Parasite corner: Sessile peritrichs

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Parasite corner: Sessile peritrichs

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The mere thought of a parasite infection makes many people feel uncomfortable and is certainly not a topic for discussion over the dinner table. And yet, most people are blissfully unaware that they play host to their own community of parasites.

For example, around a third of people under 20, half of all adults, and two thirds of elderly people carry parasitic Demodex mites, commonly referred to as eye lash mites, which inhabit our hair follicles and associated sebaceous glands. Parasites are more common than many think. In fact, a free-living organism that does not harbour several parasitic individuals or a number of species is a considered something of a rarity. One should not be surprised, therefore, to encounter parasites during a routine health assessment of a sample of fish and it is important to bear in mind that their presence does not infer the presence of disease or that a treatment must be administered. Having encountered a parasite, it is imperative to have a proper identification and a clear understanding of its life-cycle before taking appropriate action.

Here we take a closer look at some particular microscopic, ciliate protozoans of fish, which although common are frequently overlooked – the sessile peritrichs or non-mobile ciliated protozoans. Peritrichs typically possess a prominent ring of cilia that arises from a centrally positioned, oral cavity and extends in an anti-clockwise direction around the circular or bell-shaped anterior end of the parasite. These cilia function to catch and transfer organic matter towards the oral cavity, at the centre of which, is a contractile vacuole. There is, however, great diversity within this sub-class of parasites and they can be highly mobile, obligate parasites of aquatic organisms (i.e. those that need a host to complete their life-cycle, e.g. Trichodina), sessile, obligate ectocommensals (e.g. ApiosomaEpistylis – which use a stalk to attach themselves to an appropriate substrate and can be either solitary or produce branched colonies), or facultative (i.e. those that can demonstrate parasitic activity but are not dependent on a host to complete their life-cycle, e.g. TetrahymenaZoothamnium etc).

The sessile peritrichs Apiosoma (formerly Glossatella) and Ambiphyra attach themselves to the skin and gills of their fish and amphibian hosts. Apiosoma typically has a barrel-shaped body (30 × 50 µm) with cilia at the distal end (Fig. 1b), a large rounded macronucleus and are solitary organisms rather than colony formers. Ambiphyra spp. are also barrel-shaped but possess an equatorial, ciliary fringe. Neither peritrich has a stalk but maintains a superficial attachment to its host’s epithelium by means of a scopula – a modified attachment organ. Both parasites reproduce by binary fission with new ciliated, motile individuals budding off that can then colonise new hosts. They do not feed off the fish but on suspended organic debris borne in the water current. Both are generally considered as harmless ectocommensals, however, high numbers can cause inflammation, increased mucus production, hyperplasia, necrosis and ultimately ulceration of the skin and degeneration of gill tissues. Poor water quality, e.g. high organic loading, can allow parasite numbers to rapidly proliferate and cause problems.

By comparison, Epistylis and Zoothamnium are colonial ciliates. Epistylis is an obligate parasite whilst Zoothamnium is primarily free-living on a range of other aquatic substrates and are opportunist settlers. Both peritrichs attach by means of a long stalk which may exceed 1 mm, which is contractile in Zoothamnium and non-contractile in Epistylis. Once attached, one parasite will divide and spread to form large colonies. When present in large colonies, the attachment sites can cause a lesion that can become inflamed and necrotic and eventually ulcerated – commonly referred to as “red sore disease”. The lesion is then vulnerable to invasion by secondary pathogens.

Finding a large number of sessile peritrichs on your fish, however, does not automatically infer the presence of disease and that they must be treated. Instead, the first course of action would be to improve water quality, to reduce the level of organic loading and then, following an evaluation of the host, decide upon whether a concurrent treatment should be applied or whether a period of monitoring, to see if parasite numbers decrease, is appropriate.

A thick growth of colonial Zoothamnium on a marine crab.

Source: Andy Shinn, Leo Galli, Don Griffiths, Pikul Jiravanichpaisal, Nattawadee Wattanapongchart, Oran Fish Vet Group Asia Limited

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