by Chan Lai-ping. / Thesis (M. Phil.)--Chinese University of Hong Kong, 1981. / Bibliography: leaves 203-237.

No description available.

Disease development of wild-type and Rag1 mutant zebrafish was evaluated after challenge with Snakehead Rhabdovirus (SHRV), a novirhabdovirus. Rag1 mutants lack T and B lymphocytes and thus lack lymphocyte-based acquired immunity. Wild-type zebrafish became more disease resistant as they aged (4 months and older) and at an elevated temperature (28°C) but mutants remained sensitive at all ages and temperatures tested. Quantitative reverse transcription polymerase chain reaction (RT qPCR) demonstrated that interferon gamma and MxA expression significantly increased in both types of fish at 2 days post-infection with subsequent dwindling of expression. The high interferon gamma expression suggests activation of natural killer cells (and/or T lymphocytes in wild-type fish), and the up-regulation of MxA expression indicated an activation of type 1 interferon response. The development of protection in virus exposed fish was evaluated by lethal challenge at 3 weeks post vaccination. Vaccinated wild-type fish showed significant protection and while most of the mutant groups showed no protection. One vaccination treatment group of the mutants demonstrated a significantly slower mortality and less overall mortality. The results suggest that lymphocyte based immunity imparts a robust protective response to SHRV while low-level protection can develop in the absence of lymphocytes. A cell mediated cytotoxicity assay was established. Cell lines were developed from the inbred fish populations and class I MHC U lineage genes were compared. The mhc1uba and mhc1uca genes were found in the mutant and cells but no class I MHC U lineage genes were detected in the wild-type fish or cells. These cells were used as targets in cytotoxicity assays. Kidney-marrow cells of vaccinated mutant or wild-type zebrafish killed more SHRV infected target cells than did those from non-vaccinated fish with the wild-type effectors showing higher cytotoxicity. The lymphocyte component appears responsible for the temperature and age associated resistance. This helps explain why novirhabdoviruses cause higher losses in young fish and at low temperatures. Further studies are needed to understand the relative contribution of the cellular components that play important role in SHRV resistance, but the establishment of cytotoxicity assays is an important first step in dissecting the cellular defenses in zebrafish.

Marine and freshwater aquatic invasive species are a huge problem in the United States and its territorial waters with myriad ecological and economic impacts. Current management methods have fallen short. One possible solution is to harvest them for human consumption. This should be done through small scale, carefully regulated, scientifically based programs that are part of a larger overall management strategy. Four case studies (northern snakehead, European green crab, Asian carp, Chinese mitten crab) assessing edibility are included.

Aphanomyces invadans is the pathogen that causes epizootic ulcerative syndrome (EUS), an economically devastating fish disease in southern Asia. The present thesis considered possible improvements to current methods of monitoring EUS, and examined the mechanisms of the host immune response to A. invadans in order to establish whether they could be enhanced to reduce the impact of EUS on aquaculture. Monoclonal antibody (MAb) technology was considered as a possible improvement to the histopathological methods currently used in diagnosis of EUS. Five MAbs were raised to day-old A. invadans germlings. Four gave weak reactions to A. invadans and cross-reacted with other Aphanomyces spp, though they may be useful for future studies on A. invadans. The other, designated MAb 3gJC9, only cross-reacted with the crayfish plague pathogen, A. astaci, and was used for the development of an immunohistochemistry protocol that may be of use in diagnosis. Immunohistochemistry with MAb 3gJC9, which recognised an extracellular product (ECP) of A. invadans, was specific to A. invadans in fish tissue, although it also recognised A. astaci in plague-infected crayfish. It also recognised the mycelium in fish infected with ulcerative mycosis, indicating that ulcerative mycosis is synonymous with EUS. Preliminary observations indicated that both ECPs and what appeared to be a hitherto unreported early stage of the mycelium are important in the pathology of EUS. Studies in vitro on the macrophages of EUS-susceptible giant gourami Osphronemus gouramy and silver barb Barbodes gonionotus, and EUS-resistant Nile tilapia Oreochromis niloticus, found that their macrophages were able to inhibit the growth of A. invadans. The macrophages of striped snakehead Channa striata did not inhibit A. invadans, which may account for their high EUS-susceptibility, especially as A. invadans strongly inhibited the respiratory burst of snakehead macrophages. Studies on humoral immune responses revealed that complement inhibited A. invadans in the case of snakeheads, gourami and barbs but not tilapia or swamp eels Monopterus albus. The humoral responses of the latter were very different to the four other species, and not elucidated. Low levels of anti A. invadans antibodies were found in tilapia and gourami from an EUS-endemic region, and high levels in snakehead. Snakehead antibodies appeared to be able to inhibit A. invadans even when complement was removed, but lower levels were produced at the low temperatures typically associated with EUS. A range of potential immunostimulants were screened for the ability to enhance resistance to EUS. The two successful products were administered as feed supplements to snakeheads and barbs that were subsequently injected intramuscularly with A. invadans. One, the algal extract Ergosan, showed some beneficial effects on snakeheads although the challenge was inconclusive. The other, the vitamin supplement Salar-bec, accelerated the cellular immune response and reduced mortality in snakeheads and barbs, and enhanced antibody production in snakeheads. The antibody response of snakeheads was further studied by comparing the anti- A. invadans antibody level, inhibitory activity of sera in vitro and protective capacity of sera from EUS-naïve snakeheads to that of snakeheads recently exposed to EUS and those subject to long term EUS-exposure. Sera of populations recently exposed to EUS showed an increased level of antibodies, but little improvement in inhibitory or protective activity. Sera from snakeheads that had endured long term exposure showed a wide range of antibody levels, but marked increases in inhibitory and protective activity. Antibodies cross-reacted with non-pathogenic Aphanomyces spp. in all cases.