Location: Warmwater Aquaculture Research Unit
2018 Annual Report
Objectives
1) Investigate new and emerging diseases, and develop rapid diagnostic procedures for important pathogens of channel, hybrids and blue catfish.
Subobjective 1.1 Use diagnostic case submissions to identify changes in disease prevalence and emergence of new diseases.
Subobjective 1.2 Develop molecular diagnostic tests for simultaneous detection of multiple catfish pathogens.
This objective is non-hypothesis driven and serves as an essential component of the disease surveillance programs and helps define clinical and applied research programs that address critical fish health issues facing the catfish industry.
This research will aim to determine whether molecular markers can be used to identify multiple pathogens in a single PCR reaction.
2) Study the pathology and epidemiology of significant pathogens affecting cultured catfish.
Subobjective 2.1 Evaluate the susceptibility and pathology of catfish to Edwardsiella spp.
Subobjective 2.2 Evaluate the pathology of Drepanocephalus spathans in channel catfish.
This research will determine whether blue, channel, and blue x channel catfish hybrids have varying degrees of susceptibility to Edwardsiella tarda, Edwardsiella piscicida, and Edwardsiella piscicida–like spp and determine whether the metacercariae stage of the digenetic trematode Drepanocephalus spathans is short lived (< 6 months) in channel catfish.
3) Investigate the responses of channel, hybrid and blue catfish to therapeutic agents and oral vaccines for important bacterial pathogens.
Subobjective 3.1 Evaluate susceptibility of archived and clinical bacterial isolates to antimicrobials, and correlate antimicrobial susceptibility with treatment efficacy.
Sub-objective 3.2 Determine the optimal age for vaccinating catfish against ESC using an oral attenuated E. ictaluri vaccine.
Subobjective 3.3 Determine if certain stressors will affect vaccine efficacy and safety of catfish orally vaccinated with an attenuated ESC vaccine.
Subobjective 3.4 Evaluate mortality resulting from exposure of vaccinated fish to archived and year-end E. ictaluri isolates.
Data will establish uniform standardized interpretive criteria that can be used by all aquatic laboratories that are involved with clinical disease diagnosis of catfish. The research will also examine whether antimicrobial susceptibilities determined by minimum inhibitory concentration and zone diameters of inhibition will affect the treatment efficacy of medicated feeds.
4) Develop and implement practical management strategies to improve fish health and production efficiency.
Subobjective 4.1 Evaluate effects of hybrid catfish production on incidence and prevalence of Henneguya ictaluri in catfish ponds.
Subobjective 4.2 Field evaluation of an oral ESC vaccination platform for control of E. ictaluri infections in catfish.
This research will determine if the production of hybrid catfish reduces the incidence and severity of proliferative gill disease, caused by the myxozoan parasite H. ictaluri, in catfish ponds and whether oral vaccination will improve survival and production efficiency of channel catfish raised under commercial conditions.
Approach
Diagnostic records provide critical insight to changes in disease trends and emergence of new diseases affecting animal production systems, an essential component of population health management. This information provides clinicians, caregivers and researchers a good cross-section of disease occurrence across the industry. This information can then be used to prioritize the allocation of resources in the development of rapid diagnostic procedures, disease surveillance and treatment programs and implementation of biosecurity measures to stop the spread of emerging diseases when possible.
Edwardsiella tarda has been identified as an emerging disease as indicated by diagnostic case submissions and interaction with industry advisory groups. Recently, E. tarda has undergone a reclassification, suggesting many organisms previously thought to be E. tarda are actually E. piscicida, a newly described fish pathogen closely related to E. ictaluri. Research will focus on the comparative susceptibility of channel catfish, blue catfish and hybrid catfish to E. piscicida, E. tarda and the as yet classified E. piscicida-like sp. In addition, quantitative PCR assays will be developed to detect and quantify these various pathogens in fish tissues and the pond environment.
Non-hypothesis driven clinical research will be conducted to define minimum inhibitory concentration (MIC) and cut-off values for clinical bacterial pathogens against 3 commercially available antimicrobials. This information is critical to policy review concerning antibiotic approval in the catfish industry. Separately, hypothesis-driven research will be conducted to optimize the effectiveness of oral vaccination and determine environmental factors which may limit efficacy and safety.
Field studies will be conducted to evaluate practical management strategies for controlling two diseases which account for more than half of the diagnostic case submissions to the Aquatic Research and Diagnostic Laboratory (ARDL). Research will focus on development management strategies for controlling proliferative gill disease, caused by the myxozoan parasite Henneguya ictaluri and field evaluation of an oral vaccine for control of enteric septicemia of catfish (ESC) caused by the gram-negative bacteria Edwardsiella ictaluri. These management approaches have the potential to greatly reduce the impact of these diseases, that are estimated to cost the catfish industry $60-80 million annually.
Progress Report
The Mississippi State University, Aquatic Research and Diagnostic Laboratory (ARDL), which provides services to producers as well as fish health and production researchers in southeastern United States received 861 case submissions in 2017. Of these, 831 were submitted by producers and 30 were submitted by ARS researchers at Stoneville, Mississippi, and Mississippi State University researchers. Of the catfish cases, 417 were hybrid catfish, 412 were channel catfish and 4 were blue catfish. This is an indication of the growing adoption of hybrid catfish in the industry. Bacterial diseases continue to be the predominant diagnoses with 392 cases of columnaris disease, 357 cases of enteric septicemia of catfish (ESC) caused by Edwardsiella (E) ictaluri, and 72 cases of Edwardsiella piscicida (formerly E. tarda). Of the 72 cases of E. piscicida, the vast majority were from hybrid catfish. Yersinia ruckeri, diagnosed by the ARDL for the first time in 2016, was seen in 4 more cases in hybrid catfish in 2017, suggesting a potential emergence of this coldwater pathogen in hybrid catfish culture. This information was included in the Delta Research & Extension Center annual report and will be published in the Thad Cochran National Warmwater Aquaculture Center Newsletter (NWAC).
Diagnostic centered research has improved treatment recommendations and identified bacterial and parasitic agents affecting fish health. In vitro studies were conducted to assess the effectiveness of medicated feeds against catfish infected with antibiotic resistant and susceptible E. ictaluri archived isolates. Drug resistant and susceptible field isolates, recovered from disease channel catfish, were obtained from the ARDL (Stoneville, Mississippi). Increasing drug resistance patterns resulted in lower treatment efficacies. Information is being used to better assess treatment recommendations for the control of E. ictaluri infections in catfish aquaculture. The freshwater snail Biomphalaria (B) havanensis, has been identified has a host for two pathogenic digenetic trematodes. Bolbophorus damnificus and Drepanocephalus (D) auritus both cause mortality in juvenile catfish. Also noteworthy, Biomphalaria species have been implicated in the transmission of the human parasitic blood fluke Schistosoma (S) mansoni. Experimental transmission studies suggest the B. havanensis from catfish aquaculture are refractive to infection by S. mansoni. The life cycle of D. auritus has been experimentally completed in the double crested cormorant, Ram’s horn snail and channel catfish, establishing developmental timelines associated with different stages of the parasite life cycle and providing critical baseline information for the development of management strategies to reduce digenetic trematodes in catfish aquaculture. In addition, four previously unknown trematode life cycles associated with piscivorous birds endemic to catfish aquaculture in Mississippi have been identified. The significance to catfish aquaculture is being investigated.
Yersinia ruckeri and Vibrio species (spp.), two previously unreported bacterial pathogens in catfish, have been recovered from recent disease case submissions. Historically, neither of these pathogens have been cause for concern in catfish aquaculture, but with changes in management practices and the increased utility of hybrid catfish, the potential for emergent diseases within the industry is high. The role of Vibrio spp. infections in sudden onset mortality in hatchery catfish is being investigated. The genome of these species is being sequenced and genome sequence data will be used to identify molecular markers that can be exploited by quantitative polymerase chain reaction (qPCR) for rapid diagnostics and environmental surveillance, similar to platforms that have been developed at NWAC for other important catfish pathogens.
In collaboration with Michigan State University, research has been conducted to investigate the stability of the gut microbiome of channel catfish across different ontogenetic stages. Data demonstrated gut microbiomes are strongly influenced by the pond environment. Data is being used to evaluate potential applications of probiotic feed additives and determine if Vibrio spp. Infections, observed in hatchery fish, are related to carrier states in brood fish or a result on environmental exposure after hatching.
The commercial channel catfish (Ictalurus punctatus) industry is adversely impacted by enteric septicemia of catfish (ESC), caused by Edwardsiella ictaluri. An experimental live attenuated oral vaccine and vaccine delivery system has been developed under this project and validated in large scale commercial field trials. Last year, approximately 250 million stocked catfish were vaccinated using the oral vaccination platform. Previous research has proven the vaccine is efficacious in experimental trials against the parental wild type strain used to develop the attenuated vaccine. Research was conducted to determine if the vaccine was protective against field isolates recovered from diseased channel and hybrid catfish in different geographic regions. In all trials, vaccination was shown to protect catfish against all challenge isolates, regardless of host species, geographic region (state and farm location) or isolation year. In order to differentiate between the bacterial isolates, their clonal relation was determined by molecular characterization. The homogeneous nature of E. ictaluri isolates revealed by rep-PCR and virulence gene amplification negate the need to develop multivalent vaccines to account for antigenic or genetic variations occurring over time. Plasmid profiling revealed the heterogeneity of at least four bacterial isolates but were not shown to affect vaccination efficacy.
Economic analysis was performed on data collected from producers participating in field vaccination trials. This five-year (2012-2016) dataset consists of production parameters from 97 vaccinated and 80 non-vaccinated channel catfish ponds and 126 vaccinated and 120 non-vaccinated hybrid catfish ponds. Results from the commercial trials for both channel and hybrid catfish showed that vaccination significantly improved survival, growth, feed consumption, feed conversion, and yield resulting in net economic benefits to fingerling production phase. The net economic benefit at the fingerling production stage for channels and hybrids were $3,868 and $7,063/hectare (ha), respectively. Results from whole farm mathematical programming models showed additional economic benefits in the range of $397 to $473/ha on farms that integrate fingerling production to their foodfish operations. Model suggested that the improved productivity from vaccinating fingerlings could lead to ways of maximizing profits either by intensifying production or by appropriating more of the freed up fingerling ponds to yield maximizing foodfish production strategies.
Accomplishments
1. A novel vaccine against Enteric Septicemia of catfish. Enteric septicemia of catfish (ESC) is the one of the most problematic bacterial diseases affecting the production of channel catfish fingerlings. Mississippi State University scientists at Stoneville, Mississippi, through a cooperative agreement with ARS scientists at Stoneville, Mississippi, have developed an effective vaccine and method of oral delivery against this disease. To date, approximately 500 million stocked catfish have been orally vaccinated using the developed technologies in field trials on commercial farms. The net economic benefit at the fingerling production stage for channels and hybrids were $3,868 and $7,063/hectare (ha), respectively. Results from whole farm mathematical programming models showed additional economic benefits in the range of $397 to $473/ha on farms that integrate fingerling production to their foodfish operations. Model suggested that the improved productivity from vaccinating fingerlings could lead ways of maximizing profits either by intensifying production or by appropriating more of the freed up fingerling ponds to yield maximizing foodfish production strategies.
Review Publications
Reichley, S.R., Ware, C., Steadman, J., Gaunt, P.S., Garcia, J.C., LaFrentz, B.R., Thachil, A., Stine, C.B., Waldbieser, G.C., Arias, C.R., Lock, T., Welch, T.J., Cipriano, R.C., Greenway, T.E., Khoo, L.H., Wise, D.J., Lawrence, M.L., Griffin, M.J. 2017. Comparative phenotypic and genotypic analysis of Edwardsiella spp. isolates from different hosts and geographic origins, with an emphasis on isolates formerly classified as E. tarda and an evaluation of diagnostic methods. Journal of Clinical Microbiology. 55:3466-3491.
Reichley, S.R., Waldbieser, G.C., Soto, E., Lawrence, M.L., Griffin, M.J. 2017. Complete genome sequence of Edwardsiella ictaluri isolate RUSVM-1 recovered from nile tilapia (Oreochromis niloticus) in the Western Hemisphere. Genome Announcements. 5:e00390-17.
