STURGEON DISEASES IN AQUACULTURE

Sturgeon aquaculture is economically important in many countries, for both meat and caviar production. Sturgeon is the common name for 27 species of fi sh belonging to the family Acipenseridae. Among them, only the sterlet (Acipenser ruthenus) and the Siberian sturgeon (Acipenser baerii) completes the lifecycle in fresh water. In Serbia, in the last few years, aquaculture enterprises have shown more interest in farming these fi sh species. Also, the importance of sturgeon aquaculture grows due to the rapid decrease of wild populations caused by overfi shing, water pollution and destruction of habitat. Th e development of sturgeon aquaculture activities has been accompanied by the disease outbreaks, and possibility of the emergence and rapid dissemination of several infectious disease agents may represent serious problem in sturgeon aquaculture. Several viral, bacterial, fungal and parasitic diseases have been reported worldwide. Due to the limited knowledge about epizootiology and disease control methods, infectious diseases may represent a major challenge in sturgeon aquaculture. Moreover, none of the diseases reported in sturgeon are regulated in the World Organization for Animal Health (OIE) or European Union (EU) legislations. Due to the increasing interest in sturgeon aquaculture in Serbia present study is focused on the most important pathogens that may represent a threat to sturgeon aquaculture in Serbia.


INTRODUCTION
Sturgeon farming has become an important and rapidly expanding sector of aquaculture worldwide. In the last decade, the farming of sturgeon in Western Europe has increased exponentially. Th e annual world production is allocated among 4 species: White sturgeon (Acipenser transmontanus), Siberian sturgeon (Acipenser baerii), Adriatic sturgeon (Acipenser naccarii) and Russian sturgeon (A. gueldenstaedtii). Th is applies to both the growing of fry and fi ngerlings for restocking natural reservoirs as well as the production of table sized fi sh and caviar for the market. Sturgeon farming has become increasingly important worldwide for the production of caviar and fi sh fl esh, as well as for restoration programmes set up to save endangered wild populations (Bronzi and Rosenthal, 2014). Several species are farmed, among which the most desirable are Russian (otherwise known as Danube) (Acipenser gueldenstaedtii) and Siberian (Acipenser baerii) sturgeons due to their short reproductive cycles and desired products (caviar and meat) (Williot et al., 2001). However, the success of sturgeon farming is heavily restricted due to the paucity of information related to sturgeon diseases and related control methods. It is believed that sturgeons are comparatively more resistant to fi sh diseases; nevertheless, many studies have shown that their diseases are idiopathic and involve diff erent pathogens. Intensive culture exposes the fi sh to several sources of stress, such as high stock densities and manipulations that predispose animals to a number of infectious diseases associated with viral or bacterial pathogens (Georgiadis et al., 2000). As in the culture of other fi sh species, diseases are a principal limiting factor in sturgeon farming. Among them, viral diseases often cause major damage to the industry. Disease control in sturgeon farming is diffi cult due to lack of knowledge about disease epidemiology and control methods (Ciulli et al., 2016). As a result of more intensive sturgeon aquaculture, infectious diseases that aff ect the species have emerged.

Viral Diseases
Th e intensive breeding of sturgeon has facilitated the emergence and the spread of diseases. To date, several specifi c viruses have been found in sturgeons. Almost all of them were discovered in sturgeons native to North America, both in the USA and Canada, as well as in Europe, where these fi sh species were introduced (Raverty et al., 2003, Kelley et al., 2005.Viruses of the Herpesviridae and Iridoviridae families are the major threats for the sturgeon aquaculture and presents the most reported causes of mortality outbreaks in sturgeons (Raverty et al., 2003;Shchelkunov et al., 2009;LaPatra et al., 2014). Among them, the white sturgeon (A. transmontanus) iridovirus (WSIV) and white sturgeon herpesvirus type 2 (WSHV-2, later renamed to Acipenserid herpesvirus 2, AciHV-2) are economically the most signifi cant pathogens, causing up to 95% mortality in cultured young sturgeon (Watson et al., 1995, Georgiadis et al., 2001, Kelley et al., 2005.

Acipenserid herpesvirus 2 (AciHV-2) disease
Acipenserid herpesvirus 2 (AciHV-2) also known as a white sturgeon herpesvirus 2 (WSHV-2), was fi rst isolated from ovarian fl uid of an adult sturgeon and was a cause of mortality in farmed juvenile white sturgeon in North America in the mid-1990s (Watson et al., 1995). Later, a closely related virus was isolated in Russia, suggesting that the Russian isolates may have originated from North America. Mortality in adult fi sh infected with AciHV-2 was generally less than 10%. Experimentally, the shovelnose and pallid sturgeon were susceptible to AciHV-2 but other species were refractive (Mao et al., 1999). White sturgeon herpesvirus-2 occurs in older sturgeon with small white blisters which develop into the open lesions on the body surface. Th ese lesions are frequently infected with secondary bacteria and/or ectoprotozoal parasites. Internally, the stomach and intestines are fi lled with fl uid but other tissues appear normal. Wild white sturgeon which was infected with AciHV-2 became listless and stopped eating. Current management strategies for controlling AciHV-1 and AciHV-2 are avoidance of the agent and inspection of potential carrier fi sh via cell culture assay. Sturgeon infected with AciHV-2 could be prophylactically treated with salt and parasiticides to reduce secondary infections in open ulcers.

Siberian sturgeon herpesvirus (SbSHV) disease
Siberian sturgeon herpesvirus (SbSHV) was isolated in Russia for the fi rst time in 2006 from moribund Siberian sturgeon (Acipenser baerii) and bester (beluga Huso huso × sterlet Acipenser ruthenus hybrid) fi ngerlings during acute outbreak of disease on a fi sh farm (Shchelkunov et al., 2009).Th e infection has been found widespread in cultured sturgeon species in Russia. It is the cause of an acute necrohaemorrhagic skin syndrome complicated by secondary opportunistic infections (fungal, myxobacterial or protozoan) (Shchelkunov et al., 2009). Based on sequence analysis of the viral genome, it is determined that the SbSHV is a potential member of the genus Ictalurivirus within the family Alloherpesviridae under the order Herpesvirales (Doszpoly and Shchelkunov, 2010). Two Russian types of SbSHV (I and II) were isolated, and they diff er from each other in four principal marker traits and each of the two has close genetic relationship with one or another strain of North American AciHV-2 species. It was hypothesized that the Russian type I and type II SbSHV may represent two diff erent strains or genotypes of the Acipenserid herpesvirus 2 species (Doszpoly and Shchelkunov, 2010).

Iridoviral diseases
Iridoviruses have been associated with severe disease and economic loss in fi sh with more than 50% mortality. Th e fi rst iridovirus associated with mortality outbreaks of sturgeon was the white sturgeon iridovirus (WSIV) (Hedrick et al., 1990). Other iridolike viruses have been detected in diff erent sturgeon species in North America and Europe, namely white sturgeon iridovirus (WSIV) ( (Adkison et al., 1998). Th ey can cause a lethal disease of the integumentary system in infected sturgeon, resulting in > 90% mortality within captive populations. In other instances, they are associated with a chronic debilitating wasting syndrome, resulting in severely impaired growth and reduced survival of fry and fi ngerlings (Clouthier et al., 2018). In total, nine species of sturgeon in the genera Acipenser or Scaphirhynchus of the family Acipenseridae were found susceptible to one or another virus. Outbreaks of virus disease are associated with stress factors such as rearing density, handling, fl uctuations in water temperature, levels, and fl ow rates , Georgiadis et al., 2000. Th e phylogenetic studies revealed that these viruses are only distantly related to Iridoviridae, and are included in a group of sturgeon nucleo-cytoplasmic large DNA viruses (NCLDVs). Th ey do not form part of any currently recognized virus genera or family but do belong to the order Megavirales (Colson et al., 2013). NCLDV group has not formally been adopted by the International Committee for the Taxonomy of Viruses (Clouthier et al., 2015). Th e sturgeon NCLDVs are present in hatchery-reared and wild sturgeon found across North America and northern Europe (Clouthier et al., 2015), and pose a potential disease risk for strategies designed to aid the recovery of threatened and endangered sturgeon populations worldwide (Clouthier et al., 2018). Data on sturgeon iridolike viruses in Europe are limited despite the increasing importance of sturgeon aquaculture. Iridovirus-like infection has been reported in Northern Europe in Russian sturgeon (Acipenser gueldenstaedtii) associated with mortality (Adkison et al., 1998). NCLDVs outbreaks are frequently reported by sturgeon farmers, but only few thorough investigations have been conducted for these outbreaks. Sturgeon NCLDVs appear to be endemic in populations of Acipenseridae found throughout North America (Clouthier et al., 2015). At present, an accurate picture of the geographical distribution of sturgeon NCLDV infection in Europe is not available. Also, in 2009, frog iridovirus type 3 (FIV3), a wellknown ranavirus provoked an outbreak on pallid sturgeon (Scaphirhynchus albus) in an American hatchery (Waltzek et al., 2014).
Recently, Acipenser iridovirus European (AcIV-E) has been detected in sturgeon populations in Europe. It appears that this virus is closely related to North American sturgeon iridoviruses, in particular with the white sturgeon iridovirus (WSIV) and the Namao virus (NV) (Axen et al., 2018). AcIV-E was detected and associated with clinical disease in diff erent sturgeon species (Acipenser baerii, Acipenser gueldenstaedtii, Acipenser naccari, and Huso huso). Higher mortality due to AcIV-E was recorded in Russian sturgeon compared to Siberian sturgeon -90% vs 50% -suggesting a lower susceptibility to the virus in Siberian sturgeon (Bigarré et al., 2017). Much important information, e.g., pathogenesis, transmission routes, and epidemiology of these viruses, is lacking, making virus and disease control extremely diffi cult (Axen et al., 2018).

White sturgeon adenovirus (WSAV) disease
WSAV was identifi ed in diseased juvenile white sturgeon between 1984 and 1986 (Hedrick et al., 1985) but the disease has not evolved into a serious health problem, and there has not been any report on WSAV since then.

Infectious hematopoietic necrosis virus (IHN)
Sturgeon are oft en cultured on facilities that contain other fi sh species. LaPatra et al. (1995) demonstrated that white sturgeon fry were susceptible to IHNV and that white sturgeons cultured with IHNV-infected rainbow trout had neutralizing antibodies to the virus.

Viral nervous necrosis (VNN)
Betanodavirus within the family Nodaviridae is the etiological agent of viral nervous necrosis (VNN, also known as viral encephalopathy and retinopathy or VER). Th e virus infects a large range of host species in more than 50 species of marine and freshwater fi sh worldwide. Athanassopoulou et al. (2003) reported on the presence of a betanodavirus that infected sturgeon (Acipenser gueldestaedi) in fresh water causing disease with neurological signs. Vicenova et al. (2011) reported isolation of the SVC virus in sturgeon with the clinical signs of the disease, which included internal haemorrhages, considerably enlarged bright-red spleen, and greyish-yellow liver showing tiny red spots. Th e sturgeon isolate was genetically identical to the isolate from clinically healthy koi carp collected from the same aquaculture site.

Infectious pancreatic necrosis (IPN)
Little scientifi c information is available to assess whether sturgeon can become infected and develop into potential carriers of IPNV. Only the report from France indicated that IPNV was isolated from 3% (1 of 34) of a group of Siberian Sturgeon Acipenser baerii (Nougayrede, 1988).

Aquareoviruses
Aquareoviruses are serious pathogens of aquatic animals. Reovirus particles were observed in diseased Chinese sturgeons, suggesting the existence of primary viral illness (Zhang and Gui, 2012).

Cyprinid herpesvirus-3 (CyHV-3)
According to Pospichal et al. (2016) the hybrid between sterlet and beluga seems to be susceptible to cyprinid herpesvirus 3, but the authors could not prove that they can transfer this virus to naïve koi. Also, Kempter et al. (2009) reported that Atlantic sturgeon and Russian sturgeon are susceptible to CyHV-3.

Bacterial diseases
Few mortality outbreaks have been reported in sturgeon associated with primary bacterial pathogens such as the case of lactococcosis in hybrid sturgeon, Bester (Huso huso x Acipenser ruthenus) (Chen et al., 2012). However, the isolation of bacteria from sturgeon was oft en reported as a secondary infection or as a consequence of severe stress and high stock densities. Motile Aeromonas infection (MAI) is one of the most common infection in sturgeon (Santi et al., 2019). MAI is usually associated with viral diseases in surviving sturgeon. Motile Aeromonas species have been isolated from sturgeon, generally as a consequence of severe stress and high stock densities or opportunistic, secondary to a primary viral infection. A motile aeromonas septicaemia caused by Aeromonas hydrophila frequently induced considerable losses in Persian sturgeon (Acipenser persicus) fi ngerlings in northern Iran (Soltani and Kalbassi, 2001) and also in the Harrison river sturgeon in Canada (Raverty and Nikl, 1999). Aeromonas hydrophila as a pathogenic agent has been isolated from Amur sturgeon (Acipenser schrenckii) in China (Meng et al., 2011). Aeromonas hydrophila and Aeromonas veronii were isolated during the outbreak of a disease characterized by haemorrhagic ascites and intestinal and renal haemorrhaging in cultured Chinese sturgeon (Di et al., 2018). In Turkey, Aeromonas hydrophila was detected in Russian sturgeon (Acipenser gueldenstaedtii) as the cause of bacterial haemorrhagic septicaemia and high rate of mortality (Timur et al., 2010). Aeromonas veronii was identifi ed as a pathogen and cause of mortality of Siberian sturgeon (Acipenser baerii) (Ma et al., 2009). Aeromonas veronii was isolated from reared sturgeons in Iran (Gholamhosseini et al., 2018). Aeromonas sobria was isolated from Acipenser gueldenstaedtii and Acipenser baerii in Turkey (Kayis et al., 2017). Pseudomonas spp., particularly Pseudomonas fl uorescens, are common worldwide and found mainly in cold freshwater. Th e infection was reported in Acipenser baerii by Brunetti et al. (2006) and in Acipenser gueldenstaedtii by Kayis et al. (2017). Th e pathogen Pseudomonas alcaligenes has been identifi ed in Chinese sturgeon (Xu et al. 2015). Streptococcus iniae was isolated from liver, kidney and spleen of the dy-ing sturgeons with clinical symptoms during an episode of continuous mortality of cultured hybrid sturgeons occurred on a farm in China in 2012 (Wang et al., 2014). Yersinia ruckeri was found responsible for 10% mortalities in cultured sturgeon (Acipenser baerii) in France (Vuillaume et al., 1987).
An outbreak of Pseudomonas fl uorescens was reported in young (10 g in size) farmed Siberian sturgeon (Acipenser baerii) with high mortality (40%) in northern Italy (Brunetti et al., 2006).
Lesions in Acipenser gueldenstaedtii and Acipenser baerii have been described during natural and experimental infections with Acinetobacter johnsonii and Acinetobacter baumannii (Kozinska et al., 2014).
Streptococcus dysgalactiae, a Gram-positive bacterium, has been isolated from Acipenser schrenckii in China. Haemorrhages, abdominal swelling and ascites have also been reported in cultured sturgeons ).

Parasitic diseases
Most common parasitic diseases are those in reared young sturgeons caused by ciliates of the genus Trichodina and other genera of the family Urceolariidae. Rarely occurs infections caused by Ichthyophthirius multifi lis and Chilodonella cyprini (Bauer et al., 2002). Helminthic diseases of reared sturgeons are very rare in fi sh farms, although representatives of several species of Monogenea, Trematodes, Cestodes and Nematodes are sometimes found in reared sturgeons (Bauer et al., 2002).

CONCLUSIONS
Th e increasing importance of sturgeon farming and international trade in all species of sturgeon worldwide has increased the risk of disseminating specifi c pathogens and emphasizes the need for adequate diagnostics to prevent the spread of these pathogens. Most infections in natural conditions result in unapparent or mild disease, yet various pathogens can be highly pathogenic in sturgeon aquaculture where transfer of pathogens is facilitated by high densities of naïve hosts. As sturgeon is increasingly being farmed, a better understanding of the pathogens infecting this species is crucial to the development of a sustainable industry.
Th e majority of disease conditions on sturgeon farms could be signifi cantly reduced if proper attention is paid to good husbandry and the maintenance of optimal environmental conditions, especially water quality.
In conclusion, the wide diversity of pathogens and the numerous movements of live fi sh and fi sh products have likely strongly contributed to the spread of infectious agents for years. Th e transfers of live material need to be better controlled in the future with an aim of improving fi sh health and hence their commercial and ecological value. Considering the potential of various infectious pathogens to cause severe disease in an aquaculture setting, additional studies are needed to increase the knowledge on the epidemiology of sturgeon diseases. Th is will help in controlling infections of managed stocks reared for food and also in conservation programs.

ACKNOWLEDGEMENT
Th is study was fi nancially supported by a grant from the Ministry of Education, Science and Technology development of the Republic of Serbia, under the project TR31075, TR-31011.