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Parasite Focus: Gyrodactylus

by the Fish Site Editor
04 December 2006, at 12:00am

Gyrodactylus is an interesting little parasite; species of which can be found infesting many different types of freshwater and marine fish as well as amphibians such as frogs, writes Pete Southgate, the Fish Vet Group.

It is a so-called monogenean trematode - a parasitic fluke less than one millimetre in length with a direct life cycle not involving any intermediate stages or hosts. It generally infests the skin and fins of the fish, although it has also sometimes been found on the gills (it's near relative Dactylogyrus prefers the gills). It is also viviparous, producing live young which, rather like Russian dolls, can also produce more live young almost immediately. On microscopic examination you can sometimes see a number of generations developing within one fluke. Consequently the life cycle can be very short - between 1 and 5 days - and high numbers can therefore build up very rapidly.

Feeding mostly on the mucus and epithelial cells of the skin's surface - occasionally on blood - it attaches to the host by means of a complex hook and anchor arrangement and it is this attachment which damages tissues and causes great irritation to the fish. The fish shows obvious signs of irritation and the damaged areas can develop into significant skin ulcers and fin erosions, often with secondary bacterial and fungal infections. Gyrodactylus is frequently found as a mixed parasitic infestation along with parasitic protozoans such as Trichodina which also contribute to the irritation and the damage.

Having said all that, Gyrodactylus is not often seen as a problem in fish farms and we see it more frequently infesting aquarium and ornamental pond fish where it can cause significant disease problems.

The one species that is of concern to fisheries and fish farms is Gyrodactylus salaris. This is the notifiable one which is naturally widespread in the rivers of Finland, Russia and eastern Sweden draining into the Baltic Sea and which has been identified as the cause of loss of whole populations of Atlantic salmon parr and smolts in western Sweden and Norway. Epidemics of the organism occurred when it was transferred to areas where it is not naturally found and salmon exposed to the parasite for the first time had no evolved immunity (unlike those fish in the Baltic rivers which appear to be tolerant). This resulted in heavy losses of the susceptible fish and populations of salmon have been eliminated from more than twenty Norwegian rivers with the probable inevitable loss of some strain characteristics of those populations.

The threat from G salaris to native populations of fish in Norway is considered to be so serious that all fish populations have been wiped out in river systems where the parasite has been identified. This is an extremely drastic and destructive approach and has not always proved successful in eliminating the parasite. It is, however, the only control method considered suitable and demonstrates the severity of the problem.

It is absolutely imperative that G salaris is kept out of British waters - Scottish salmon have been shown to be susceptible to the parasite - and present European legislation is in place to control the potential introduction of the organism on live fish. It is also important that anyone possibly exposed to a risk ensures that they, and their equipment, are not contaminated with the organism.

The standard method of diagnosing Gyrodactylus infestation is to carry out microscopic examination of skin scrapes from the fish. The difficulty comes when trying to distinguish one species from another and the definitive diagnosis of G salaris infestation takes specialist expertise. The parasitologists at the Institute of Aquaculture in Stirling have done some very elegant work on distinguishing G salaris from other Gyrodactylus species and staff at the Marine Laboratory in Aberdeen have developed the expertise to characterise the organism. As a notifiable disease it is important that the regulatory authorities are notified in the case of any suspicion of the presence of G salaris and specialist advice should be sought on further investigation and the submission of appropriate suspicious material.

It is also interesting that my spell check of Gyrodactylus salaris comes out as Gyro salaries - another subject dear to my heart.

Source: Fish Vet Group - 2006

the Fish Site Editor

 

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