Family: Picornaviridae

Genus: Kobuvirus

Distinguishing features

The genus is distinguished on the basis of genetic characters. 

Virion

Morphology

Unlike other picornaviruses, kobuvirus capsids show a distinctive surface morphology when observed by electron microscopy (Figure 1E. Picronaviridae). The crystal structure of Aichi virus A (AiV-A1) was recently resolved (Sabin et al., 2016)

Physicochemical and physical properties

Virions are stable at pH 3.5. 

Nucleic acid

Genome (Yamashita et al., 1998, Yamashita et al., 2003, Reuter et al., 2008, Reuter et al., 2009, Smits et al., 2013, Otomaru et al., 2016, Pankovics et al., 2016): up to 8,280 nt (5′-UTR: up to 744 nt; ORF: 7,299–7,467 nt; 3′-UTR: 136–240 nt). The genome of Aichi virus (AiV-A) has a high G+C base composition (59%). The 3′-UTR differs in the three species: members of Aichivirus A have the longest 3′-UTR at 240 nt, however, the 3′-UTRs of bovine kobuvirus (BKV, Kobuvirus B) and porcine kobuvirus (PKV, Kobuvirus C) are shorter at 177 and 170 nt, respectively. In AiV-A1, there is a 5′-proximal stem-loop involved in RNA replication and encapsidation. The IRESs of members of the Aichivirus A and Aichivirus B species are of type V, however, those of Aichivirus C (PKV) possess a type IV IRES. The cre is located in the 2A gene region of rabbit kobuvirus but has not been identified in other kobuviruses. 

Genome organization and replication

Genome layout:

VPg+5′-UTR^IRES-[L/1AB-1C-1D/2A^H-box/NC-2B-2C/3A-3B-3C-3D]-3′-UTR-poly(A)

The deduced polyprotein is of c. 2,432–2,488 amino acids. There is a leader polypeptide of unknown function, and distinctive length (170–195 aa). 1AB remains uncleaved; however, a myristoylation signal (GxxxT) is present at the amino terminus of the polypeptide. The 2A protein contains an H-Box/NC motif. 

Biology

AiV-A1 grows in cell cultures (BSC-1, Vero). AiV-A1 is thought to be a cause of human gastroenteritis. Kobuviruses have been isolated from humans, cattle, sheep and pigs. Kobuvirus genomes have been detected in faeces of dogs, cats, mice, marmots, rats, ferrets, goats, rabbits, bats, birds and in sewage. 

Twenty genetic types are distinguished by means of phylogenetic analysis (Aichivirus A: 10 types, Aichivirus B: 3 types, Aichivirus C: 2 types, Aichivirus D: 2 types, Aichivirus E: 1 type, Aichivirus F: 3 types). 

Antigenicity

Aichivirus and bovine kobuvirus have been distinguished by neutralization of infectivity and serological assays. 

Derivation of names

Aichivirus: from the Aichi prefecture in Japan

Kobuvirus: from Japanese kobu, 'knuckle, bump' (reference to surface structure of virus particle). 

Species demarcation criteria

Members of a species of the genus Kobuvirus:

• are less than 30% divergent in polyprotein aa sequence
• are less than 30% divergent in P1 aa sequence
• are less than 20% divergent in 2C + 3CD aa sequence
• share a common genome organization

The divergence (number of differences per site between sequences) of members of different Kobuvirus species ranges from 0.37–0.60 for P1 and 0.21–0.48 for 3CD. 

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations. 

* partial genome