Performing Department
(N/A)
Non Technical Summary
Lactococcosis is an emerging disease of significant concern for American aquaculture. The disease typically presents as a hemorrhagic septicemia with high mortality rates. At present, there are no efficacious vaccines or other prophylactic methods available for this disease, and treatment with antimicrobials often fails to control outbreaks. Additionally, the dynamics of disease transmission and pathogenesis remains unclear. Lactococcus petauri poses a particular problem for control efforts, as it is both highly prevalent in the Americas, and has demonstrated both high virulence and high transmission efficiency in rainbow trout cohabitation studies. Further, L. petauri may persist outside the host in both biofilm and planktonic forms. The contribution of these characteristics to pathogenesis have not been assessed collectively. We aim to use a multi-pronged approach to meet the needs for new tools and models to manage piscine lactoccosis. We will investigate the potential for horizontal transmission of L. petauri under varied aquaculture relevant conditions using in-vivo laboratory-controlled challenge models. Information garnered from these experimental studies, in combination with existing knowledge, will be used to develop an epidemiological model of lactococcal transmission and vaccine efficacy. Together, these aims combine to help identify risk factors associated with the disease and to prevent future loses to the aquaculture industry. The proposed project encompasses two of the Program Area Priorities. Specifically, the project will address: 1) "Critical disease issues impacting commercial aquaculture species", and 2) "Development of climate resilient technologies, production systems, and/or management strategies for commercial aquaculture species".
Animal Health Component
20%
Research Effort Categories
Basic
30%
Applied
20%
Developmental
50%
Goals / Objectives
Objective 1. Evaluate the role of fish stocking density, water-flow, and temperature of water in the transmission of Lactococcus petauri to rainbow trout, channel catfish and Nile tilapia. Objective 2: Develop an agent-based spatial explicit epidemiological model of Lactococcus petauri transmission for cultured rainbow trout using field and laboratory-controlled data to evaluate multiple eco-epidemiological and intervention scenarios.
Project Methods
Methods: Objective 1 will investigate horizontal transmission of Lactococcus petauri through co-habitation models at different environmental conditions. Methods by Madetoja et al. [52] will be used with modifications. Infectivity trials will be conducted with two different sub-types of L. petauri as determined by whole genome sequencing (Figure 1). One of these isolates was recovered from rainbow trout in the outbreaks occurring in California in 2020, and the other from hybrid catfish in Alabama [12,13]. Briefly, different stocking densities of ICe injected "shedder" trout, and naïve "cohabitant" trout (2:8, 3:12, 4:16 shedder:cohabitants, respectively) will be tested, maintaining the same ratio of infection (0.20). Fish will be housed in 35 gallon flow-through tanks at two different temperatures (13 or 18°C) and three different water-flow rates (turnover rate of half an hour, one hour, or two hours). In total, 18 different infected treatments will be evaluated per isolate. Similarly treated sham-infected negative controls will be utilized at each condition. Each treatment will consist of 5 replicate tanks. Morbidity and mortality will be evaluated daily for 42 d post-challenge. At days 1, 7, 14 and 21, 28, 35, and 42 bacterial load in tank water will be evaluated by qPCR [15]. At the end of the study period, 10 cohabitant fish and 5 "shedder" fish will be arbitrarily selected per treatment and subjected to complete necropsy. Total and culturable bacterial loads in the posterior kidney and brain will be assessed by qPCR and plating in Tryptic Soy Agar supplemented with 5% sheep blood (SBA), respectively. Mortalities will only be removed once per day. To assess the level of bacterial shedding by mortalities, a maximum of 10 "fresh-dead" fish per treatment will be placed in 1L of freshwater at their respective temperature (13 or 18°C) and shed bacterial loads quantified using qPCR [15].Corresponding studies will be conducted for Nile tilapia at UC Davis, as well as channel and hybrid catfish at Mississippi State University. Temperature conditions will be adjusted to ~25 and ~30°C to account for the range in which these fish species are typically cultured and where outbreaks of lactococcosis in them have been reported [12,13].Methods: 2.1. Pre-Processing Data: Analyze and cleanse the provided field and laboratory-controlled data for utilization in the model. 2.2. Model Construction and Calibration: Develop an agent-based spatial-explicit model that not only represents individual trout as agents and uses interactions to depict L. petauri transmission dynamics, but also captures the complexity of the environment and diversity of trout production systems. The model will simulate different scenarios of production processes across different systems, including varying farming practices, environmental conditions, and population densities. This comprehensive approach will ensure that the model encapsulates the heterogeneity seen in real-world trout farms, thus enabling it to offer valuable insights regardless of the specific farming system in question. Care will be taken to incorporate all known factors influencing the spread of L. petauri provided in Objectives 1 such as water temperature and population density, among other environmental characteristics and potential stressors (i.e., water characteristics and quality, handling, etc.). This nuanced representation of the trout farming industry will allow for a more accurate and applicable epidemiological model. The model will integrate the effects of various disease control methods. It will model the administration of orally and immersion-administered live-attenuated vaccines in combination with different vaccination strategies, including their timing, frequency, and coverage, to understand how these factors might impact the spread of L. petauri. Lastly, the model will simulate the effects of farm-specific practices, such as management and biosecurity measures on L. petauri transmission which will provide valuable insights about the cost-effectiveness of implementing these practices in various scenarios. 2.3. Model Validation: we will validate the model against historically observed real outbreaks, quantifying its predictive accuracy. 2.4. Development of a User-friendly Interface: we will integrate the validated model into an online user-friendly tool (using R and shiny) that aims to assist aquaculturists in managing L. petauri risks. Expected outcomes: We expect to provide a comprehensive agent-based epidemiological model of L. petauri transmission among cultured rainbow trout. By simulating different vaccination strategies and farm-specific practices, the model will provide critical insights into effective measures to control L. petauri transmission and reduce piscine lactococcosis outbreaks. The agent-based model will provide a deeper understanding of the key factors influencing L. petauri transmission across a diverse range of farming systems and conditions. Additionally, the model will be integrated into a user-friendly tool to evaluate the cost-effectiveness of different production systems, varying farming practices, environmental conditions, and disease control strategies. This user-friendly interface will allow aquaculturists to input their specific parameters to simulate various environmental scenarios, and devise risk-based and more effective farm-specific strategies for disease control. Both the model and the user-friendly tool will be open-access and will be easy to update and expand to changing data or epidemiological scenarios (e.g., climate change) and adapt to other fish pathogens.