Investigating Disease Dynamics of Koi Herpesvirus in Wild Carp in Minnesota, USA.

 

 1,2Isaiah E. Tolo, Soumesh K. 1,2Padhi, 1,2Peter J. Hundt, 1Sophie Halvorson, 1,2Przemyslaw G. 1,3Bajer, Sunil K. Mor, 1,2Nicholas B. D. Phelps.

1 Minnesota Veterinary Diagnostic Laboratory and Department of Veterinary Population Medicine, University of Minnesota, 1333 Gortner Avenue, St. Paul, MN, 55108; 2 Minnesota Aquatic Invasive Species Research Center and the Department of Fisheries, Wildlife, and Conservation Biology, College of Food, Agriculture, and Natural Resource Sciences

ABSTRACT

Koi herpes virus (KHV) is a virulent pathogen of common/koi carp (Cyprinus carpio, carp). The importance of carp as both an aquaculture species and an ecologically damaging invasive species worldwide makes KHV of high significance though the rationale for studying pathogens specific to carp are often at odds between different stakeholders. For example, conservation biologists may be most interested in KHV’s potential impact on reducing the density of invasive carp populations while aquarists and fish farmers may be most interested in developing disease treatment and prevention strategies. Since 2017, 10 mass mortality events of wild carp have been reported in Minnesota, USA in association with detection of KHV. As part of an effort to understand the disease ecology of KHV in wild carp, we have conducted surveys of wild carp in five lakes with previous histories of KHV. Tissue samples of brain, gill, and kidney from a total of 300 carp were screened for the presence of KHV using specific qPCR and a total of 160 tissue pools were subjected to cell culture on Koi fin (KF-1) and common carp brain (CCB) cell lines. A subset of 55 carp in one lake (Lake Elysian) were also screened for anti-KHV antibodies via ELISA performed at the University of California Davis. Active infections of KHV were detected by qPCR at <2% prevalence in three lakes and higher prevalence (12% and 23%) in two lakes. KHV was isolated on CCB cells from Lake Elysian, which was determined to have been undergoing a KHV-associated mortality event at the time of sampling. Live carp with apparent acute infections with KHV were captured during sampling of Lake Elysian and housed alive for screening by qPCR. Additional attempts at virus isolation on CCB cells from disease free carp challenged with KHV positive tissue homogenates obtained from lake Elysian were also successful. 60% of carp from Lake Elysian, tested for anti-KHV antibodies, were positive. Paired testing with qPCR and ELISA was used to differentiate between acute, persistent and latent infections with KHV. Detection of acute infections were significantly higher in young male carp. This study provides additional information on the distribution and demographic prevalence of KHV in wild carp in Minnesota. Additionally, Isolation of a North American reference strain of KHV will facilitate investigations of KHV disease dynamics and host-range in lab-based trials.