Diseases of mediterranean fish species: Problems, research and prospects

Seabream Sparus aurata and seabass Dicentrarchus labrax still dominate the production of the Mediterranean fish farming industry, but new species, Mediterranean and otherwise, are constantly being added to the list. Significant progress has been made in the farming of Atlantic bluefin tuna (Thunnus thynnus) in Spain, Malta and Croatia. Health management in the domestication of new species remains a high priority, while knowledge gaps are gradually bridged. The major diseases which confront farmers vary in prevalence according to the geographic latitude of each country around the Mediterranean basin, while the methods by which the animals are cultured greatly influence type and severity of diseases. “Old” pathogens (Lymphocystis and encephalitis viruses, “Pasteurella” piscicida, Vibrio spp., Amyloodinium ocellatum) still cause serious economic losses, and once sporadic diseases (caused by Mycobacterium marinum, Streptococcus iniae, Lactococcus garvieae, Enteromyxum [formerly Myxidium] leei, Kudoa sp.) are emerging. Sophisticated biotechnologies have been or are being developed and many microbial pathogens are identified today on a molecular basis. A direct PCR method for detection and identification of M. marinum based on the 16S rRNA gene sequence was successfully developed ten years ago in Israel, and has become established for other bacterial fish pathogens. Randomly Amplified Polymorphic DNA (RAPD) and Amplified Fragment Length Polymorphism (AFLP), by using the entire DNA rather than gene sequences, have provided an even higher level of precision in genotyping. Disease diagnosis can thus be very rapid and accurate, and taxonomic positions assigned with much greater confidence. At the same time, however, higher precision has made the need for major taxonomic shifts and re-classifications more frequent. The advent of genetic engineering has provided the fish industry with recombinant vaccines, based on selected antigens. Unfortunately, immunocompetence in very young stages is virtually non-existent, the specific immune response in fish is generally slow to build up and characterized by a short memory. Circulating antibodies may not be protective, so that high levels of specific antibodies often do not correlate with protection or disease status. These facts make the fish rely more heavily on the nonspecific immune system as the initial and primary defense against microbial invasion. Endogenous peptides with powerful antimicrobial properties (endobiotics) have recently been detected, purified and characterized in fish. Three members of a novel family of such endobiotics (“piscidins”), expressed in mast cells of various families of fish in the suborder Percoidei, display a broad antibacterial spectrum and are effective against protistan ectoparasites such as the dinoflagellate Amyloodinium ocellatum and the ciliate Cryptocaryon irritans. The factors affecting modulation of piscidins are being studied. Potentiating the natural, non-specific, non-adaptive defenses of the host will limit use and dependence on treatment drugs and chemicals (antibiotics in particular) and is the innovative strategic course that fish health research has taken in the past few years.