A parasite is an organism that lives on or in a host organism and derives nourishment and protection from other living organisms at the expense of its host. They are living organisms, which depend on a living host for their nourishment and survival. Their multiplication or development undergo in host.
Parasites can be classified as:
Ectoparasites inhabit only the body surface of the host without penetrating the tissue. Lice, ticks, and mites are examples of ectoparasites. The term infestation is often employed for parasitism with ectoparasites.
Many ectoparasites are known to be vectors of pathogens, which the parasites typically transmit to hosts while feeding or defecating (occasionally).
A parasite, which lives within the body of the host and cause an infection is called an endoparasite. Most of the protozoan and helminthic parasites causing human disease are endoparasites. E.g. Tapeworm, flatworms etc. An intercellular parasite lives in the spaces within host’s body within host’s cells.
They rely on a third organism, known as vector, or carrier for transmittance (vector transmits endoparasite to the host). Mosquito is a vector for many parasites e.g. Plasmodium, which causes malaria.
Free living parasite
It refers to nonparasitic stages of active existence, which live independent of the host, e.g. cystic stage of Naegleria floweri.
These feed on other parasites in a relationship known as hyperparasitism. A flea lives on a dog, but flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite in epiparasitic relationship.
Endoparasites can further be classified as:
- Obligate parasite: The parasite, which cannot complete its life-cycle without a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. , e.g. Toxoplasma gondii and Plasmodium.
- Facultative parasite: Organism which may either live as parasitic form or as free living form. They may have parasitic activity, but does not absolutely rely on any host for completion of its life cycle. Naegleria fowleri infection in humans.
- Accidental parasites: Parasites which infect an unusual host are known as accidental parasites. Echinococcus granulosus infects man accidentally giving rise to hydatid cysts.
- Aberrant parasites: Parasites which cannot reach normal destination and infects a host where they cannot develop further are known as aberrant or wandering parasites. E.g. Toxocara canis (dog roundworm) infecting humans.
Parasitic infections can be caused following organisms:
Protozoa are microscopic, single-celled organisms free-living or parasitic that can live and multiply inside host. Some infections caused by protozoa include giardiasis, toxoplasmosis, leishmaniasis, balantidiasis, and cryptosporodiasis.
Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal-oral route (contaminated food or water or person-to-person contact) while those in blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through bite of a mosquito or sand fly).
Helminths are large, multi-celled organisms that are generally visible to the naked eye in their adult stages. They are more commonly known as worms. They include flatworms, tapeworms, thorny-headed worms, and roundworms.
Helminths can be:
- Flatworms (platyhelminths) which includes trematodes (flukes) and cestodes (tapeworms).
- Roundworms (nematodes) whose adult form an reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues.
- Thorny-headed worms (acanthocephalins) which are thought to be intermediate between cestodes and nematodes.
Diagnosis of parasitic infections can be made by analyzing various specimens and detection of these parasites or their different forms.