Investigation of Koi Herpes Virus (KHV) Disease Periods and Factors Influencing KHV Transmission in a Low Stocking Density Infection Trial

Isaiah E. Tolo1,2, Przemyslaw G. Bajer1,2, Tiffany Wolf3, Sunil K. Mor1,3, Nicholas B. D. Phelps1,2

 1University of Minnesota, Minnesota Aquatic Invasive Species Research Center, St. Paul, Minnesota, USA; 2University of Minnesota, College of Food, Agriculture and Natural Resource Sciences, Department of Fisheries, Wildlife and Conservation Biology, St. Paul, Minnesota, USA; 3University of Minnesota, College of Veterinary Medicine, Department of Veterinary Population Medicine, St. Paul, Minnesota, USA

ABSTRACT

Koi herpesvirus (KHV, syn Cyprinid herpesvirus 3) remains an impactful pathogen of aquaculture and wild populations of common carp worldwide. Though many advances have been made in understanding the pathology and ecology of KHV, the mechanisms of transmission and kinetics of KHVD are not well understood. Understanding the relative contributions of direct and indirect transmission of KHV as well as the factors that drive KHV transmission can clarify the importance of environmental disease vectors and would also be valuable for informing disease modeling efforts. To study the mechanisms and factors driving KHV transmission we used infection trials to determine the kinetics of KHVD and to determine the contributions of direct and indirect forms of KHV transmission as well as the contributions of contact rate, viral load, disease burden and contact type. The incubation period of KHVD (the time interval between exposure to KHVD and onset of clinical signs) was 5.88+1.75 days and the symptomatic period (time interval between onset of symptoms and experimental endpoint) was 5.31+0.87 days. Direct transmission was determined to be the primary mechanism of KHV transmission (OR = 25.08, 95%CI = 10.73-99.99, P-value = 4.29E-18) and transmission primarily occurred during the incubation period of KHVD due to the high contact rate of shoaling carp. Direct transmission decreased in the symptomatic period of disease due to behavioral changes of symptomatic carp and uninfected conspecifics. Transmissibility of KHV and indirect transmission increased during the symptomatic period of disease, correlating with increased viral loads. These findings inform the mechanisms and factors driving KHV transmission in a low stocking density disease trial that and are relevant to low density aquaculture and wild populations of carp. Additionally, potential virulence-transmission tradeoffs and disease avoidance behaviors relevant to KHV transmission are identified.