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Parasitism is an ecological relationship between to different organisms, one designated parasite and the other one the host (Crofton, 1971). The parasite absorb food intended for the host i.e. they are metabolically and physiologically dependent upon the host. The reproductive potential of parasite is higher than that of their host and a heavily infected host will be killed by their. Parasites have an incredibly bad reputation in the animal kingdom, as they utilize virtually every organ and tissue on their host’s body as niches to survive (Poulin ; Morand, 2000).
There are two types of parasites, the ectoparasites and the endoparasites. The ectoparasites are the parasites that live outside the body and the endoparasites live inside the body of their host. There are many categories of parasites that can affect an organism and the most studied ones are those that affect humans. They include protozoa, helminths, and arthropods
Parasites are not all harmful. Some of them can be beneficial to humans, environment and other living creatures. Parasites are important to the nature order like any other living creatures, and some scientists have put forward the fact that removing parasites from modern life can have serious effects on human health. Humans have long been involved in parasites and getting them to heal us, and now we have a better understanding that parasites can be able to be used in farming and other applications.

Medically important parasites fall under the kingdom Protista and Animalia.
Protozoans play a vital role as a link in the food chain and ecological balance of many communities in wetlands ; aquatic environments. Important as well in biological sewage treatment, involving both anaerobic digestion and aeration. In addition, protozoans are important laboratory organisms in research areas, their asexual reproduction enables clones to be established with the same genetic make-up. These appear to be useful in the study of cell cycles and nucleic acid biosynthesis during cell division.
Parasitic worms, often result in horrible illness and diseases, they also appear to have medical properties as well. The importance of parasitic worms has come to light in regards of treating various diseases, hence humans benefit from their presence. The most common use of parasitic worms as medicine, is in the use against diseases characterized by an overactive immune response. This symptom is often seen in individuals with allergies and hay-fever. Parasitic worms have the ability to damp down the immune system, which promotes an environment where they can thrive without being attached, this damping down of the immune system is beneficial as it may prevent development of allergies. Parasitic worm infection results in an increase in eosinophils, thus promoting control of glucose maintenance (sugar balance).

Some species of parasites derive a mutual benefit to their host and this brings a confusion because parasite is supposed to harm its host. One of the most straightforward examples of this involves the parasitic tapeworm genera Anthobothrium and Paraorigmatobothrium. Just like any other tapeworm, they absorb nutrients intended for the host while sitting inside animal guts. However, instead of sitting inside the guts of dogs, cats or humans, these worms spend their time inhabiting the guts of sharks, getting the nutrients by absorbing them before the sharks can. A research study conducted by Masoumeh Malek at the University of Tehran in Iran and a team of colleagues found that these parasitic worms might provide a critically valuable service to their hosts. They dissected 16 white cheek sharks (Carcharhinus dussumieri) found in the Persian Gulf and they removed tapeworms that they found. They then compared the concentrations of different compounds that they found inside the tissues of the sharks and the worms. They found out that the shark had an amazing 278 to 455 times higher levels of the toxic metals cadmium and lead inside their little bodies than the sharks did. Anthobothrium and Paraorigmatobothrium as they feed from the shark’s gut, they also function as filters that protect the predator from becoming poisoned by heavy metals (Malek et al. 2007).

Even though parasites seems to have a bad reputation, but in medicine they have proved to be useful. To investigate into the human medical kingdom, leeches have been used for bloodletting for the last 2500 years and in modern medicine, Hirudo medicinalis has been used to reduce swelling and restore blood circulation (Thearle, 1998). The anticoagulant of leeches is also a fertile ground of research for surgeries in which an incision must be kept open. In addition, leech saliva is found to contain powerful antibiotics and anaesthetics which no doubt will prove useful in future medicinal practice (Sawyer, 1986). However, if we move into the biological world and take a step back to observe the impacts of parasites on a community, or even within an ecosystem, we can begin to understand the overall importance of these organisms in the structuring of populations (Hudson et al. 2006).
Researchers state that humans have co-evolved with a host of parasites, and at some point they can rely on exposure to these parasites to effectively regulate our immune system. Whipworms are typically considered a scourge, but they have benefits. In monkeys, whipworms can restore intestinal bacterial balance and enables the monkey to avoid dangerous overreaction from immune systems. A dose of these worms can cure a chronic diarrhea in monkeys, this raises questions of what worms might do for human. Parasites, especially worm, can prevent a variety of anti-inflammatory responses (stimuli from the host), to be able to stay in our bodies. This worms awake the regulatory side of the immune system, that will in turn help turn off immune responses that are not needed.

Under the helminthes we have parasitic nematodes which have several benefits. Nematodes are microscopic, found mostly in moist soil, and are best predators of pests that spend any stage of their life cycle in the soil. Included among the group are fleas, cut worms, ants, root weevils and grubs they are just a few of more than 250 difficult to control pests. Nematodes move through the ground consuming other insects without harming the environment. They are harmless to humans, animals, plants and healthy earthworms, beneficial nematodes aggressively pursue insects. This nematodes can control a broad range of soil inhabiting insects and ground insects in their soil inhabiting stage of life. They carry an associated Xenorhabdus bacteria which produces enzymes that kills insects fast within 48 hours. This parasites are so effective, they can operate in the soil to kill the immature stages of garden pests before they become adults. They have such a wide host range that they can successfully control other numerous insect pests. Their indirect life cycle (require an intermediate host to be completed) enables them to infect a large number of insect species. Eggs of this parasites can make tropical ants swell and discolor to resemble barriers, birds eat this ants, other ants forage the bird’s waste and get infected. Nematodes are used to test the biological effects of spaceflight, such as the genetic damage that can be caused by exposure to cosmic radiation. Scientific tests suggested that more than 65 percent of human disease genes have equivalents in the genome of the Caenorhabditis (genus of nematodes which live in bacteria rich environment) elegans nematode.

Biological control conservationists seek to preserve and enhance populations of resident beneficial parasites in cropping systems. When a crop environment is friendly to beneficial arthropods, biological control provides endemic populations of predators and parasites to contribute substantially to pest management. Beneficial arthropods can often provide partial and, in rare instances, complete control of spider mites and aphids, depending on the population densities of pest and prey, environmental conditions and grower cultural practices. Adults of lady beetle species may consume 100 aphids per day.
Lady beetles can be important to natural suppression of hop aphids in areas where high temperature do not keep aphid population below damaging levels. Attraction and conservation of lady beetles is more effective and sustainable than the purchase and introduction of Hippodamia Convergens, which tend to rapidly disperse from hop yards after release. Despite feeding primarily on aphids and spider mites, these lady beetles can also feed on thrips, and other small insects, thus contribute at some level to overall biological control.
Lady beetles can be monitored by simply walking through yards and conducting timed counts. Adult lady beetle represents a significant population capable of responding to aphid population increases. Lady beetles are compatible with many new, selective insecticides and miticides but are negatively affected by older, broad-spectrum pesticides.
Insects belonging to hemiptera are predatory bugs, they possess shield-like structure, thickened forewings and suck out the body contents of their prey through tubular, stylet-like mouthparts. All of the predatory bugs found on hop feed on more than one type of prey, consuming the eggs, juveniles, and adults of a wide variety of prey including mites, aphids, caterpillars, and thrips.
Parasitic wasps can be monitored by placing a light-colored tray or cloth directly under a bine and shaking the bine vigorously to dislodge pests and wasps out of the canopy and onto the tray.
Close observation can reveal the tiny parasites. Yellow sticky traps may also be used to monitor wasp parasites. Wasp parasites are important in the biological control of hop looper and other caterpillar pests of hop. They also play a role in controlling hop aphid, but usually only on the overwintering of Prunus species. Host of this pest. A number of fly species from at least five families are known as predators or parasites of hop pests in the Pacific Northwest. In particular, outbreaks of Bacillus thuringiensis (a bacterial infection) and viruses occasionally result in population crashes of hop looper. Once pathogens take hold, they can almost eliminate hop looper populations.

Predatory midges are most often found feeding amongst aphids, thrips, and the eggs of other insects and mites. Predatory midges are most frequently seen during pest outbreaks. In some parts of the Pacific Northwest, a predatory midge species Feltiella sp. adapted to feeding on spider mites. Mites and aphids may also succumb to pathogens, but the incidence of this is generally low in the Pacific Northwest, unless the season is unusually cool and wet. Other species may occur, including Aphidoletes spp., which adapted to feed on aphids. Adult predatory midges feed on nectar and honeydew and lay 70 to 200 eggs near aphid or mite colonies. A larva during development consumes 40 to 100 mites or aphids. Naturally occurring diseases sometimes contribute to management of hop pests.
Diseased caterpillars are easy to spot, they are dark brown to black and hang from one pair of claspers or are draped over leaves. They emit a foul odor and basically become liquefied, releasing endospores of B. thuringiensis to infect other caterpillars.

Although parasites have a bad reputation and are considered ‘reprehensible citizens’ of an ecosystem, they have been shown to be incredibly important in the ecology and evolution of these ecosystems (Poulin, 2007). Because of these important roles, they make incredibly useful species to monitor changes that are happening in our ecosystems. Parasites can be used as effect indicators and as accumulation indicators because of their many ways in which they respond to anthropogenic pollution. According to Borcherding ; Wolf (2001), effect indicators can provide valuable information about the chemical status of their environment by changes of their physiology and behavior. If, for example, mussels change their shell-opening frequency, these alterations can indicate the presence of toxins within the water. This opening behavior can be checked online using the commercially available mussel monitor with Dreissena polymorpha or Mytilus edulis as indicator species for monitoring contamination in freshwater or marine habitats, respectively (Schuurmann ; Makert, 1998).
The usefulness of parasites in environmental monitoring has long been established with three publications summarizing results and identifying trends within the literature by utilizing quantitative methods (Blanar et al. 2009; Lafferty 1997; Poulin 1992). What these studies discovered is that eutrophication and metal contamination were the two types of pollution to illicit a significant response by parasite communities, particularly within the digeneans and monogeneans. Majority of these pollution types had a negative effect on the parasite population by impacting the community richness. According to MacKenzie (1999), there are three reasons why parasites make such excellent indicators of ecosystem health. Firstly, there are more parasitic than free living species that demonstrate an incredible biological diversity as they have had to adapt to a variety of hosts and living environments. Secondly, many parasitic species have complicated life history strategies, often involving highly sensitive, short lived, free living, developmental stages that are incredibly sensitive to environmental change. Finally, there are also parasites that are more resistant than their hosts to environmental change and tend to increase in number when ecosystems become polluted.
This has allowed researches to identify parasites as important indicators of environmental health and several authors have listed criteria to identify parasitic species that would be suitable for monitoring ecosystem change (e.g. MacKenzie 1999; Overstreet 1997; Williams ; Mackenzie, 2003).

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2. Borcherding J, ; Wolf J. 2001. The in?uence of suspended particles on the acute toxicity of 2-chloro-4-nitro-aniline, cadmium, and pentachlorophenol on the valve movement resoponse of the Zebra mussel (Dreissena polymorpha). Arch. Environ. Contam. Toxicol. 40, 497–504.
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