Harnessing Evolutionary Ecology to Manage Aquatic Infectious Diseases

Andrew R. Wargo

Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA  23062, USA

Our research program utilizes the principles of ecology and evolution to manage infectious diseases in aquatic environments.  Much of this work has focused on infectious hematopoietic necrosis virus (IHNV), which causes devastating losses to global salmonid conservation, fisheries, and aquaculture.  We have demonstrated that virulence confers a fitness advantage to IHNV, which has historically let to the evolution of increased virulence in the field.  We have explored how practices such as vaccination, selective breeding, and culling might be used to manage virulence evolution in this system.  We have also investigated how environmental factors, such as temperature, coinfection, and microplastic pollution, may exacerbate virulence and affect management efficacy.  Our work has also delved into other systems, such as emerging pathogens in American eels in the Chesapeake Bay.  We employ an interdisciplinary approach that combines large-scale in vivo experiments, field studies, and mathematical models. This talk will provide a brief overview of our innovative aquatic health research program at the Virginia Institute of Marine Science.  

*Due to technical difficulty, the first ~1min of the presentation was not recorded. Sorry!

50 Years of Virulence and Shedding Evolution of an Emergent Virus in Salmonid Hosts

1Malina M. Loeher, 2Gael Kurath, 2William N. Batts; 1Hannah N. Brown, 2Joanne E. Salzer, 3David A. Kennedy, 4Rachel B. Breyta, 1Andrew R. Wargo

1 Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA 23062, USA; 2 U.S. Geological Survey, Western Fisheries Research Center, Seattle, WA 98115, USA; 3 The Pennsylvania State University, University Park, PA 16802, USA; 4 University of Washington, Seattle, WA 98195, USA

A fundamental question in evolutionary ecology is how pathogen virulence will evolve after emerging in a new host species. Infectious hematopoietic necrosis virus (IHNV) in salmonid fish provides a unique set of resources to investigate viral evolution post host-jump, in the novel environmental conditions of intensive aquaculture. After theorized millennia of co-evolution with its ancestral host species of sockeye salmon (Oncorhynchus nerka) in its endemic range of the Pacific Northwest region of North America, IHNV rapidly diversified over the past 50 years since its emergence in a novel host, rainbow trout (O. mykiss). The evolution and geographic spread of IHNV is associated with the intensification of trout aquaculture across the northern hemisphere. Effective disease mitigation in aquaculture and conservation hinges on understanding whether viral traits such as virulence and transmission are linked, the strength of the relationship, and how IHNV may evolve in the future. Building on our previous work which investigated virulence before and after the host jump, we conducted a matching set of in vivo assays to examine how viral shedding kinetics have evolved alongside virulence. Using an array of 16 archived IHNV isolated selected to represent evolution through both time and genetic space, our previous work documented the continued evolution of increased virulence among emergent isolates in the novel host. In this set of studies, we quantified the shedding profiles of 15 of these isolates over four weeks post-exposure in rainbow trout and peak shedding for 25 additional isolates to investigate differences among genetic subgroups. In all novel isolates, shedding peaked within three days post-exposure and diminished within seven days. Viral shedding success also appears to be associated with virulence. Together these data describe the phenotypic diversification of IHNV from the 1970s to the present and give insight into mechanisms of virulence evolution.

Implementation of Digital PCR to Quantify a Novel Viral Shedding Dose Response (SD50)

Hannah N. Brown1, Malina M. Loeher2, Isabelle A. Danforth3, Andrew R. Wargo3

1North Carolina State University, Raleigh, North Carolina  27695; 2U.S. Geological Survey, Western Fisheries Research Center, Seattle, WA 98115, USA; 3Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA  23062, USA

We previously demonstrated that infectious hematopoietic necrosis virus (IHNV) has continued to evolve increased virulence since it made a host jump from sockeye salmon (Oncorhynchus nerka) into rainbow trout (O. mykiss) in the 1970s.  Our data indicates that an increase in viral shedding was the primary driver behind this virulence evolution.  To assess this further, we quantified the viral dosage needed to result in infection and shedding in 50% of fish, which we have termed “SD50”, akin to an ID50 value.  Rainbow trout were exposed to one of seven viral isolates, spanning the time of the host jump to the present day.  For each viral isolate, 4 exposure dosages were used (2 x 102,  2 x 103, 2 x 104, 2 x 105 pfu/ml).  The fish (20 per treatment group), were then isolated into individual tanks, and water samples were collected 3 days post infection to capture the peak of viral shedding.  We developed a novel digital PCR assay using the QIAcuity system to quantify viral loads in the water samples.  We showed that the SD50 value has continued to evolve downwards since the IHNV host jump event, indicating increased infectivity of the virus.  This provides further support that viral transmission was a primary driver of IHNV virulence evolution in the field. 

The Impact of an IHNV DNA vaccine on Transmission and Virulence Evolution in Rainbow Trout (Oncorhynchus mykiss)

1Isabelle A. Danforth, 1Megan M. Tomamichel, 2Angelica R. Ramos, 1Andrew R. Wargo

1Virginia Institute of Marine Science (VIMS), 1370 Greate Rd, Gloucester Point, Virginia 23062; 2Universidad Ana G. Ménendez, C278+9FH, PR-190, Carolina, Puerto Rico 00983

The economic burden of infectious diseases has increased with the industrialization of the aquaculture industry. Vaccines offer a viable solution, facilitating effective disease management and improving aquaculture productivity. In salmonid aquaculture, a commercially licensed DNA vaccine against infectious hematopoietic necrosis virus (IHNV) provides high levels of disease protection, against this economically devastating pathogen. However, recent studies indicate that the IHNV DNA vaccine may not block viral transmission. Consequently, vaccinated fish may act as undetected viral reservoirs and allow for viral evolution to occur. In agricultural systems, vaccines which reduce host mortality but do not prevent transmission (often termed imperfect vaccines), are theorized to allow for the continued circulation and proliferation of highly virulent (lethal) viral strains. Evolutionary theory predicts that imperfect vaccines prolong the infectious period of infected hosts by preventing mortality without concurrently reducing transmission. As such, hypervirulent viruses are expected to outcompete less virulent viral strains in vaccinated host populations due to the fitness benefits provided by virulence. While field and experimental evidence in Marek’s disease virus (MDV) of poultry provides support for these predictions, the consequences of imperfect vaccine use in aquaculture settings remains unclear. Previously, we quantified the fitness of 6 IHNV isolates ranging in virulence in vaccinated or unvaccinated rainbow trout (Oncorhynchus mykiss). Preliminary results suggested that while vaccination significantly reduced mortality across all isolates, it failed to block transmission and had variable effects on recovery rate. Notably, the most virulent isolate was fittest in both vaccinated and unvaccinated hosts, suggesting that the current vaccines allow for lethal viral strains to persist. Therefore, we sought to determine whether viral shedding could be more effectively reduced by increasing vaccine dose. Rainbow trout were vaccinated intramuscularly with10ng, 50ng, 100ng, 250ng, and 1000ng of the IHNV DNA vaccine diluted in phosphate buffered saline (PBS) and exposed to 1 of 2 IHNV isolates. Another group of fish were similarly sham-vaccinated with PBS alone. Surprisingly, viral shedding intensity was equivalent across all vaccinated fish, indicating no suppression in viral shedding with increasing vaccine dose. Additional studies are required to pair shedding protection efficacy and mortality data. Ultimately, our findings suggest that the IHNV DNA vaccine is incapable of curtailing viral circulation even at high vaccine doses. These findings highlight the importance of developing transmission blocking IHNV vaccines to ensure the sustainability and resilience of the salmonid aquaculture industry.