A novel Asfarvirus-like virus identified as a potential cause of mass mortality of abalone

Abstract

A novel Asfarvirus-like virus is proposed as the etiological agent responsible for mass mortality in abalone. The disease, called abalone amyotrophia, originally was recognized in the 1980s, but efforts to identify a causative agent were unsuccessful. We prepared a semi-purified fraction by nuclease treatment and ultracentrifugation of diseased abalone homogenate, and the existence of the etiological agent in the fraction was confirmed by a challenge test. Using next-generation sequencing and PCR-based epidemiological surveys, we obtained a partial sequence with similarity to a member of the family Asfarviridae. BLASTP analysis of the predicted proteins against a virus database resulted in 48 proteins encoded by the novel virus with top hits against proteins encoded by African swine fever virus (ASFV). Phylogenetic analyses of predicted proteins of the novel virus confirmed that ASFV represents the closest relative. Comparative genomic analysis revealed gene-order conservation between the novel virus and ASfV. In situ hybridization targeting the gene encoding the major capsid protein of the novel virus detected positive signals only in tissue from diseased abalone. The results of this study suggest that the putative causative agent should be considered a tentative new member of the family Asfarviridae, which we provisionally designate abalone asfa-like virus (AbALV). African swine fever virus (ASFV) is the causative agent of African swine fever (ASF). The virus causes a hemorrhagic fever with high mortality, with rates approaching 100% in domestic pigs 1. The virus infects domestic pigs and their relatives and ticks 2. ASF outbreaks had been recorded in Africa and Europe, but in recent years the disease has spread to China, Vietnam, Cambodia, Mongolia, Hong Kong, and Korea, becoming a threat to the swine industry worldwide 3. ASFV is a member of nucleocytoplasmic large DNA viruses (NCLDVs) with an average diameter of 200 nm. Although some related viruses, such as faustovirus 4 , kaumoebavirus 5 , and pacmanvirus 6 , have been reported, ASFV is the only member of the Asfarviridae family 7. In the present paper, we describe a virus likely to be the closest ASFV relative found to date; this novel virus was isolated as the presumptive causative agent of abalone amyotrophia. Mass mortalities of abalone have been reported since the early 1980s, during seed production in Japan. The disease was designated abalone amyotrophia because diseased abalone develop muscle atrophy in the mantle and foot 8. Diseased abalone show reduced ability to adhere to the substrate, and some diseased abalone exhibit incisions on the front margin of the shell and brown pigmentation inside of the shell 9. Histopathological evaluation has revealed the presence of abnormal cell masses that are produced extensively, primarily in the ganglion and peripheral nerve of the foot muscle 9. Cumulative mortality can reach 50% and higher 10. Abalone herpesvirus (AbHV) 11,12 and abalone shriveling syndrome-associated virus (AbSV) 13 also cause mortality ac...

High Transcriptional Activity and Diverse Functional Repertoires of Hundreds of Giant Viruses in a Coastal Marine System

“…We also included all Nucleocytoviricota genomes available in NCBI RefSeq as of June 1st, 2020. Lastly, we also included several Nucleocytoviricota genomes from select publications that were not yet available in NCBI, such as the cPacV, ChoanoV, and AbALV viruses that have recently been described (42, 47, 64).…”

Comparative Genomics and Environmental Distribution of Large dsDNA Viruses in the Family Asfarviridae

“…For this study, we analyzed 35 Asfarvirus MAGs generated in two previous studies (Moniruzzaman et al, 2020a;Schulz et al, 2020) and complete genomes of five Asfarviruses (Reteno et al, 2015;Silva et al, 2015;Bajrai et al, 2016;Andreani et al, 2017;Matsuyama et al, 2020). MAGs were quality-checked using ViralRecall v. 2.0 (default parameters), with results manually inspected to ensure that no large non-NCLDV contigs were present (Aylward and Moniruzzaman, 2021).…”

“…Using amoeba as the host, two other Asfarviridae , Kaumoebavirus and Pacmanvirus, were isolated ( Bajrai et al, 2016 ; Andreani et al, 2017 ). Lastly, a culture independent study in early 2020 reported Asfar-like virus (AbalV) causing mass mortality in abalone ( Matsuyama et al, 2020 ). Together, these studies have begun to show that the Asfarviridae are likely a diverse family of NCLDV that are globally distributed and infect both protist and metazoan hosts.…”

Host Range and Coding Potential of Eukaryotic Giant Viruses

“…In addition to Mimiviridae and the two purely amoebal families, Marseilleviridae and Pithoviridae , viruses that infect Acanthamoeba or Vermamoeba have often been placed within the two NCLDV founding families Asfarviridae and Phycodnaviridae ( Figure 1 ). Asfarviridae , with its name derived from ASFVs—notorious viruses that kill pigs and boars, can be transmitted through tick (Chelicerata, Metazoa) vectors, and have close relatives infecting abalones (Mollusca, Metazoa) [ 64 , 78 ] ( Table S1 )—has recently been joined by related amoebal viruses with genomes smaller than 500 kb [ 13 , 35 , 38 , 39 ] and possibly also a marine algal virus infecting Heterocapsa (Dinoflagellata, Alveolata) [ 53 ]. Phycodnaviridae has lost some alga-infecting members, but at the same time, it has been joined by amoebal viruses characterized by the largest known genomes—pandoravirus.…”