Koi sleepy disease (KSD) is a disease with increasing importance in global common carp aquaculture. Despite the fact that carp edema virus (CEV) is most likely the causative agent of KSD, the disease often presents itself as multifactorial with several parasites and bacteria species present on gills, skin or in internal organs. Therefore, in this study, we analysed and presented initial results on an interaction of flavobacteria and CEV in the development of clinical KSD in carp suffering from proliferative gill disease. We examined selected field samples from Germany and Hungary and confirmed the presence of CEV and flavobacteria co-infections in subset of the samples. In several infection experiments, we studied the transfer and dynamics of both infections. Furthermore, we analysed which Flavobacterium species could be isolated from KSD-affected fish and concluded that Flavobacterium branchiophilum is a possible copathogen. Antibiotic treatment experiments showed that CEV seems to be the primary pathogen causing an insult to the gills of carp and by these enabling other pathogens, including F. branchiophilum, to establish co-infections. Despite the fact that F. branchiophilum co-infection is not required for the development of clinical KSD, it could contribute to the pathological changes recorded during the outbreaks.

The infection of common carp and its ornamental variety, koi, with the carp edema virus (CEV) is often associated with the occurrence of a clinical disease called 'koi sleepy disease'. The disease may lead to high mortality in both koi and common carp populations. To prevent further spread of the infection and the disease, a reliable detection method for this virus is required. However, the high genetic variability of the CEV p4a gene used for PCR-based diagnostics could be a serious obstacle for successful and reliable detection of virus infection in field samples. By analysing 39 field samples from different geographical origins obtained from koi and farmed carp and from all 3 genogroups of CEV, using several recently available PCR protocols, we investigated which of the protocols would allow the detection of CEV from all known genogroups present in samples from Central European carp or koi populations. The comparison of 5 different PCR protocols showed that the PCR assays (both end-point and quantitative) developed in the Centre for Environment, Fisheries and Aquaculture Science exhibited the highest analytical inclusivity and diagnostic sensitivity. Currently, this makes them the most suitable protocols for detecting viruses from all known CEV genogroups.

Carp edema virus (CEV) is the causative agent of koi sleepy disease (KSD), a serious gill disease affecting common carp, Cyprinus carpio, and its ornamental variety, koi. After recent detections of the virus in various countries around the world, KSD has emerged as a new global disease in carp. However, the prevalence of the infection in carp populations in a given geographical region has not been studied thoroughly. The present communication reports an investigation into the presence of CEV in carp and koi populations in Germany. For this purpose, gill samples collected from carp and koi populations suffering from gill diseases or collected for a routine examination of their health status were tested for the presence of CEV by PCR. In total, 651 fish samples from 401 carp or koi cases were examined in 2015 and 2016, additional 118 samples from previous studies were included in the examination. CEV was detected in archive samples from carp dating back to 2007, and in koi samples dating back to 2009. From 2015 to 2016, CEV was detected in 69% of cases from carp populations examined from the main carp‐producing areas in Germany, and in 41% of the examined cases from koi populations from all over Germany. Clinical KSD occurred mainly from April to June in carp populations at water temperatures ranging from 8 to 12°C and in koi populations at water temperatures ranging from 18 to 22°C. Most fish from clinically affected carp or koi populations harboured high virus loads of above 10,000 copies of CEV‐specific DNA per 250 ng DNA, while gills from fish of other fish species from the ponds, including goldfish, grass carp and European perch were found CEV negative or harboured a low virus load. A phylogenetic analysis revealed the presence of multiple CEV variants from genogroup I in carp and genogroup II in koi populations in Germany. Genetically identical genogroup I isolates were detected in carp from different geographical locations in Germany and in other European carp populations. Some German genogroup II variants were identical to variants previously recorded from koi in Asian and other European countries. The data presented here show that CEV is highly prevalent in German common carp and koi populations and implies the spreading of this virus by intense trading of common carp and koi without necessary risk mitigating measures. As infections with this virus may induce serious disease, CEV diagnostic should be included in health surveillance and disease monitoring programmes.