing parasites, invade and multiply within red blood cells.

Intercellular Parasitism:

• The parasite lives in the spaces between the host’s cells.
• Example: Filariasis-causing nematodes, which live in the lymphatic system of humans.

Temporary Parasitism (Intermittent):

• The parasite lives in or on the host for only a part of its life cycle.
• Example: Bedbugs (Cimex spp.), which feed on the host’s blood but do not reside on the host.

Permanent Parasitism:

• The parasite remains with the host for a significant portion or the entirety of its life cycle.
• Example: Lice (Pediculus humanus), which live and reproduce on the human scalp.

Brood Parasitism:

• A form of parasitism where the parasite relies on the host to raise its offspring.
• Example: Cuckoo birds, which lay their eggs in the nests of other bird species, leaving the host birds to incubate and raise the cuckoo chicks.

Kleptoparasitism:

• The parasite steals food or other resources collected by the host.
• Example: Some species of spiders steal prey caught in the webs of other spiders.

Hyperparasitism:

• The parasite itself is parasitized by another parasite.
• Example: The parasitoid wasp Aphidius colemani, which parasitizes aphids that may themselves be hosts to another parasitic fungus.

Zoonosis:

• An infectious disease that is transmissible from animals to humans.
• Example: Toxoplasmosis.

Vector-borne Disease:

• An illness caused by pathogens and parasites in human populations.
• Example: Dengue fever transmitted by Aedes mosquitoes.

Life Cycle:

• The series of stages in form and functional activity through which an organism passes between successive recurrences of a specified primary stage.
• Example: The life cycle of Plasmodium includes stages in both the mosquito and human hosts.

Incubation Period:

• The time between exposure to the parasite and the appearance of the first symptoms.
• Example: The incubation period for malaria is typically 10-15 days.

Pathogenesis:

• The development of disease and the chain of events leading to that disease.
• Example: The pathogenesis of malaria involves liver infection followed by red blood cell invasion.

Trophozoite:

• The active, motile feeding stage of protozoa.
• Example: The trophozoite stage of Giardia intestinalis.

Cyst:

• The dormant, non-replicating form of some parasites that is resistant to environmental conditions.
• Example: The cyst stage of Entamoeba histolytica.

Larva:

• An immature form of a parasite that often looks different from the adult.
• Example: The larval stage of Ascaris lumbricoides.

Oocyst:

• A hardy, thick-walled spore stage of some protozoans that is released in the feces of infected hosts.
• Example: Oocysts of Cryptosporidium.

Sporogony:

• A process of spore formation in the life cycle of certain protozoans.
• Example: Sporogony in Plasmodium occurs in the mosquito.

Merogony:

• A form of asexual reproduction in some protozoans involving multiple fission.
• Example: Merogony in the liver stage of Plasmodium.

Schizogony:

• A type of asexual reproduction by multiple fission, found in some protozoa.
• Example: Schizogony in Plasmodium’s life cycle.

Bradyzoite:

• A slowly dividing stage of some protozoan parasites.
• Example: Bradyzoites of Toxoplasma gondii in tissue cysts.

Tachyzoite:

• The rapidly multiplying stage of some protozoan parasites.
• Example: Tachyzoites of Toxoplasma gondii.

Hypnozoite:

• A dormant stage of some protozoan parasites.
• Example: Hypnozoites of Plasmodium vivax in the liver.

Epimastigote:

• A developmental stage in the life cycle of Trypanosoma, found in the insect vector.
• Example: Epimastigotes in the tsetse fly.

Amastigote:

• A non-flagellated stage in the life cycle of some protozoans.
• Example: Amastigotes of Leishmania in human macrophages.

Promastigote:

• A flagellated stage of some protozoans.
• Example: Promastigotes of Leishmania in the sandfly.

Kinetoplast:

• A specialized mitochondrial DNA found in some protozoans.
• Example: Kinetoplast in Trypanosoma brucei.

Merozoite:

• A stage in the life cycle of Plasmodium that emerges from liver cells and invades red blood cells.
• Example: Merozoites released into the bloodstream during malaria infection.

Sporozoite:

• The infective stage of Plasmodium that is transmitted to humans by mosquitoes.
• Example: Sporozoites injected by Anopheles mosquitoes.

Cercaria:

• The larval form of trematodes that is released from the snail host.
• Example: Cercariae of Schistosoma.

Miracidium:

• The free-swimming larval stage of trematodes that hatches from an egg.
• Example: Miracidia of Schistosoma haematobium that infect snails.

Microfilariae:

• The early larval stage of filarial worms that circulates in the bloodstream of the definitive host.
• Example: Microfilariae of Wuchereria bancrofti causing lymphatic filariasis.

Fomites:

• Objects or materials that can carry infection, such as clothes, utensils, and furniture.
• Example: Bed linens contaminated with lice.

Chemoprophylaxis:

• The administration of a medication to prevent infection or disease.
• Example: Taking antimalarial drugs before traveling to a malaria-endemic area.

Anthelmintic:

• A type of drug that expels or destroys parasitic worms.
• Example: Albendazole used to treat infections by various nematodes.

Proglottid:

• A segment of a tapeworm containing both male and female reproductive organs.
• Example: Proglottids of Taenia solium.

Xenodiagnosis:

• A diagnostic method for certain parasitic infections, where a vector is allowed to feed on a suspected host and is later examined for the presence of the parasite.
• Example: Xenodiagnosis for detecting Trypanosoma cruzi in Chagas disease.

Parasitic Agents: Host, Habitat, Disease