Carp edema virus surveillance in the koi trade: early detection through shipping environment sampling and longitudinal monitoring of CEV outbreaks in a wholesaler facility.

Abstract

Carp edema virus (CEV), a member of the Poxviridae family, has been a significant pathogen in koi and common carp since its initial identification in Japan during the 1970s. CEV, the causative agent of Koi Sleepy Disease (KSD), can cause high mortality rates and has been reported in many countries and is often linked to the fish trade. The virus is typically detected through DNA analysis of gill tissues, where the highest viral loads are found. However, traditional sampling methods, such as gill sampling, are lethal, complicating routine surveillance, particularly in asymptomatic or high-value koi. This study aimed to evaluate nonlethal sampling methods for CEV surveillance in the koi trade. We analysed various shipping environment samples, such as shipping water and fish bag swabs, alongside gill swabs from anaesthetised fish and gills from naturally deceased fish. Using qPCR, we found that the sensitivity of environmental samples, particularly shipping water, was greater than that of direct fish samples. Latent class modelling estimated that the sensitivity associated with 1.5 mL shipping water samples was greater than 89%, making them a reliable alternative for early detection. All detected variants belonged to genogroup II. Some post-import outbreaks shared variants with earlier outbreaks or shipping environment samples, suggesting that the detected DNA generally reflected infectious particles rather than just free environmental DNA and indicating that CEV can go unnoticed for several months after importation. These findings highlight the utility of environmental samples for effective, non-invasive surveillance and improved biosecurity management in the koi trade.