This is an extract of our Parasitism document, which we sell as part of our Population Dynamics and Ecosystems Notes collection written by the top tier of University Of Manchester students.
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Parasitism Organisms that live on or within a host, to the host's detriment, can be divided into two groups:
1. Macroparasites which include nematodes (tapeworms, flukes etc.)
2. Microparasites which are pathogens or disease-causing organisms (bacteria, fungi, viruses, prions). Both groups may affect plants or animals, effects can be sub-lethal (common) or lethal (rare). Sub-lethal effects usually much more important in ecological terms - if you depend on a host to survive then you don't want to kill it. Microparasites May have direct transmission, in the case of the influenza virus, venereal diseases or food-borne micro-organims; or may have indirect transmission using vectors. For example, sleeping sickness is caused by trypanasomes, which are transmitted using the tsetse fly as a vector; plasmodium (responsible for malaria) is transmitted by the mosquito. Macroparasites Some may have complex life cycles which include more than one host. o Tapeworms parasitise humans and their eggs are laid in the faeces. They may have intermediate hosts of insects or other mammals. o Schistostomes (flukes) show alternate infection between humans and snails o Filarial nematodes have insects as intermediate hosts, eg. Blackflies which carry the river blindness nematode. o Plant pathogens of certain species (eg the black stem rust alternates between wheat and Berberis). Others may show direct transmission - e.g intestinal human nematodes and fungal parasites of plants. Plant parasites Plants are parasitized by rusts and mildews, which are fungal infections. They show symptoms of macro-parasites but are they macro or micro parasites?
Plants can even parasitise other plants. Holoparasitic plants such as dodder do not produce chlorophyll and are obligate parasites of other plants. Hemiparasitic plants like mistletoe and rattles have poorly develop root systems but can photosynthesise without the aid of other plants. Parasite specificity Parasites may either be host-specific (in which they parasitise only one species) or generalists (in which they can parasitise a number of different hosts). Whether they are specific or generalist depends a lot on their lifestyle. Depends on their ability to move between hosts - immobile parasities spend their whole life on one host. The majority of these are host-specific. Those that are highly mobile tend to be generalists, with more than 3 hosts.
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