MANAGING CATFISH HEALTH IN AQUACULTURE

<b>Project Methods</b><br> All trials will be conducted with established protocols using sound experimental design subject to parametric statistical analysis. Objective 1: Identify emergent pathogens in catfish aquaculture and develop disease diagnostic methodologies.1.1 Diagnostic cases submitted through the Aquatic Research and Diagnostic Laboratory (MSU-CVM, Stoneville, MS) will be used to track changes in disease trends, identify emergent pathogens, and assess antibiotic resistance patterns. Potential emergent pathogens will be genetically characterized and documented as pathogens of catfish through fulfillment of Koch's or River's postulates. Assembled genomes of newly identified pathogens will be mined for discriminatory gene targets that can be exploited by quantitative PCR and provide a rapid confirmatory diagnostic test in tissue and environmental samples.1.2. Recently Yersinia ruckeri, isolated from diseased hybrid catfish, and a non-toxigenic strain of vibrio cholera, isolated from hatchery fry have been identified as potential emergent pathogens. Hybrid and channel catfish will be exposed by IP injection to archived cultures of Y. ruckeri and V. cholera. Moribund fish will be subjected to diagnostic evaluation and recovered bacteria will be PCR confirmed to fulfill Koch's postulates. In addition, two channel catfish variants have been identified along with a blue catfish herpesvirus isolated from a diseased blue catfish. Virus will be propagated in tissue culture and used to infect fish. Virus will be re-isolated in tissue culture and confirmed by PCR sequencing to fulfill River's postulates.Objective 2: Optimize treatments and management strategies to minimize diseases in catfish aquaculture.2.1.A. An experimental live attenuated E. ictaluri vaccine and method of oral delivery has been developed an extensively field tested. Since the vaccine is delivered orally and daily feeding activity can vary, reapplication may be necessary to ensure adequate vaccine coverage. Vaccination trials will be conducted to determine vaccine safety in fish receiving multiple vaccine dosages. Fish will be fed the vaccine feed mixture once or on consecutive days and challenge with a wild type isolate to assess vaccine efficacy. Vaccine safety will be determine by the occurrence of post vaccination mortality. If adverse vaccine reactions to over dosing does not occur, the study will be repeated in experimental ponds.2.1.B. Preliminary work indicates the experimental ESC vaccine cross protects against a closely related bacterial pathogen, E. piscicida. However, recently at least 5 genetic E. piscicida variants have been identified which could alter the cross-protective potential of the vaccine. Fish will be vaccinated by oral delivery and later challenged with wild-type E. ictaluri and E. piscicida. Cross reactive serology and protection against infection will be used to assess the cross-protection potential of the E. ictaluri vaccine against identified E. piscicida variants.2.2.A. The myxozoan parasite H. ictaluri is the causative agent of proliferative gill disease in channel catfish. Recent work has demonstrated hybrid catfish suffer from the acute stages of infection but serve as a dead end host for the parasite, preventing further propogation of the parasite within the pond environment. However, in some instances, PGD is developing in hybrid catfish production systems. This suggests that there are other myxozoan species causing PGD or the presence of other disease vectors. Myxozoan actinospores will be collected from D. digitata isolated from benthic sediments of ponds with active PGD and used in infectivity challenges with channel and hybrid catfish. Fish will be sampled for identification of pre-sporogonic stages characteristic of H. ictaluri induced PGD and for the presence of mature myxospores. Mature myxospores will be sequenced to identify the presense non-H. ictaluri life stages causing PGD.2.2.B. Work done with other myxozoan species suggests there is a intraspecific variability of the oligochaete host with some lineages being refractory to myxozoan infections. Oligochaetes will be collected from channel and hybrid catfish ponds with and without active PGD outbreaks in the spring of the year during periods of peak PGD incidence. Myxozoan positive and myxozoan negative oligochaetes will be characterized by 16S mtRNA sequencing to determine relationships, if any, to 16S mtRNA sequence types and myxozoan actinsopore release in Dero digitata.Objective 3: Epidemiology of infectious diseases in catfish aquaculture and economic evaluations of disease management. 3.1. Spatial and temporal analysis of aAh isolates from catfish producing states in the southeastern United States will be evaluated for genetic variation. This work will genotype historical archived aAh isolates using previously established PCR protocols to identify geographical or temporal relationships among the two variants. The ARDL repository contains over 200 archived catfish isolates from the southeastern U.S., as well as non-catfish isolates from other states. In addition, new isolates will be added to the data set as they become available. Currently two distinct aAh variants have been identified in commercially raised hybrid and channel catfish (Rasmussen-Ivey et al. 2016), one of which is prominent in Mississippi (MS-aAh), while the other appears to be endemic to west Alabama (AL-aAh) (Griffin, unpublished data). Challenge trials will be conducted to determine pathological and serological differences between genetic variants in channel and hybrid catfish. Vaccination trials using an experimental live attenuated vaccine developed by Auburn University will be evaluated against each genetic variant. 3.2. Environmental and community data will be used to modeling programs to identify risk factors associated with aAh infections in catfish aquaculture. Pond models will be created using a combination of R Studio v 0.99.903 (R Development Core Team) and STELLA 10.0.2 (ISEE Systems). Logistic regression will be used to assess associations between dichotomous outcomes such as vAh outbreak occurrence. Generalized linear mixed models (GLMMs) will be fitted, using random effects to account for dependence of multiple measurements from the same pond and variability among farms.Graphical modeling will occur using STELLA modeling software. STELLA allows for visual modeling of systems through the use of flows and stock variables. Environmental and community data will be coupled with production data (i.e. feeding rates, stocking densities, etc.) collected from the literature and participating farms. Positive vAh outbreak scenarios will also undergo a sensitivity analysis to evaluate the importance of each predictive variable.3.3. Economic evaluation of a live attenuated E. ictaluri vaccine will be conducted on commercial operations involving participating farms and over 350 million fish. A partial-budget analysis that quantifies the economic cost and benefits of vaccinating channel and hybrid catfish on commercial fingerlings operations will be conducted to evaluate the net economic benefit of ESC vaccination on catfish fingerling operations. Whole-farm mathematical programing will be employed to estimate the economic effects of vaccination on catfish operations that integrates hatchery, fingerling, and foodfish operations. The models will be developed to capture the resource-use efficiency arising from vaccination improvements in fingerling-production phase. Profit-maximizing linear-programming models will be developed to obtain the optimal allocation of land, fingerlings, feed, capital etc.