Selected features of the responses by fish to helminth parasites are discussed and comparison is made where appropriate with mammals. These include: (i) Factors influencing host specificity and consideration of the mechanisms that underpin the restriction of some parasites in their host spectrum, (ii) How fish leucocytes kill helminth larvae, with emphasis on the role of released oxygen (ROS) and nitrogen (RNS) free radicals from macrophages, (iii) Immune evasion strategies used by fish helminths, including invasion of immunologically privileged sites, encystment, adsorption of host proteins on the parasite surface, and high surface membrane turnover, (iv) Potential immunogens for vaccination and use for immunodiagnosis of infection, and (v) Natural and induced protection against helminths, with emphasis on the potential for future vaccination strategies.

Recirculating systems create unique environments for fish culture which may provide favorable conditions for disease occurrence or the reproduction of opportunistic microorganisms. Stressful conditions in recirculating systems, such as poor water quality or high stocking densities in the culture tanks, may contribute to disease outbreaks. Non-infectious problems, including high levels of ammonia, nitrites, carbon dioxide, suspended solids, or ozone residual levels have also caused mortalities in recirculating systems. The diseases encountered in rainbow trout (O. mykiss) cultured in recirculating systems include: those caused by bacteria (bacterial gill disease, furunculosis, bacterial kidney disease, fin rot), parasites (Gyrodactylus, Chilodonella, Trichodina, Epistylis, Trichophrya, Ichthyopthirius, Ichtyobodo, proliferative kidney disease, amoebic gill infestation, Coleps), fungi (Saprolegnia), and viruses (infectious pancreatic necrosis, viral hemorrhagic septicemia, and infectious hematopoietic necrosis). Treatments with chemotherapeutants in the water or feed in a recirculating system present special considerations; the main one is whether the biofilter will be treated and how the chemicals could affect its function. Management practices designed to prevent the occurrence of diseases or the degradation of water quality are critical to a successful recirculating facility. The introduction of known pathogens with infected fish should be prevented either by hatching eggs at the facility from disease-free broodstock, or by purchasing fingerlings from disease-free certified broodstock and by creating a quarantine period. Each recirculating facility should design a protocol for prevention of and control of fish diseases with the aid of a fish health professional, based on the generally accepted principles of fish health management.

Educational programs in fish health have developed in several different settings during the 20th century. Two prominent programs which have played major roles in providing educational opportunities developed independently during the last 40 years yet display numerous similarities in their goals and approaches. One arose primarily from a freshwater fisheries biology perspective while the other found its roots in traditional veterinary medical education. Not surprisingly, their pathways converged in due time. The following articles provide a brief view of the origins, philosophies, and accomplishments of these two particular programs. Other programs exist and still others are needed to fully serve those seeking to become a part of the aquatic animal health field.

Virus surveillance and certification procedures for finfish have traditionally relied upon isolation of replicating agents in cell culture and identification using serological procedures. However, accurate monitoring may also be achieved using techniques to detect fish antibodies against viral disease agents. The serological procedures most used for detection of fish antibodies are the serum neutralization test and immunofluorescence. Other techniques such as enzyme linked immunosorbent assays (ELISA) have been less commonly used. Using infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) as examples, this paper reviews the serological test procedures used for rhabdoviral surveillance and the applications of this methodology to viral epizootiology and certification of finfish.

Pasteurellosis, caused by Pasteurella piscicida, is one of the most threatening diseases of wild and cultured marine fish, and has been reported from many geographical areas including the USA, Japan and the Mediterranean countries. The objective of this article is to construct a picture of the current state of knowledge about this bacterial pathogen and the pathogenesis of the disease it causes. We review some important questions such as the controversial taxonomic position of the bacterium, and its main virulence mechanisms. The epidemiology of the disease, the routes of transmission and the putative reservoirs of P. piscicida in the environment are also discussed. Finally, a detailed survey of the strategies for controlling the disease is performed, including new diagnostic procedures, chemotherapy, employment of immunostimulants, and improvements in immunization programs.

Recirculating systems create unique environments for fish culture which may provide favorable conditions for disease occurrence or the reproduction of opportunistic microorganisms. Stressful conditions in recirculating systems, such as poor water quality or high stocking densities in the culture tanks, may contribute to disease outbreaks. Non-infectious problems, including high levels of ammonia, nitrites, carbon dioxide, suspended solids, or ozone residual levels have also caused mortalities in recirculating systems. The diseases encountered in rainbow trout (O. mykiss) cultured in recirculating systems include: those caused by bacteria (bacterial gill disease, furunculosis, bacterial kidney disease, fin rot), parasites (Gyrodactylus, Chilodonella, Trichodina, Epistylis, Trichophrya, Ichthyopthirius, Ichtyobodo, proliferative kidney disease, amoebic gill infestation, Coleps), fungi (Saprolegnia), and viruses (infectious pancreatic necrosis, viral hemorrhagic septicemia, and infectious hematopoietic necrosis). Treatments with chemotherapeutants in the water or feed in a recirculating system present special considerations; the main one is whether the biofilter will be treated and how the chemicals could affect its function. Management practices designed to prevent the occurrence of diseases or the degradation of water quality are critical to a successful recirculating facility. The introduction of known pathogens with infected fish should be prevented either by hatching eggs at the facility from disease-free broodstock, or by purchasing fingerlings from disease-free certified broodstock and by creating a quarantine period. Each recirculating facility should design a protocol for prevention of and control of fish diseases with the aid of a fish health professional, based on the generally accepted principles of fish health management.

Piscirickettsia salmonis is the first of the previously unrecognized rickettsial pathogens of fish to be isolated, characterized, and demonstrated to be the etiologic agent of an epizootic disease. However, since the isolation of P. salmonis in 1989, the scope and impact of these pathogens has become increasingly apparent. This growing awareness of the role of the rickettsiae in fish health has led to documentation of rickettsial diseases in diverse species of fish and in disparate geographic locations and aquatic environments. Considerable work remains in order to establish the source, reservoir, and normal mode(s) of transmission of these agents and to devise appropriate methods of disease prevention and control.

To generate an adaptive response from the mammalian immune system requires that antigen bind to cognate receptors on T and B cells, a process which activates intracellular signaling pathways. Crosslinking the B cell antigen receptor (BCR) ultimately activates cell proliferation in both higher and lower vertebrates. Recent studies suggest that many functional components of these intracellular pathways were evolutionarily conserved among the vertebrates. Antibody-mediated crosslinking of surface immunoglobulin leads to tyrosine phosphorylation on presumptive accessory molecules of the teleost BCR as well as several intracellular proteins. Crosslinking the teleost BCR also triggers calcium influx and activation of protein kinase C (PKC) which are hallmark components of the phosphatidyl inositol signal transduction pathway in mammalian lymphocytes. The activation of teleost PKC ultimately generates dually-phosphorylated forms of mitogen activated protein kinase. The latter enzyme is viewed as a key cytoplasmic control point for integrating signals arriving from several kinase/phosphatase pathways in mammalian cells. Preliminary evidence suggests that intracellular signaling mediated through antigen receptor complexes may be very sensitive to external factors, including heavy metals such as mercuric chloride which can alter calcium flux and tyrosine phosphorylation patterns in teleost leukocytes. As the process of lymphocyte activation in teleost fish is better understood, it may be possible to provide aquaculturists, environmental regulators and fisheries managers with better information on those natural and man-made conditions which interfere with the development of protective immune responses in natural and captive finfish populations.