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what is a parasite?
The word “parasite” will make most people cringe, probably because they associate it
with little worms living inside them. This is not completely wrong, as we will see later. In the ancient Greek, however, the term “parasitos” was initially used to describe community officials who went to public events and spent the community’s money for their meals and entertainment .
Over time, “parasite” evolved to describe any person who lives off of other people’s riches. It was not until the seventeenth century that the biologist Carolus Linnaeus used the term parasite to describe tapeworms. Nowadays, a parasite refers to any microorganism,
most parasites kill their hosts
A parasitic relationship is one in which one organism, the parasite, lives off of another organism, the host, harming it and possibly causing death. The parasite lives on or in the body of the host.
A few examples of parasites are tapeworms, fleas, and barnacles. They get food by eating the host’s partly digested food, depriving the host of nutrients. Fleas harm their hosts, such as dogs, by biting their skin, sucking their blood, and causing them to itch. The fleas, in turn, get food and a warm home. Barnacles, which live on the bodies of whales, do not seriously harm their hosts, but they do itch and are annoying.
Usually, although parasites harm their hosts, it is in the parasite’s best interest not to kill the host, because it relies on the host’s body and body functions, such as digestion or blood circulation, to live.
Some parasitic animals attack plants. Aphids are insects that eat the sap from the plants on which they live. Parasitic plants and fungi can attack animals. A fungus causes lumpy jaw, a disease that injures the jaws of cattle and hogs. There are also parasitic plants and fungi that attack other plants and fungi. A parasitic fungus causes wheat rust and the downy mildew fungus attacks fruit and vegetables. Some scientists say that one-celled bacteria and viruses that live in animals and harm them, such as those that cause the common cold, are parasites as well. However, they are still considered different from other parasites. Many types of parasites carry and transmit disease.
A parasite and its host evolve together. The parasite adapts to its environment by living in and using the host in ways that harm it. Hosts also develop ways of getting rid of or protecting themselves from parasites. For example, they can scratch away ticks. Some hosts also build a symbiotic relationship with another organism that helps to get rid of the parasite. Ladybugs live on plants, eating the aphids and benefiting by getting food, while the plant benefits by being rid of the aphids.
Parasites Metabolic Dependency on the Hosts Could be in Several Ways:
a) Nutritional: As in Flukes, tapeworms such as Diphyllobothrium latum
b) Developmental stimuli: As in P. intergerrinum and Opalina ranarum , (parasites of frog),
c) Digestive Enzymes: As in tapeworms such as Taenia sp and Echinococus sp
d) Shelter: As in endoparasites such as Plasmodium sp , Entamoeba sp, Ascaris sp e.t.c
e) Movement: As in ectoparasites such as lice, fleas, ticks and mites
f) Control of maturation: As in P. intergerrinum and Opalina ranarum
Which invertebrates are parasites?
Many invertebrate groups have parasitic members. Some well known parasites are fleas, ticks, parasitic mites, leeches, worms (e.g. round worms) and some parasitic flies (e.g. mosquitoes). Stylops are parasites of wasps, bees and bugs. The female is larvae-like and spends its entire lifecycle within the host. The winged, free-living males locate females by scent and mate with the small portion of the female protruding from the host.
In nature, when two individuals of different species often live in close association with each other, this leads to a phenomenon called symbiosis.
There are three types of symbiosis:
- Mutualism is a win-win situation for both organisms because both benefit from the relationship.
- With commensalism, one organism benefits while the other is unaffected.
- Then there’s parasitism, where one organism (the parasite) benefits at the expense of the other (the host).
Parasites can be protists, bacteria, viruses, fungi, plants or animals. An estimated 40 per cent of animal species are parasites. Some parasites live on their hosts (ectoparasites) while others live inside them (endoparasites).
Effects of Parasites on their Hosts:
Parasites May Injure their Host in a Number of Ways
Toxins: Apart from disease and death, parasites can produce poisonous substances in the form of secretions, excretions or other products such as proteolytic enzymes and pigments. These can harm or sensitize the host, for example, Schistosome cercariae, Entamoeba histolytica and Malaria parasites .
Mechanical Effects: Mechanical damage due to big size or number of parasites occurs such as the hydratid cysts of Echinoccocus species. Intestinal obstruction, blockage and entangling of worms also occur such as in Ascaris lumbricoides. Parasites can perforate vital organs when migrating as in hookworms and Ascaris. Adhesive structures of parasites also cause mechanical damage as in Taenids.
Absorption of Food: Parasites can deplete the host nutritional level to reach disease level for the host. For example, Diphyllobothrium latum, the broad fish tapeworm absorbs a great quantity of vitamin B12 reaching to megaloblastic anaemia. Hook worm absorbs iron daily leading to iron deficiency anaemia.
Destruction of Host Tissues: Skin penetrating parasites cause skin destruction such as the larvae of hookworm and Schistosomes which causes swimmer’s itch and the larvae of myasis producing flies. Microfilariae of Onchocerca volvulus causes skin onchodermatitis, nodules, leopard skin and lizard skin. The skin lesions become the site of secondary bacterial infection .
Ingestion of Host’s Body Constituent: Some parasites such as hookworms and microfilariae of filarial worms feed on the body fluids (blood and lymph), and epithelia cells causing blood and fluid loss.
Gigantism: Some parasites such as larval stages of trematodes enhance growth of their snail intermediate host .
There are three main types of parasites.
Protozoa: Examples include the single-celled organism known as Plasmodium. A protozoa can only multiply, or divide, within the host.
Helminths: These are worm parasites. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke.
Ectoparasites: These live on, rather than in their hosts. They include lice and fleas.
Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal-oral route (for example, contaminated food or water or person-to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly).
The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:
- Sarcodina – the ameba, e.g., Entamoeba
- Mastigophora – the flagellates, e.g., Giardia, Leishmania
- Ciliophora – the ciliates, e.g., Balantidium
- Sporozoa – organisms whose adult stage is not motile e.g., Plasmodium, Cryptosporidium
Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans. There are three main groups of helminths (derived from the Greek word for worms) that are human parasites:
- Flatworms (platyhelminths) – these include the trematodes (flukes) and cestodes (tapeworms).
- Thorny-headed worms (acanthocephalins) – the adult forms of these worms reside in the gastrointestinal tract. The acanthocephala are thought to be intermediate between the cestodes and nematodes.
- Roundworms (nematodes) – the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues. Alternatively, the immature (larval) states can cause disease through their infection of various body tissues. Some consider the helminths to also include the segmented worms (annelids)—the only ones important medically are the leeches. Of note, these organisms are not typically considered parasites.
Although the term ectoparasites can broadly include blood-sucking arthropods such as mosquitoes (because they are dependent on a blood meal from a human host for their survival), this term is generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months). Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality from the diseases they cause.