A parasitic disease, also known as parasitosis, is an infectious disease caused or transmitted by a parasite. Parasites have been on Earth for billions of years and are part of Nature’s evlutionary process. Many parasites do not cause diseases as it may eventually lead to death of both organism and host. Sometime they can be useful for the human microbiota. Bacteria can be parasites. (See Microbiota file). And they make up around 40 percent of the Earth’s bio-mass. Parasitic diseases can affect practically all living organisms, including plants and mammals. The study of parasitic diseases is called parasitology. Some parasites like Toxoplasma gondii and Plasmodium spp. can cause disease directly, but other organisms can cause disease by the toxins that they produce.
The three main types of organisms causing parasitic diseases are protozoa (causing protozoan infection), helminths (helminthiasis), and ectoparasites.
Protozoa and helminths are usually endoparasites (usually living inside the body of the host), while ectoparasites usually live on the surface of the host. Protozoa are single-celled, microscopic organisms that belong to the kingdom Protista. Helminths on the other hand are macroscopic, multicellular organisms that belong to the kingdom Animalia. Protozoans obtain their required nutrients through pinocytosis and phagocytosis. Helminths of class Cestoidea and Trematoda absorb nutrients, whereas nematodes obtain needed nourishment through ingestion. Occasionally the definition of “parasitic disease” is restricted to diseases due to endoparasites.
Signs and symptoms
Symptoms of parasites may not always be obvious. However, such symptoms may mimic anemia or a hormone deficiency. Some of the symptoms caused by several worm infestations can include itching affecting the anus or the vaginal area, abdominal pain, weight loss, increased appetite, bowel obstructions, diarrhea, and vomiting eventually leading to dehydration, sleeping problems, worms present in the vomit or stools, anemia, aching muscles or joints, general malaise, allergies, fatigue, nervousness. Symptoms may also be confused with pneumonia or food poisoning.
The effects caused by parasitic diseases range from mild discomfort to death. The nematode parasites Necator americanus and Ancylostoma duodenale cause human hookworm infection, which leads to anaemia, protein malnutrition and, in severely malnourished people, shortness of breath and weakness. This infection affects approximately 740 million people in the developing countries, including children and adults, of the tropics specifically in poor rural areas located in sub-Saharan Africa, Latin America, South-East Asia and China. Chronic hookworm in children leads to impaired physical and intellectual development, school performance and attendance are reduced. Pregnant women affected by a hookworm infection can also develop aneamia, which results in negative outcomes both for the mother and the infant. Some of them are: low birth weight, impaired milk production, as well as increased risk of death for the mother and the baby.
Mammals can get parasites from contaminated food or water, bug bites, or sexual contact. Ingestion of contaminated water can produce Giardia infections.
Parasites normally enter the body through the skin or mouth. Close contact with pets can lead to parasite infestation as dogs and cats are host to many parasites.
Other risks that can lead people to acquire parasites are walking with barefeet, inadequate disposal of feces, lack of hygiene, close contact with someone carrying specific parasites, and eating undercooked foods, unwashed fruits and vegetables or foods from contaminated regions. Parasites can also be transferred to their host by the bite of an insect vector, i.e. mosquito, bed bug, fleas.
Parasitic infections can usually be treated with antiparasitic drugs. Albendazole and mebendazole have been the treatments administered to entire populations to control hookworm infection. However, it is a costly option and both children and adults become reinfected within a few months after deparasitation occurs raising concerns because the treatment has to repeatedly be administered and drug resistance may occur.
Another medication administered to kill worm infections has been pyrantel pamoate. For some parasitic diseases, there is no treatment and, in the case of serious symptoms, medication intended to kill the parasite is administered, whereas, in other cases, symptom relief options are used. Recent papers have also proposed the use of viruses to treat infections caused by protozoa.
In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. However, in the human gut, some experts claim that parasites can be useful to the human organism, as long as they are controlled and balanced. (See microbiome file)
The entomologist E. O. Wilson has characterised parasites as “predators that eat prey in units of less than one”. Parasites include protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes.
Like predation, parasitism is a type of consumer-resource interaction, but unlike predators, parasites, with the exception of parasitoids, are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing Plasmodium species, and fleas.
Parasites reduce host fitness by general or specialised pathology, from parasitic castration to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.
People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations.
Major evolutionary parasitic adaptation strategies
There are six major parasitic strategies, namely parasitic castration, directly transmitted parasitism, trophically transmitted parasitism, vector-transmitted parasitism, parasitoidism, and micropredation. These apply to parasites whose hosts are plants as well as animals. These strategies represent adaptive peaks; intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable. A perspective on the evolutionary options can be gained by considering four questions: the effect on the fitness of a parasite’s hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, the major evolutionary strategies of parasitism emerge, alongside predation.
Honing in on Parasitic castrators
Parasitic castrators partly or completely destroy their host’s ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host’s other systems are left intact, allowing it to survive and sustain the parasite. Parasitic crustaceans such as those in the specialised barnacle genus Sacculina specifically cause damage to the gonads of their many species of host crabs. In the case of Sacculina, the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to have gained female secondary sex characteristics such as broader abdomens, smaller claws and egg-grasping appendages. Various species of helminth castrate their hosts (such as insects and snails). This may be directly, whether mechanically by feeding on their gonads, or by secreting a chemical that destroys reproductive cells; or indirectly, whether by secreting a hormone or by diverting nutrients. For example, the trematode Zoogonus lasius, whose sporocysts lack mouths, castrates the intertidal marine snail Tritia obsoleta chemically, developing in its gonad and killing its reproductive cells.
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