Gyrodactylus are a monogenean parasite of fish that gives birth to live young, which can be seen within the body of the adult worm, and frequently are skin parasites of fish.
Monogenean trematodes, which have direct life cycles, are common, highly pathogenic, obligatory parasites of the skin and gills. They are ~0.1-0.8 mm long and are best seen microscopically. The worms can be identified by their characteristic hold-fast organ, the haptor, which is armed with large and small hooks. Aquarium and cultured fish are subject to a rapid buildup of parasites by continuous infection and worm transfer to other fish in the tank or pond. Although many species are host-specific, the more common types seen in aquaria are less selective.
Members of the genus Gyrodactylus are hyperviviparous. Embryos develop within each other inside the mother’s uterus and asexual reproduction alternates with sexual reproduction. This rapid reproduction in close relationship with the host, together with the high host specificity is thought to promote co-evolution between host and parasite. At first sight, the absence of a free-living larval stage might decrease the chance to encounter other host species, minimising the opportunities for host switching. However, it has been shown that gyrodactylids can survive for short periods independent of their host and a ‘swimming behaviour’ has been described. This ‘active’ dispersion capacity of Gyrodactylus in combination with the ability to produce a viable deme from only one pregnant individual might increase the chance for speciation by host switching. Auto-infection and the high level of host specificity might also enhance sympatric speciation. The succession of several generations on a single host specimen ensures the continuity of a population, but increases the chance of inbreeding.
Infection site for this parasite are the skin, fin, gill. Infected fish usually exhibits no external abnormality. Heavily infested fish shows anorexia, lethargy and haemorrhages.
Parasitized fish exhibit excessive mucus secretion and petechial haemorrhagic lesions. In case of Atlantic salmon, an epidermal layer becomes thin due to the decrease in density of the mucous cells. Since this parasite is not infectious to human, it is harmless in food hygiene.
Individual aquarium fish may be treated with fenbendazole.
Gyrodactylids are small and low numbers may appear to have little effect on their hosts, but initial appearances are misleading. Even low infection levels can cause host mortality and higher parasite burdens can cause secondary pathology, such as fin clamping, which presumably inflicts severe fitness costs to the host. The actual cause of host death is unknown, but may be induced by secondary infections and/or osmotic problems associated with the puncture wounds inflicted by gyrodactylids during attachment and feeding. Most host populations show evidence of an immune response against gyrodactylids (reviewed by Buchmann) with considerable within and between population heterogeneity in resistance.
With its high fecundity and transmission potential, G. salaris can rapidly colonize an entire river system and even re-colonize waterways after treatment with the rotenone. Rotenone poisoning of entire river systems was used, largely unsuccessfully, for many years, and now control measures involving heavy metal (aluminium) dosing are being trailed in Norway. In the current global economy, the risk of further introductions of gyrodactylid monogeneans is an indisputable threat. The biology of these pathogens predilects them for introduction and numerous species have been introduced to Europe. Unfortunately, the impact of such introductions to the ecosystem is currently unpredictable, and factors influencing the pathogenic-potential of Gyrodactylus spp. remain largely unknown.
- Appleby, C., (1996) Population dynamics of Gyrodactylus sp. (Monogenea) infecting the sand goby in the Oslo Fjord, Norway. J Fish Biol 49:402–410
- Cable, J., et al (2002) Behavior favoring transmission in the viviparous monogenean Gyrodactylus turnbulli. J Parasitol 88:183–184
- Johnson, B. O. and A. J. Jensen (1991) The Gyrodactylus story in Norway. Aquaculture 98:289-302
- Llewellyn, J., Green, J.E., Kearn, G.C., (1984) A checklist of monogenean (Platyhelminth) parasites of Plymouth hosts. J Mar Biol Assoc U.K. 64:881–887
- Merck Veterinary Manual