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During the winter of 2013 and 2016, several Croatian fish farms experienced mortalities in the fry of European sea bass, Dicentrarchus labrax. Affected fish showed abnormal swimming behaviour and reduced appetite, and death ensued several days after the onset of clinical signs of disease. Necropsy revealed pale liver, empty digestive tract, distended gall bladder, and hyperaemia and congestion of the meninges. Routine bacteriological examination tested negative, and virological examination ruled out nodavirus infection. Histological examination revealed multifocal necrosis and extensive inflammation in the brain with abundant cellular debris in the ventricles. Inflammatory cells displayed intra-cytoplasmic basophilic vacuoles leading to suspicion of Piscirickettsia salmonis infection. Fluorescent in situ hybridization using an oligonucleotide probe targeting Domain Bacterium applied to tissue sections tested positive. The pathogen was identified by 16S rRNA gene sequencing of brain material, and the sequence showed 99% similarity with P. salmonis. This result enabled the design of an oligonucleotide probe specifically targeting P. salmonis. In 2016, P. salmonis was successfully isolated on CHAB from the brain of an affected specimen and identified using 16S rRNA gene sequencing and MALDI-TOF. This study describes the first outbreak of disease caused by P. salmonis in sea bass in Croatia, while new diagnostic tools will enable further research on its epidemiology and pathogenicity., (© 2021 John Wiley & Sons Ltd.)

Common carp (Cyprinus carpio) is a very important fish species for warm-water aquaculture in Croatia. All Croatian carp farms are subjected to a surveillance programme for the presence of koi herpesvirus (KHV), causing a deadly disease called koi herpesvirus disease (KHVD). However, there is no surveillance for other viral pathogens of importance like carp edema virus (CEV), a causative agent of koi sleepy disease (KSD). During regular testing within the KHVD surveillance programme, we tested samples for CEV simultaneously. The screening indicated possible outbreaks of KHVD and KSD. During 2016, KHVD broke out in an isolated area and soon thereafter a KHV eradication programme was successfully performed. However, during 2018 and 2019, two additional mortality events occurred in lakes in the southern part of Croatia during the spring. Samples from both events tested positive for CEV. An epidemiological investigation confirmed the introduction of infected carps from an infected farm to one of the lakes. To prevent the spreading of CEV into open waters, it is of utmost importance to introduce CEV testing before fish movement or to perform regular testing of all carp farms in the country to determine CEV prevalence for the purpose of implementation of control measures., (© 2020 The Authors. Journal of Fish Diseases published by John Wiley & Sons Ltd.)