Parasitism: The Variety of Parasites


Associations between animals of different species can take a variety of forms, ranging from commensalism where the partners live independent lives but benefit each other, to parasitism where one partner of the relationship benefits at the expense of the other, the host. In between, we have mutualism where the partners have an intimate relationship which is obligatory for both partners but is also beneficial to both. The majority of parasites in the animal kingdom are invertebrates. Amongst the protozoa, parasites of medical or veterinary importance are found amongst the amoebae (e.g. Entamoeba histolytica), the flagellates (e.g. trypanosomes) and the sporozoa (such as the malarial parasites), all of which are parasitic. The parasitic worms (helminths) are very important parasites of man and his domestic animals including the flukes (e.g. schistosomes causing bilharzia), round worms or nematodes such as Ascaris lumbricoides, the large round worm of humans, and the tapeworms or cestodes (it is the larval stages of tapeworms which are the cause of morbidity). Finally, the arthropods include important parasites such as ticks and mites (arachnids), and insects such as lice and fleas. Many insects such as mosquitoes, of course, act as vectors of parasites. Parasites are also identified as either endoparasites or ectoparasites depending on whether they are located internally or externally with respect to the host. Parasites often have incredibly complicated life cycles and employment of control measures against parasites requires detailed knowledge of these life cycles.

Key Concepts:

  • Associations between different species of animals can take various forms ranging from commensalism, through mutualism to parasitism.

  • In commensalism both partners live independently, in mutualism there is an obligatory intimate relationship between the partners which is beneficial to both and, in parasitic relationship one partner benefits at the expense of the other.

  • Most parasitic species are invertebrates but there are some vertebrate parasites such as the cuckoo.

  • Parasitic species are found in both single‐celled organisms, the protozoa, and the multi‐celled metazoan organisms.

  • There are important medical and veterinary parasites in all the major groups of protozoan and metazoan animals.

  • Parasites can be found inside their host and are known as endoparasites whereas some found outside their hosts and are known as ectoparasites.

  • Devising strategies to control parasites and avoid infection depends on an understanding of the life cycle of the parasites.

Keywords: protozoa; helminths; insects; arthropods; endoparasites; ectoparasites; control; macroparasites; microparasites

Figure 1.

Some examples of parasitic protozoa. Trichonympha is found in wood eating termites and digests cellulose in the termites wood diet; Trichomonas vaginalis is found in the human vagina and is transmitted by sexual intercourse; Giardia is found in the small intestine and causes diarrhoea; Trypanosoma brucei is the tsetse fly transmitted agent of sleeping sickness in humans and the disease is fatal unless treated; E. histolytica can cause amoebic dysentery and in serious cases invades the liver. Undulating membrane, UM (for locomotion); nucleus, N; flagellum, F.

Figure 2.

Life cycle of the blood fluke Schistosoma mansoni which causes bilharzia. The fluke has two hosts – humans and a freshwater snail. Humans become infected when in water containing the infective cercariae, which are released from the infected snail. The adult worms live in the veins around the gut and the eggs are released in the faeces of the infected individuals.



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Further Reading

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Phillips, R Stephen(Sep 2012) Parasitism: The Variety of Parasites. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003156.pub2]