In effect, although the annihilation of the host by a parasite could be depicted as a crude example of "the survival of the fittest," situations where the host dies rapidly often amount to a pyrrhic victory because parasitic colonies harboured within it may die as well before the transmission stage.
So why would natural selection favour high virulence if this results in both the host's and the pathogen's deaths? From the last quarter of the nineteenth century to the late s, it was often considered that, all else being equal, pathogen's evolution towards harmlessness was the expected outcome of long-term biological associations, as it would ensure the survival of both species. Frequently dismissed today as naively adaptationist, this perspective was however widely defended by some of those who pioneered an ecological, even evolutionary view of disease in the early-twentieth century, and was often ascribed to American bacteriologist and comparative pathologist Theobald Smith DOI: Abstract PDF.
Parasitism is an association between two different species of organisms, where only one partner which is always the parasite derives benefits metabolically. The other partner is the host and gets nothing in return but harm, damage or death [1]. A parasite lives in a particular way called parasitism.
Parasitism as a one-sided partnership in which the host receives no benefits but instead some degree of harm, or death is an ingratitude and is synonymous the parasite committing matricide! Both the parasite and host make the best of a bad job. There is usually a major nutritional interface with the host such as the digestive-absorptive epithelium in platyhelminthes which has enormous increase in surface area by development of folds and microvilli such as tubercules, spines and pores of flukes and tapeworms [5].
This can be more damaging than the infection itself [5,6]. 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 [5]. 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. Microfilariae of Onchocerca volvulus causes skin onchodermatitis, nodules, leopard skin and lizard skin.
The skin lesions become the site of secondary bacterial infection 6, 7. We can now combine mathematical modelling with lab experiments of evolving microbes to answer intriguing questions about how other species evolve in response to host protection. Does the host evolve to harbour the protective parasite, and is this how we developed a symbiotic relationship with some of our gut bacteria? Can more harmful parasites evolve to overcome host protection?
Answering questions like these can help us find new ways to treat infectious diseases. The results of our research shed light on a fascinating biological phenomenon about which we still know very little. Portsmouth Climate Festival — Portsmouth, Portsmouth. Edition: Available editions United Kingdom.
Become an author Sign up as a reader Sign in. Some E. Coli protect humans from more harmful strains. Ben Ashby , University of Bath. Friends, for now. Signal Corps Bacteria that live in our gut can occasionally cause problems, but they also prevent colonisation by more harmful microbes such as Salmonella enterica , which causes food poisoning.
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