Parasitology has been broadly defined as “a study of symbiosis or speaking “living together”. mistily speaking, parasitology is defined as the scientific study of parasites.
What then are parasites???? And how living together can be harmful.
A parasite is defined by the advanced as “an animal or plant living in or on another and getting its food from it”.
Taking the broad definition of parasitology into consideration, the word “symbiosis” used, raises another question.
The word itself is taken from the Ancient Greek language meaning “living together” i.e close and long-term interactions between two or more different biological species.
In 1877, Albert Bernard Frank used the word symbiosis to describe the mutualistic relationship existing among lichens. This usage conforms to the definition of symbiosis given by the Oxford advanced English dictionary depicting people living together in a community.
In 1879, Heinrich Anton de Bary, a German Mycologist defined symbiosis as “living together of unlike organisms”. The meaning of the word symbiosis has become controversial among Scientists. Whereas some Scientists believe symbiosis should refer to relationships that are beneficial to both parties, (ie mutualistic relationships), others believe it should apply to any type of persistent biological interaction.
• The science that deals with the occurrence, diseases caused, mode of transmission, prevention of the parasite and its cure is called Parasitology.
• Parasitism is an association in which two organisms of different species live together where one enjoys all benefits like nourishment, Shelter, reproduction etc. at the expense of the other.
• The benefited organism is called the parasite and the organism harboring the parasite is called the host.
Types of host-parasite relation;
Mutualism
Commensalism
Parasitism
Symbiosis
Now let us discuss each type of relationship in detail
Mutualism:
-> Previously termed as symbiosis. Mutualism is an association between organisms of two different species in which each benefit. Mutualistic arrangements are most likely to develop between organisms with widely different living requirements. In this case, both of the species derive benefits and there exists a close and often permanent and obligatory relationship which is more or less essential for the survival of each.
-> For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut. The termite benefits from the ability of bacterial symbiont within the protozoa to digest cellulose. The
termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbiont benefit by having a protective environment and a constant supply of food from the wood-chewing actions of the termite.
->Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria. As these symbionts grow together, the glucose produced by the algae provides nourishment for both
organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae.
Commensalism:
->Commensalism, in biology, is a relationship between individuals of two species in which one species obtains food or other benefits from the other without either harming or benefiting the latter. The commensal—the species that benefits from the association—may obtain nutrients, shelter, support, or locomotion from the host species, which is unaffected. The commensal relation is often between a larger host and a smaller commensal. The host organism is essentially unchanged by
the interaction, whereas the commensal species may show great morphological adaptation. This relationship can be contrasted with mutualism, in which both species benefit.
->One of the best-known examples of a commensal is the remora (fish) that rides attached to sharks and other fishes. Remoras have evolved on the top of their heads a flat oval-sucking disk structure that adheres to the bodies of their hosts. Both remoras and pilot fishes feed on the leftovers of their host’s meals.
->Other examples of commensals include bird species, such as the great egret (Ardea alba), that feed on insects turned up by grazing mammals or on soil organisms stirred up by ploughing. Various biting lice, fleas, and louse flies are commensals in that they feed harmlessly on the feathers of birds and sloughed-off flakes of skin from mammals.
PARASITISM:
->Parasitism is a kind of relationship between species, where one organism lives on or in another organism, causing some harm to the host, and the parasite is adapted structurally to this way of life. A parasite is an organism that lives in or on another living organism and derives nutrients from it.
->In this relationship, the parasite benefits, but the organism being fed upon, the host is harmed. The
host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle by spreading to another host.
->Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between the parasite and its host. Unlike commensalism and mutualism, the parasitic relationship harms the host,
either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Within that scope is many possible ways of life parasites are classified in a variety of different but overlapping
schemes, based on their interactions with their hosts and on their life cycles, which are sometimes
very complex.
- Ecto-parasite (Ectozoa): Lives outside on the surface of the body of the host. E.g ticks and mites
- Endo-parasite (Entozoa): Lives inside the body of the host: in the blood, tissues, body cavities, digestive tract and other organs. E.g Ascaris, Plasmodium etc
- Temporary Parasite: Visits its host for a short period. E.g mosquito
- Permanent Parasite: Leads a parasitic life throughout the whole period of its life. E.g Ascaris
- Facultative Parasite: Lives a parasitic life when an opportunity arises.
- Obligatory Parasite: Cannot exist without a parasitic life. E.g Plasmodium
- Occasional or Accidental Parasite: Attacks an unusual host.
- Wandering or Aberrant Parasite: Happens to reach a place where it cannot live.
- Epiparasite: It is a parasite whose host, often an insect, is also a parasite and their association is called hyperparasitism.
Types of Hosts
->Host is the organism upon or inside which the parasite lives or the organism which lodges the parasite. Before discussing the various types of hosts.
->Let us take an example of the malaria-causing protozoan parasite Plasmodium. For this parasite, there are two hosts, the man and female Anopheles mosquito. In mosquitoes, the parasite undergoes sexual reproduction to produce infective stages. When this mosquito bites the man, infective stages get transmitted to the man. Here in the second host the parasite undergoes asexual reproduction and produces a large no of parasites. Some of the parasites get matured into gamete-producing parasites. So when a mosquito bites the infected man, gamete-producing parasites get transmitted to the mosquito where sexual reproduction takes place and the cycle continues.
The various types of hosts are as.
1. PRIMARY HOST (Definitive host): It is the host that harbours the adult stage or sexually mature stage of a parasite or the host in which the parasite undergoes sexual reproduction. e.g., Man is the
primary host for Wuchereria bancrofti (Filarial Worm), and the Female anopheles mosquito is the primary host for malaria-causing protozoan Plasmodium vivax.
2. SECONDARY HOST (Intermediate host): Is the host that harbours the developing larval or immature or asexual stages of a parasite or the host in which the parasite undergoes asexual reproduction. e.g,
Man is the secondary host for plasmodium vivax, Female culex mosquito is the secondary host for Wuchereria bancrofti.
3. Paratenic Host (A carrier or transport host): An intermediate host which is not needed for the development of the parasite, but serves to maintain the life cycle of the parasite. It is a host, where the parasite remains viable without further development. A paratenic host is similar to
an intermediate host, except that it is not needed for the parasite’s development cycle to progress.
Paratenic hosts serve as “dumps” for non-mature stages of a parasite in which they can accumulate in high numbers.
4. Reservoir Host: Also known as a temporary host. It is the host that lodges the infective stages of a parasite in its body when the main host is not available. In the reservoir host, the parasite neither
undergoes development nor causes any disease. In the absence of a regular host, some parasites survive in the reservoir hosts. Reservoir hosts become the source of infection for regular hosts. Reservoir hosts
are not essential for the parasite to complete its life cycle. e.g, Monkey is the reservoir host for Plasmodium
5. Vector Host: Vector is an organism which transfers the infective stages of a parasite from one main host to another. It is a host in which part of the life cycle of the host takes place and is instrumental in the transmission of infective stages of the parasite from one host to another.
Vectors are of two types, namely;
A) MECHANICAL VECTOR: It is the vector, which merely transfers the infective stages of a parasite but no part of the parasitic development takes place in it. e.g., Houseflies and Cockroaches in the case
of Entamoeba.
B) BIOLOGICAL VECTOR: It is the vector in which the parasite undergoes a part of the development
before it gets transferred to another host. e.g., Female anopheles mosquito in the case of Plasmodium and Female culex mosquito in the case of Wuchereria.
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