Parasites in cultured and feral fish

Abstract

Parasites, causing little apparent damage in feral fish populations, may become causative agents of diseases of great importance in farmed fish, leading to pathological changes, decrease of fitness or reduction of the market value of fish. Despite considerable progress in fish parasitology in the last decades, major gaps still exist in the knowledge of taxonomy, biology, epizootiology and control of fish parasites, including such ‘evergreens’ as the ciliate Ichthyophthirius multifiliis, a causative agent of white spot disease, or proliferative kidney disease (PKD), one of the most economically damaging diseases in the rainbow trout industry which causative agent remain enigmatic. Besides long-recognized parasites, other potentially severe pathogens have appeared quite recently such as amphizoic amoebae, causative agents of amoebic gill disease (AGD), the monogenean Gyrodactylus salaris which has destroyed salmon populations in Norway, or sea lice, in particular Lepeophtheirus salmonis that endanger marine salmonids in some areas. Recent spreading of some parasites throughout the world (e.g. the cestode Bothriocephalus acheilognathi) has been facilitated through insufficient veterinary control during import of fish. Control of many important parasitic diseases is still far from being satisfactory and further research is needed. Use of chemotherapy has limitations and new effective, but environmentally safe drugs should be developed. A very promising area of future research seems to be studies on immunity in parasitic infections, use of molecular technology in diagnostics and development of new vaccines against the most pathogenic parasites.

Introduction

Fish are an indispensable source of proteins for humans, notwithstanding their importance as an object of sport fishery and pets in the case of ornamental fish. Development of aquaculture during the last decades has resulted in much greater attention being paid to problems posed by parasites and their importance for fishery leading to constraints in the productivity of aquaculture (Kennedy, 1994a). Besides direct losses caused by mortality, parasites may have considerable impact on growth and behaviour of fish, their resistance to other stressing factors, susceptibility to predation, etc.; their presence may also reduce marketability of fish (Crowden and Boom, 1980, Brassard et al., 1982, Lom and Dykova, 1992, Williams and Jones, 1994, Kumaraguru et al., 1995, Woo, 1995).

Important progress in fish parasitology has been achieved in the last years and several comprehensive textbooks dealing with fish parasites have recently appeared (Lom and Dykova, 1992, Moravec, 1994, Pike and Lewis, 1994, Williams and Jones, 1994, Woo, 1995). Nevertheless, there remain numerous problems to be solved. Due to space limitations, only a few selected parasites of each major groups of fish parasites, i.e. protozoans, myxosporeans, helminths and parasitic crustaceans, will be dealt with in more detail in the present paper and some contemporary research activities and topics of possible future interest will also be briefly discussed.