Nidoviruses in Aquatic Organisms-Paradigm of a Nascent Concern

ORF: Open Reading Frame; nt: Nucleotides; WBV: White Bream Virus; FHMNV: Fathead Minnow Nidovirus; PCR: Polymerase Chain Reaction; RT-LAMP: Reverse TranscriptaseLoop Mediated Isothermal Amplifi cation; CCNV: Crucian Carp Nidovirus; CSNV: Chinook Salmon Nidovirus; YHV: Yellow Head Virus; GAV: Gill-Associated Virus; EsRNV: Eriocheir sinensis Ronivirus; BWCoV: Beluga Whale Coronavirus; BdCoV: Bottlenose dolphin Coronavirus; CoV: Coronavirus; SARS: Severe Acute Respiratory Syndrome; MERS: Middle East Respiratory Syndrome


Introduction
The Order "Nidovirales" [1] was introduced for the fi rst time by the International Committee on Taxonomy of Viruses in the year 1996. The Latin term 'nidus' means nests inferring the nested assemblage of sub-genomic mRNA [2].
Nidoviruses possess a wide variety of morphological and molecular structures which justify assigning these viruses into different families. The intricate molecular genetics completely discriminating the aquatic nidoviruses from other groups of RNA virus which enables them to survive in different ecological niches. Group IV positive-sense single-strand RNA nidovirus is grouped under two categories based on their genome size; large nidovirus and small nidovirus. Large nidovirus comprises three families Coronaviridae, Roniviridae, Mesoniviridae but the small nidovirus consists only a single family, viz., Arteriviridae [3][4][5]. The nidoviruses can infect a wide range of hosts and can acclimatize to adverse environmental conditions due to their higher frequency of recombination and mutation [6]. As a result, the taxonomy of nidovirus is constantly changing.
Thus, the establishment of complex replication-transcription model of all nidoviruses has been one of the thriving areas of research. It is projected to be a progressive fi eld of study with the commencement of Nidoviruses in aquatic animals after breaking down the perception that Nidoviruses are restricted to terrestrial animals i.e. particularly humans and livestock animals. It affects a broad range of hosts from marine mammals to fi shes as well as shrimps and crabs. Among these, Citation: Kumar

Life-cycle
Infection begins when ligand of nidovirus particle attaches

White bream virus (WBV) :
The virus was fi rst characterized from a healthy white bream or silver bream in Germany [7]. It encompasses 26.6 kb long genome consisting of fi ve ORFs. It encodes non-structural protein i.e. replicase polyprotein and structural proteins like spike protein, membrane protein and  [24]. As it is not showing detrimental effects on fi sh; listlessness, anorexia, excess mucus production, pseudofaeces and petechiae are common nonspecifi c clinical signs [25].

Fathead minnow virus (FHMNV)
In 1997, Fathead Minnow Virus was fi rst reported from moribund fathead minnows (Pimephales promelas) in United States [9]. The viral genome consist of approximately 27 kb RNA and the structural organization of genome resembles to WBV genome. The fi sh infected with FHMNV exhibited skin haemorrhage, eye haemorrhage, gill palor, multifocal petechial haemorrhages on the abdomen and lesions in liver, spleen, kidney. Fishes challenged with this pathogen suffered 90% mortality. Histopathology is exemplifi ed by nuclear pyknosis displaying marginated chromatin and severe multifocal areas of necrosis in the anterior kidney, liver and spleen [9]. In the study of experimental infection, morbidity becomes apparent by changes in behaviour and appearance characterized by initial erratic swimming followed by listlessness at the tank bottom and head faced upward at the water surface [9]. It had also been detected in other fi shes such as muskellunge (closely related to northern pike) [26], golden shiner and spotfi n shiner [27]. The diseased spotfi n shiner exhibits moderate echymotic haemorrhage on the isthmus and severe multifocal echymotic haemorrhage along the caudal to the dorsal fi n whereas golden shiner infected with FHMNV showed severe diffuse haemorrhage in brain and severe multifocal pinpoint haemorrhage along the spine just below the dorsal fi n [28].
For diagnosis and surveillance of this pathogen, the development of PCR procedure for FHMNV was accomplished in 2012 [10]. Subsequently, RT-LAMP test with a sensitivity of around 5 copies of RNA [29] and reverse transcriptase PCR assay [26] were standardized for precise diagnosis.
In 2019, Xiao-yu, et al. [30], isolated a nidovirus from crucian carp and named it as Crucian carp nidovirus (CCNV) having the highest resemblance with Chinook salmon nidovirus (CSNV). Another novel nidovirus was reported from cultured chinook salmon [31], posing a threat to the wild stock of salmon as well as in aquaculture Figures 4,5.

Crustacean nidoviruses
Yellow head virus (YHV): Yellow head disease (YHV genotype 1), among the eight known genotypes of the yellow head complex of viruses, was fi rst identifi ed to be associated with the mass mortality in cultured Penaeus monodon in Thailand in 1990 [32]. During 1992-1993, yellow head disease left a devastating effect over shrimp farming and caused an economic loss of over 40 million U.S. dollars [33]. Subsequently, the disease spread over other Asian countries popular for shrimp farming [34]. It's genome consists of four long ORFs with 26,662 nt. In general, juvenile to subadult P. monodon were more susceptible to infection. Due to the bleached appearance of the infected shrimp as well as the yellowish cephalothorax this disease is referred to as Yellow Head Disease. Yellowish cephalothorax is  It was fi rst detected in wild and farmed giant tiger shrimp in Queensland, Australia in 1994 [16]. The virus could be found over the whole life-span of wild shrimp either in a dormant or chronic infection state [16,37]. GAV and YHV are two different kinds of virus because they show approximately 80-85% homology in DNA sequence and 96% homology in amino acid composition [38,39]. Reddening of body and appendages, biofouling with ectoparasites, emaciation, pink to yellow

Eriocheir sinensis ronivirus (EsRNV):
It causes an infection called, "sighs" disease (SD) in Chinese mitten crabs displaying respiratory problems. The crabs manifested by viruses respire bubbles that sound like 'sigh' in the serene of night. It is associated with black gill syndrome in freshwater Chinese mitten crab. Anorexia and sluggishness are non-specifi c clinical symptoms. The enveloped rod-shaped virus possesses approximately 22 kb long ssRNA genome. There is a close evolutionary relationship between EsRNV and okaviruses based on structure and genome characteristics but the whole genome sequencing is required to assign this new virus either to an existing genus or to a new genus [40]. The disease damaged over 28,000 hectares land in China causing a huge loss of about 30 lakh euros to the economy in 2002. Histopathology is marked by 1-5 μm diameter clusters of pale to deeply basophilic necrotic cells exhibiting nuclear hypertrophy, karyorrhexis or pyknosis and by 200-800 nm diameter cytoplasmic inclusion bodies. Necrotic cells were observed in the lymphoid organ and connective tissues of different organs such as hepatopancreas, gut, gills, heart and testes [17,41] Figures 6-8.

Crab Oka-Like Viruses:
The rod-shaped Rhabdolike-virus A (RhVA) was isolated from the Atlantic blue crab (Callinectes sapidus) in 1970s [43,44]. The infection of this virus was spotted in Y organ or mandibular glands (also known as ecdysial gland). Therefore, later it was renamed as ecdysial gland virus 2 (EGV-2) [45,46]. Soon after, it was noticed that EGV-2 seen to be present along with EGV-1, another rod-shaped viral particle ornamented with peplomers [45]. It was apparent that EGV-1 and EGV-2 appeared conspicuously after the eyestalk ablation. Therefore, it is indicating that disease occurrence is due to stress response to the pre-existing infections [45]. As the diameter of the documented okaviruses [40] and bafi niviruses [3] are greater than the rod-shaped RhVA and EGV-2 virions [3], complete genome sequencing is needed to establish their evolutionary relationships.

Nidoviruses in aquatic mammals
Harbor seal coronavirus: Acute necrotizing enteritis had occurred in the Pacifi c harbor seal (Phoca vitulina) due to suspected coronavirus infection [18]. It is a distinct form of But a major distinguishing feature between these two viruses was observed in their structures and composition of spike glycoproteins. The largest genome among all coronaviruses were detected in BdCoV due to multiple distinct ORFs, the estimated size being around 32000 nt. It is assumed that BdCoV   and BWCoV had evolved around 60 years back from a common ancestor of coronavirus [20] Figure 9.
The COVID-19 pandemic has created a global crisis affecting the world economy [49], its deadly nature is similar to SARS coronavirus outbreak in 2002 [50] and MERS coronavirus outbreak in 2013 [51]. In all cases, bats are considered as reservoir hosts of CoVs because of its ability to migrate over extensive areas due to its strength of fl ight. Due to several recombinations in various CoVs, new viral strains can emerge. Consequently, there will be a tremendous possibility of migrating the pathogens from bats to other animals resulting in the adaptation of new host followed by the transmission to livestock animals or humans [52] (Table 2). [55,56]. The present point of view is that it is necessary to stop the propagation of the virus and forthcoming "zoonotic spill overs". Different CoVs with zoonotic importance are moving around the wild population but Bat CoVs are the most diverse group. There are ample chances of evolution and recombination of zoonotic CoVs leading to the burgeoning of new CoVs strain which could be more communicable and fatal in humans in near future. Though SARS-CoV-2 have an animal source, it has been noted that the principal route of transmission of coronavirus seems to be human interaction as per the World Animal Health Organisation (OIE) [57]. The implementation of interdisciplinary 'One Health' concept representing a holistic approach including human, animal and environment is the need of the day.

Conclusion
Hitherto, fi sh Nidoviruses are most prevalent in cyprinids and salmonids. The most common Nidovirus FHMNV severely affects the economically important freshwater baitfi sh industry in North America. It can also infect hatcheries where baitfi sh are used as a source of feed to fulfi ll the protein requirement of farmed fi shes. Therefore, some rigorous biocontrol measures such as quarantine, screening and certifi cation are required to control the transfer of infectious pathogen FHMNV to other farmed animals. Roniviridae, genus Okavirus (YHV) causes substantial economic losses in crustacean farming, therefore, the World Organization for Animal Health has declared it as an OIE notifi able disease. The systematic surveillance of aquatic mammal populations maintained in zoos and aquaria is crucial for ensuring the public health safety. Simultaneously, the discovery of new nidoviruses through continuous screening from the wild and cultured aquatic animals will aid to understand the mechanism of mutation and recombination in the viral genome. Such investigations will provide understanding of the evolutionary biology of Nidoviruses and their pathogenesis. Furthermore, the health of humans is intricately connected with the health of animals and the environment. Thus, a holistic approach, called "One Health" needs to be developed taking into account the global health issues, climate change, migration and its interconnections. Hence, it is important to understand the need of "One Health" concept and work on all the associated aspects to protect the health of humans, animals, and the environment.

Zoonotic potential
Zoonotic potential is high. The probable route of transmission is from reservoir host bat to intermediate host pangolin to fi nal host human [49].
No zoonotic potential has been recorded yet.

Major difference between Human coronavirus and Aquatic coronavirus
Zoonotic diseases are coupled with rapidly changing environment and human behaviour. To date, nidoviruses of aquatic organisms have not been reported to have any zoonotic potential nor has fi sheries proven to be associated with the transmission of coronavirus to humans worldwide in any report. Bondad-reantaso, et al. [53] also stated that SARS-CoV-2 might not infect aquatic animals. Therefore, the fi sh and fi shery products consumption are safe as per the statement given by Department of Fisheries, Ministry of Fisheries, Animal Husbandry and Dairying in 2020 [54]. However, there are chances of transmission of CoV to fi sh and fi shery product, if an infected person handles these products in processing plant or at wet market. The wet markets make an entire squall for cross-species transmission of pathogens. The nidoviruses are suspected to remain in the latent state in many animals