There are no living organisms, besides maybe viruses, that do not carry their own parasitic organisms. Yet, parasitism is a question of specificity, and the unravelling of the complex interactions of species to host, species to vector, and sometimes vector to host, continues to captivate many biologists. Three types of parasitic associations can be contemplated. True parasitism is where the parasites live at the expense of their hosts and always have the potential to harm them, either directly, by depriving them of essential material or by destroying tissues, or indirectly, by damaging their hosts by the liberation of toxins. This is the case with pathogenic parasites. (Sometimes the host-parasite interaction does not initially induce a pathogenic state due to the capacity of the host immune system to hold it at bay.) In the event where this balance is tipped against the host it may lead to the parasite gaining
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Figure 2.1 The current models of the tree of life. (A) Consensus model with only a few of the 'kingdoms' of the bacteria shown. Mitochondrial and chloroplast endosymbioses are indicated by lower and upper diagonal arrows, respectively (from Doolittle 1999). (B) Molecular tree of life based on ssRNA sequences and modified by trimming with other sequences (from Pace 1997). Reprinted with permission of the editors.
the upper hand.) Commensalism is when the eukaryote acquires nourishment from the host but does not hurt it and is not metabolically dependent on it. Good examples of this are the usually harmless intestinal amoebae, Entamoeba dispar or Entamoeba coli, which probably do not remove anything of value to man. Symbiosis, called by some as mutualism, is an interaction where both hosts benefit. A good example is the role of the prokaryotic bacterium Wolbachia which has been shown to infect filarial worms such as Brugia malayi which parasitizes man. The interaction of the bacterium with the filaria seems important for the development of the latter, as suggested by the lack of sexual development of the worm when the Wolbachia are eradicated. Thus, the interaction between the human host and the filarial worm is parasitic, while that between the bacterium and the worm is a form of symbiosis. This fascinating example is one of many interactions of parasites with their hosts. Those parasites that harm the host are called pathogens, while those that do not are non-pathogenic (commensals). Those that do not depend on the host during the entire life-cycle but also live outside of it are facultative parasites (e.g. Strongyloides spp.), while obligate parasites are unable to survive without their hosts. Some parasites are ectoparasites as they live on the body surfaces. This definition is applicable no matter how brief the period of time, as occurs, for example, with arthropods such as ticks or mosquitoes. The latter form of parasitism is called infestation. Helminths or protozoa living in the body of the host cause infections. They are endoparasites. Hosts are also placed into different categories depending on where the parasite reaches sexual maturity. A definitive host is one in which sexual maturity and reproduction occur, while an intermediate host is one in which they do not; the mosquito is unexpectedly, therefore, the definitive host of the malaria parasite, while man is the intermediate host. Medical parasitology includes the study of parasites, their vectors, definitive and intermediate hosts, and factors of ecological or epidemiological importance relevant to a disease. The delicate balance of these interactions is what determines whether a parasitic disease remains endemic or is eradicated. Thus, climatic and ecological changes that affect the vector may drastically alter the incidence of a given disease. In the same way, a break at any point in the complex chain may be enough to eradicate some parasitic diseases. It is worth noting that diseases such as malaria, which are often defined as being 'tropical' in nature, were prevalent in northern countries prior to the 1930s. However, changes, including those in the social structure, apparently broke the chain and thus the adequate transmission of Plasmodium. A narrow definition of parasitology, such as that used for medical parasitology, may be preferred by those studying parasitic infections, while population geneticists may favour, for example, a more comprehensive definition. Parasitism, in the broader sense, embraces every gradation of a hostparasite relationship, from interactions in which the partners are mutually and equally beneficial to cases in which the parasite is pathogenic or even lethal to its host.
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