Carboviruses have so far only been detected in pythons (Hyndman et al., 2018). They are currently known only from genome sequences.
Physicochemical and physical properties
By analogy to other members of the family Bornaviridae, jungle carpet python virus (JCPV) and southwest carpet python virus (SWCPV) genomes are assumed to consist of negative-sense non-segmented single-stranded RNA (about 8.8 kb) (Hyndman et al., 2018).
Based on sequence comparisons, carbovirus genomes encode homologs of orthobornavirus proteins N, X, P, M, G, and L (see Table 1.Orthobornavirus in the Orthobornavirus genus section).
Genome organization and replication
The genome organization of carboviruses resembles that of culterviruses, having the gene order 3′-N-X/P/G-M-L-5′ (Figure 1.Bornaviridae). This organization differs from that of orthobornaviruses and other mononegaviruses (3′-N-X/P-M-G-L-5′). Carbovirus transcription units have not been validated experimentally, though conserved sequence motifs similar to orthobornavirus transcription initiation and termination sites have been identified at the expected locations in carbovirus genomes. Putative termination sites with the sequence 3′-UAA[A/G]AAA-5′ are present at the ends of the predicted N, G, and L ORFs. Putative transcription initiation sites with the sequence 3′-CCGAA[A/C][A/C]A-5′ have been identified upstream of the N, X, and M ORFs. Conserved splice donor and acceptor sites have not yet been identified in the genome sequences and no evidence of splicing was identified in Illumina sequence datasets, though it is unclear whether depth of coverage was sufficient for detection of spliced molecules.
The replication of carboviruses has not yet been described.
The presumed reservoir hosts of carboviruses are reptiles. JCPV and SWCPV were first found in a jungle carpet python (Reptilia: Pythonidae: Morelia spilota cheynei Wells & Wellington, 1984) and a south-west carpet python (Reptilia: Pythonidae: Morelia spilota imbricata (L.A. Smith, 1981)), respectively. Also, sequences have been detected in pythons (Reptilia: Pythonidae) of a variety of species of the genus Antaresia and a number of different Morelia spilota (Lacépède, 1804) subspecies (Hyndman et al., 2018).
Infection and Pathogenicity
Some pythons testing positive for carbovirus RNA have neurological signs of disease (e.g., incoordination, progressive caudal paresis). Some, but not all, snakes with neurological signs have had nonsuppurative encephalitis (Hyndman et al., 2018).
Controlled infection studies have not been reported but carbovirus RNA has been detected in blood as well as oral and cloacal swabs from infected snakes. Consequently, carbovirus transmission is presumed to occur via oral or cloacal secretions or both (Hyndman et al., 2018). The possibility of carbovirus transmission by hematophagous ectoparasites has not been investigated.
Species demarcation criteria
Criteria for bornavirus species demarcation are based on genomic characteristics, including PAirwise Sequence Comparison (PASC), in combination with biological characteristics, such as antigenic relationships and natural host ranges. In agreement with these additional criteria the bornavirus species differentiation cut-off for PASC of coding-complete genome sequences was defined as 72–75% (Kuhn et al., 2015). The JCPV and SWCPV genomes share 69% pairwise nucleotide identity.