Virus discovery and ecology in common carp
Common carp (Cyprinus carpio) are one of the world's most ecologically damaging aquatic invasive species - and a huge problem in Minnesota. While many tools exist to manage populations, most have harmful non-target impacts, are cost-prohibitive, or the effectiveness is limited to narrow situations. It may be possible that a species-specific pathogen could be used as a part of integrated pest management strategies to effectively and efficiently reduce population sizes. The primary goal of this project is to identify and evaluate pathogens that can be safely and effectively used to control common carp.
More information about common carp can be found here and information about pathogen biocontrol research at MAISRC here.
Identifying carp viruses:
Our approach has been to identify local pathogens (primarily viruses) that cause disease of the invasive common carp but not to native fishes. Over several field seasons, we have collected samples from populations of apparently healthy carp, populations of carp undergoing mortality events, and populations of native fishes undergoing mortality events. We then tested those samples for known and novel viruses using molecular tools.
This first phase of the project identified many novel viruses of healthy and moribund carp, providing information on the disease causing potential of endemic viruses. In addition, we identified many undescribed viruses in the fish kills of native species, with remarkably little overlap with the viruses of carp. Not only useful for consideration of biocontrol agents, understanding the viromes of fish is important for broader fish health management.
Maunscript in prep.
Funding: The Minnesota Aquatic Invasive Species Research Center; Minnesota Environment and Natural Resources Trust Fund.
Identification of KHV and CEV in wild carp populations:
Koi herpesvirus (KHV) and carp edema virus (CEV) cause disease outbreaks in wild carp and ornamental koi carp (same species as common carp). Following two years of surveillance of carp mortality events, we have determined that these two important viruses are widespread in the Upper Midwest. The outbreaks are the first reports of KHV in MN and PA, and CEV in IA, MN, and WI. In addition, this is the first detection of confections of KHV and CEV in wild carp. It is unknown if the apparent widespread distribution is the result of repeated spillover from koi releases or if the viruses were long ago introduced with the carp - it is interesting to speculate.
Funding: The Minnesota Aquatic Invasive Species Research Center; Minnesota Environment and Natural Resources Trust Fund.
Focusing on Koi herpesvirus:
Following an extensive literature review, local and international consultation, and multiple seasons of region-wide fish disease surveillance, we have determined that koi herpesvirus (KHV) is a candidate pathogen worthy of further investigation due to its species specificity, high mortality rate and ability to generate long term disease reservoirs. However this preliminary work has also identified several key research gaps that must be addressed in order to continue evaluation of KHV as a biocontrol agent in MN. The next steps of this project will work to obtain and characterize local isolates of KHV in order to select highly virulent strains for disease trials. Additionally, an important aspect of KHV biocontrol would be the control of carp recruitment (production of young). This stage of the project will investigate the virulence of KHV in multiple life-stages of carp in lab disease trials and use survey of populations of carp with histories of KHV outbreaks to uncover information about the ecology of KHV in wild carp and its potentially effects on carp recruitment. Information from this project will generate the disease data needed to better inform an integrated model, that in combination with host population and environmental data, can be used to predict the outcome of KHV biocontrol.
We understand the risks/concerns of a viral biocontrol strategy for common carp. At this time, we have no plans to experimentally release KHV, or any other pathogen, into a wild population of carp - there are many questions that must be addressed before that could/should be done. We are focused on understanding the ecology and dynamics of KHV in naturally exposed wild populations to fill knowledge gaps that are useful to understanding biocontrol potential, and fish health management more broadly.
We are proceeding with an abundance of caution and open to input on research questions and collaborations that should be pursued.
Isolating KHV from wild carp:
Host range, transmission, and mortality rates are the drivers of disease dynamics in natural populations and can often be estimated in lab-based disease trials. To do so, we must first isolate and propagate the pathogen so that it can be manipulated in the lab.
In 2019 we were able to isolate KHV from wild carp in lake Elysian - the first North American isolate from wild carp. Having the virus isolate in hand, we immediately began testing the susceptibility of native (fathead minnow) and invasive (carp, goldfish) fishes to understand the host range. We found that carp were the only susceptible species tested, providing evidence of the host-specificity of KHV, an important consideration for the safety of KHV-biocontrol.
Tolo, I. E., S. K. Padhi, K. Williams, V. Singh, S. Halvorson, S. K. Mor, N. B. D. Phelps. 2021. Susceptibility of Pimephales promelas and Carassius auratus to a strain of koi herpesvirus isolated from wild Cyprinus carpio in North America. Scientific Reports 11:1985.
Funding: The Minnesota Aquatic Invasive Species Research Center; Minnesota Environment and Natural Resources Trust Fund.
Host range and update of CEV outbreaks:
Although KHV is a more promising virus as a biocontrol agent for carp, CEV has also been implicated in carp-only mortality events in Minnesota. We identified new outbreaks of CEV in multiple lakes in Minnesota and Pennsylvania in 2019 and used this as an opportunity to study the host range of the virus in a single lake (Swartout) in Minnesota. After testing 150 native fishes/amphibians (of 5 different species), we determined that despite close contact with infectious carp, none had been infected with CEV during a mass mortality event in Lake Swartout. Interestingly, we did fine evidence of CEV DNA in the reguretants of double-crested cormorants feeding on juvenile carp. Though we are only beginning to understand the significance of CEV-associated mass mortality events of wild carp, his study provided evidence of the species specificity of CEV.
Tolo, I., S. K. Padhi, P. J. Hundt, P. G. Bajer, S. K. Mor, N. B. D. Phelps. 2021. Host range of carp edema virus (CEV) during a natural mortality event in a Minnesota lake and update of CEV associated mor-tality events in the USA. Viruses 13, 400.
Funding: The Minnesota Aquatic Invasive Species Research Center; Minnesota Environment and Natural Resources Trust Fund.
