The piscine immune system is normally quite efficient in protecting the host (innate and acquired immunity) from parasitic infections. Innate immunity may occur at two distinct levels — between host species and within a host species. If the resistance is at the host species (or a higher taxonomic group) level, then it is inter-host innate immunity. For example, Oncorhynchus mykiss can be infected with the pathogenic hemoflagellate, Cryptobia salmositica isolated from Oncorhynchus spp. but cannot be infected with Cryptobia catostomi from Catostomus commersoni. At the next level, there are individuals within a susceptible host species that are resistant to infection — this is intra-host innate immunity; e.g. some Salvelinus fontinalis are resistant to C. salmositica infection while others are not. This resistance to infection is not dependent on age or size of the fish; it is inherited and is controlled by a dominant gene. Protection at both levels of innate immunity is via the activation of the alternative pathway of complement activation to lyse the parasite. Also, S. fontinalis can be infected with the pathogenic C. salmositica have very high parasitaemias but they do not suffer from the disease as O. mykiss. This resistance to disease is related to high levels and rapid production of α2-macroglobulin which is one of two natural antiproteases. The α2-macroglobulin in the blood neutralises the metallo-protease secreted by the pathogenic C. salmositica. Acquire immunity was shown in fish that survived infections of pathogenic flagellates. Fish that have recovered from Amyloodinium ocellatum, C. salmositica, Cryptobia bullocki, and Trypanosoma danilewskyi are protected. This protection requires prior exposure to the pathogen and/or its antigens. Humoral (e.g. complement fixing antibodies to lyse the parasite) and cell-mediated (e.g. T-cell cytotoxicity, phagocytosis) are part of the protective mechanism in acquired immunity. Also, an attenuated live C. salmositica vaccine has been developed and it protects juvenile and adult salmonids from cryptobiosis for at least 2 years.
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.
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.
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.
Whirling disease is caused by the myxozoan parasite Myxobolus cerebralis, Hofer. It has been associated with a serious decline in wild rainbow trout populations in the western United States. The alternate host is the tubificid oligochaete Tubifex tubifex, and perhaps other tubificids. Tubifex is a very adaptable taxon, being found in various morphological forms. Polyploidy has been demonstrated. There is some suspicion that sibling species may be found to exist. Infection rates in tubificids are low. Related myxosporeans are associated with a variety of aquatic oligochaete worms belonging to three families, Tubificidae, Naididae and Lumbriculidae, and also with polychaetes. Myxozoans are now recognized as members of the phylum Cnidaria. Other fish parasites, such as the cestodes Archigetes and Caryophyllaeus and the nematode Eustrongylides, are transmitted by tubificids. The tubificid worm population in an ecosystem may provide an infective reservoir as the disease persists for long periods in the worm. Control of worm populations might be achieved by reducing organic inputs, lowering erosion, increasing flow and removing objects that accumulate pockets of silt. The use of pesticides is unlikely to be effective because of a lack of specific impact and consequent disruption of the whole benthic community, at least.
To treat immune systems and how environments affect them is a unique challenge especially when the environment is considered in its broadest perspective: internal and external. Internal focuses on relationships between immune, nervous and endocrine systems (neuroendocrine) and how they interact to maintain homeostasis. External considers physical and chemical influences that act to change the internal. Using animal models is based upon phylogeny which focuses on invertebrates, fish, amphibians, and reptiles, including mammalian results and relationships to humans. Emphasizing primitive animals is due to a growing interest in using them as models, sentinels, surrogates—predictors of what may happen when the environment is disturbed. They are inexpensive, socially acceptable, and since they live in diverse habitats under natural conditions, what may happen to them may be more applicable to humans than to laboratory reared models (sometimes inbred) whose controlled habitats may not be considered as natural. Humans do not live in controlled laboratories but like numerous animals, we do live in various climates, under different conditions of light, temperature, crowding, seasons and in different habitats that determine the quality of life including the susceptibility to disease. The immune system is affected by these influences, and it in turn is responsible for the body's surveillance against these threats.
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.
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.
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.
The sea cucumber Apostichopus japonicus is a flourishing aquaculture species in China. However, there are challenges for sea cucumber aquaculture, one of which is the high temperature in summer. In this study, we explored the transcriptome expression profiles with seasons (APR, JUN and JUL) in the muscle tissue of A. japonicus. The temperature of the natural coast was 13 °C, 21 °C and 25 °C respectively when sampling. Compared with APR group, changes of expression profiles were more significant in JUL group than that in JUN group. A total of 46 differential expressed genes (DEGs) involved in both innate and adaptive immunity were highlighted, including 27 up-regulated and 19 down-regulated genes. They were further grouped into 10 sub-classes: heat shock, coagulation cascades, antigen processing and presentation, inflammatory response, transporter activity, immunoglobulin, lectin C, cell adhesion, reactive oxygen species (ROS) scavenging, apoptosis and autophagy. The study will offer deep insights of the molecular mechanisms underlying the physiological responses to seasonal high temperature in A. japonicus. Particularly, knowledge about the immunological effects of seasonal temperature on the species is critical for the optimal management practices for both wild and aquaculture populations.
Virus-associated mass mortalities among several marine mammal populations inhabiting industrialized coastal areas have generated an interest in wildlife immunotoxicology. Despite the isolation of previously uncharacterized viruses from victims, a contribution of immunotoxic contaminants to the severity of the outbreaks could not be ruled out. Fish-eating marine mammals, including seals, occupy high trophic levels in the aquatic food chain, and accumulate high levels of contaminants including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychorinated dibenzofurans (PCDFs). Such chemicals have been found to be immunotoxic at low doses in studies of laboratory animals. While associations have been established between environmental contaminants and various adverse biological effects in certain free-ranging seal populations, evidence for immunotoxicity has, until recently, been lacking. To this end, we carried out an immunotoxicological study, in which captive harbor seals were fed herring from either relatively uncontaminated sites of the Atlantic Ocean, or from the highly contaminated Baltic Sea. In this review, we summarize the contaminant-related immunosuppression observed in the captive group of seals fed herring from the Baltic Sea, and discuss these results in the context of what is currently known about outbreaks of virus infection, comparative immunology, and environmental contaminants. We also describe two parallel studies, in which laboratory rats exposed as adults or perinatally to the contaminants in the Baltic Sea herring, exhibited immunotoxicity. On the basis of these and other studies, we conclude that complex mixtures of environmental contaminants may represent a real immunotoxic risk to free-ranging marine mammals in many areas of Europe and North America.
The reason for analyzing tunicate recognition systems is two-fold. First, they can be established as primitive models for understanding fundamental immunological mechanisms by analyzing either their individual cells in vivo or in vitro. Discovered mechanisms could provide alternatives to traditional mammalian (mouse, rat) and emerging models (fish, amphibians) in answering basic questions concerning immunity and disease. Moreover, their advantages lie in: (a) the simplicity of primitive systems (minimal hemopoietic sites), allowing a more effective dissection of variables; (b) their limited expense when compared to experimentation using vertebrates; and (c) their being socially non-controversial. Second, techniques of cell and molecular biology are equally applicable to tunicates as invertebrate models. Despite the considerable advances of recent years, many fundamental or conceptual aspects of immunological reactivity remain unresolved. A clear understanding of the system's evolutionary past will help to elucidate mechanisms which are complex and difficult to decipher in mammals, as we postulate, the immunorecognition systems of vertebrates evolved from simpler systems. Then tunicate immunodefense responses are excellent examples for analyses.
Ciliates are highly evolved protists comprising a phylum of diverse species, many of which are opportunistic or obligate parasites. Ciliates parasitic to fish consist of salt and freshwater forms with endo- or ectoparasitic modes of infection. Some of the more commonly encountered genera include Chilodonella, Brooklynella, Ophryoglenina, Ichthyophthirius, Cryptocaryon, Uronema, Tetrahymena, Epistylus, and Trichodina. Species range from obligate parasites and commensals to opportunistic, facultative forms. Some parasitic ciliates are highly pathogenic and fishes in closed environments such as aquaria and farm ponds are particularly susceptible to high mortalities. Nevertheless, fish have evolved an immune system capable of mounting an effective protective response against parasite challenge. Much of the experimental research on immunity against ciliates has been carried out with Ichthyophthirius multifiliis, on obligate parasite that invades surface epithelia of virtually all freshwater fish species. Interest in the immune response against I. multifiliis stems from the fact that convalescent fish become resistant to subsequent challenge (suggesting the possibility of immunoprophylaxis), and the need to curtail severe losses caused by this parasite in intensively farmed fishes. Furthermore, I. multifiliis has proven to be a useful experimental model because it is amenable to study under laboratory conditions. In this review cellular and humoral factors involved in both innate and acquired immunity against ciliates are covered and include natural killer cells, phagocytic cells, and antibody responses. Current ideas on the mechanisms of antibody-mediated cutaneous immunity against I. multifiliis are discussed and approaches toward the development of vaccines against this and other ciliate parasites are presented.
IPNV is a medium-sized, unenveloped bisegmented dsRNA-containing virus in the family Birnaviridae. Genome segment A (3097 bp) contains two overlapping open-reading frames (ORFs). A large ORF encodes a 106 kDa polyprotein (NH2-pVP2-NS protease-VP3-COOH) which is cotranslationally cleaved by the protease to generate the major capsid proteins VP2 and VP3, and a second, small ORF which overlaps the amino end of the large ORF but in a different reading frame, and encodes a 17 kDa arginine-rich minor polypeptide. Genome segment B (2784 bp) encodes a minor internal capsid polypeptide VP1 (94 kDa), which based on its size, low copy number and the presence of several conserved domains associated with RNA-dependent RNA polymerases (RdRp) of other RNA viruses, is the putative virion-associated RdRp. VP1 is present in the virion in two forms: as a free polypeptide and as a genome-linked protein (VPg) covalently attached to the 5′ ends of both genome segments. During in vitro RNA transcription, VP1 serves as a primer and remains attached to the 5′ end of the RNA thereby becoming a VPg. Transcription follows a semi-conservative, strand-displacement mechanism. In infected cells two genome-length 24S viral mRNAs lacking 3′ poly A tracts are synthesized that can hybridize to the two denatured genome segments. In vivo protein synthesis involves both polyprotein processing and internal initiation of translation at some of the in-phase methionine codons. The virus-coded protease functions only in cis and its insensitivity to a number of proteinase inhibitors suggests that it may be a novel viral protease. The putative cleavage sites on the polyprotein have been mapped to within a few amino acids but the exact boundary between pVP2NS and NSVP3 has not been established. A universal, group-specific epitope has been mapped to near the amino terminus of VP2, whereas a serotype-specific epitope was found to be located in the middle of the polypeptide.
Fish mycobacteriosis is a chronic bacterial disease that has the potential for infecting most fish species from both freshwater and saltwater habitats. The causative agents of fish mycobacteriosis are three species of Mycobacterium: M. chelonae, M. fortuitum, and M. marinum. Infections of fish by these bacteria result in a slow, degenerative disease with symptoms that include wasting, fading of color, exophthalmia, and frequently, granulomas and other types of skin ulcers. The disease is transmissible from fish to fish and also from fish to man, where the bacteria produce a potentially serious medical condition. Little is known about the molecular biological aspects of the disease or the host-pathogen interactions. Using the knowledge gained from recent advances in the study of M. tuberculosis and other human mycobacterial pathogens, we suggest a set of strategies to begin a molecular biological study of fish mycobacteriosis. Although no conclusions may be drawn at this early stage in understanding this disease, the prospects are very good that the techniques of molecular biology will shed light on this longstanding and misunderstood disease.
The aquatic birnavirus IPNV is commonly found in association with apparently healthy, mature salmonids. Some birnaviruses cause lethal diseases in fry; however, many of those discovered in fish may not be pathogenic for the species from which they were isolated. More pathogenic virus may be produced from the acinar cells of the pancreas; less pathogenic virus by skin and gut cells. Skin infection could explain the lack of virus clearance after development of circulating antibody; the less pathogenic form of the virus may not induce protective antibodies. True vertical transmission to progeny fish would seem not to occur, but virus may adhere to egg cases and spread to fry which ingest them at the time of first feeding. When viruses are found with moribund mature fish, alternative causes of death must be considered. Atlantic salmon Salmo salar smolts, which have high levels of virus during pre-smolting, can develop pancreatic lesions (possibly mediated by the immune system) on transfer to salt water. IPNV infection may be linked to immunosuppression; possible controlling genes have been found.
Computer software has become an important tool for fish health managers, diagnosticians, veterinarians, researchers and educators. This article reviews selected software applications which are either currently available, under development, or which may serve as inspiration for future development of appropriate applications in the fields of fish health. Computer-based multimedia is exemplified using the interactive fish anatomy and necropsy program FishGuts. The utility of other applications including programs for fish health (Aqua Medic, Aqua-Vet, AquaPath), data archiving ("Animal Care System" by Relevant using 4th Dimension, Nutshell Plus II, Filemaker Pro), and general resources and entertainment (LIFEmap, OceanLife, World of Sharks, Fishes of the Red Sea, Fishes of the Caribbean, Oceans Below, Aquazone, Undersea Adventure, Sakana Hakkei) are also discussed. Development costs, effort, and the compartmentalized expertise of educators/scientists and programmers tend to hinder the production of quality software products. Collaboration and networking are necessary and fundamental to the expansion of utilitarian computer-based applications in aquatic animal health.
The fish pathogen, Aeromonas salmonicida, has been the focus of a number of molecular genetic studies designed to characterize the microorganism and its pathogenesis. The paracrystalline surface protein layer (A-layer) of A. salmonicida has been studied in considerable detail. The A-layer gene, vapA, has been cloned and sequenced and studies have been performed on its regulation. The secretion pathway specific for the A-layer subunits has also been partially characterized as has the general protein secretion pathway. Other genes involved in the biogenesis of the A. salmonicida surface include abcA, asoA and asoB. The abcA gene encodes a protein which is involved in lipopolysacharide O-chain synthesis and secretion and may have a role in the regulation of vapA gene expression. A. salmonicida also possesses plasmids of various sizes which exhibit a high degree of conservation and can encode antibiotic resistance elements. Insertion sequence elements have been identified in two strains of A. salmonicida and are capable of transposing within a cell to cause mutations that affect virulence. Molecular biology techniques have also been applied to the problem of detection of low levels of A. salmonicida in natural environments and carrier fish. The development of tools such as specific DNA probes and PCR primer pairs allows the detection of extremely low numbers of A. salmonicida even in the presence of high numbers of other bacteria. The development of vaccines against A. salmonicida has incorporated some new techniques such as the generation of specific mutations in the chromosome or the production of large quantities of particular proteins, such as the outer membrane porins, in expression systems. Another approach involves the use of an avirulent A. salmonicida strain as a shuttle system to express fragments of genes from viral pathogens with a view to providing protective immunity against multiple diseases with a single vaccine.
