EMERGING VIRAL DISEASES OF CYPRINIDS

The intensive aquaculture remains the world’s fastest growing sector producing food of animal origin. In fact, it is the only animal food-producing sector growing faster than the human population, and provides an acceptable supplement to and substitute for wild fish. A number of cyprinid diseases have emerged globally and their study has become increasingly important. The expansion of aquaculture, which has relied heavily on the movement of animals and farming species new to aquaculture, has been paralleled with disease emergence. In the last few years several emerging or re-emerging fish diseases have been detected in cyprinid fish populations in Serbia. In this paper, the authors overview the major viral threats for cyprinid fishes in Serbia.


INTRODUCTION
Diagnostic testing of cyprinids often results in the occurrence of the cytopathic effect on cell culture and virus isolation, but fortunately, they are mostly benign. For a long time, spring viremia of carp was single viral disease of cyprinids listed by the OIE. In 2007 koi herpesvirus disease (KHVD), caused by the Cyprinid herpesvirus 3 (CyHV-3), was listed by the World Organization of Animal Health (OIE) and listed as a non-exotic disease in the European Union, related to the Directive 2006/88/EC. Recently, concern has been shown about diseases caused by other viruses, primarily herpes and pox viruses. Goal of this paper is to present an overview of current viral threats for cyprinid fish in Serbia.

Carp Edema Virus (CEV)
Although known for a long time as a cause of disease in koi carp in Japan, carp edema virus (CEV) has recently been recognized as a global threat to the koi and the common carp aquaculture. Carp Edema Virus (CEV) is the etiological agent of Carp edema virus disease (CEVD), firstly described in Japan in the 1970's as a viral edema of juvenile color Cyprinus carpio koi and later morphologically identified as a poxvirus. In koi, diseased fish exhibit erosive or hemorrhagic skin lesions with edema of the underlying tissues, thus the disease was originally named "viral edema of carp" (Oyamatsu et al., 1997). The disease has also been referred to as "koi sleepy disease" (KSD) because infected koi become lethargic and unresponsive (Miyazaki et al., 2005). After Japan, disease outbreaks in koi at import sites and in hobby ponds has been detected in USA since 1996.
In In Serbia, episodes of disease characterized by a typical sleepy behaviour, enophthalmia, generalized oedematous condition and gill necrosis, leading to hypoxia and mortality of up to 20% were observed during 2015 and 2016 in spring time on many carp farms, but causative agent was detected in 2017 (Radosavljević et al., 2018). During April and May 2017 unusual mortalities occurred in common carp at water temperatures between 9°C and 15°C. Diseased fish showed similar clinical signs and experienced about 20 percent mortality. Fish were lethargic, congregated around pond margins or at the water surface and became increasingly unresponsive.

Herpesviral hematopoietic necrosis (HVHN)
Cyprinid herpesvirus 2 (CyHV-2, Goldfish herpesvirus, GHV) is the causative agent of herpesviral haematopoietic necrosis (HVHN) in goldfish (Carassius auratus). The disease was first identified in Japan in 1992 and 1993 in goldfish (Carassius auratus) (Jung and Miyazaki, 1995). Until recently, Cyprinid herpesvirus 2 (CyHV-2, Goldfish herpesvirus, GHV) was described as the etiological agent of a disease named Herpesviral hematopoietic necrosis (HVHN), which occurred only in goldfish (Carassius auratus). This disease was first described in western Japan in the 1990s, causing high mortality in goldfish and occurring during the spring and autumn (Jung and Miyazaki, 1995). Since then, CyHV-2 has been reported in North America (Goodwin et al, 2006) as well as Taiwan (Chang et al, 1999) (Haenen et al., 2016). Clinically, affected goldfish display signs such as apathy, pale gills, increased respiratory rate, and ascites. Histopathological consistent finding in CyHV-2-infected goldfish is necrosis of the hematopoietic tissue in kidney and spleen. The disease in goldfish was not detected in Serbia, but outbreak of a disease caused with CyHV-2 was recorded in Prussian carp (Carassius gibelio) in 2017.
Recently, the virus has been detected in Prussian carp (C. gibelio) in China (Wang et al., 2012), and also in Czech Republic (Daněk et al., 2012), the Netherlands (Haenen et al., 2016) and in Serbia (Radosavljević et al., 2018). In Serbia, the outbreak of herpesviral hematopoietic necrosis in Prussian carp lasted for 1 week, at a water temperature of 26°C, and did not affect other fish species. Some fish showed a whitish slime layer over their eyes and an erythema of their skin, sometimes with haemorrhagic scales. Since the reports suggest that this virus has been spreading, and also CyHV-2 has the potential to infect other species of the genus Carassius as a host, understanding the properties and the preventive measures of this disease has become important.

CONCLUSIONS
Understanding of the emerging cyprinid viruses is still evolving. It is very probable that more cyprinid viruses will be detected as improved diagnostic methods come into general use. However, for detection of different emerging viruses of cyprinids, advanced diagnostic techniques have had to be used. The present findings indicate that the prevalence and spread of CEV and HVHN must be closely monitored in our country to avoid potential economic losses.

ACKNOWLEDGEMENT
This study was financially supported by a grant from the Ministry of Education, Science and Technology Development of the Republic of Serbia, under the projects TR31075 and TR31011.