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Torchivirus A
Unassigned torchiviruses
  Picorna-like viruses have been isolated from different species of tortoise and have been named virus “X”. The most common tortoise species affected have been spur-thighed tortoises (Testudo graeca) and marginated tortoises (T. marginata). Other species in which similar agents have been detected include Hermann’s tortoises (T. hermanni), leopard tortoises (Geochelone pardalis) and Egyptian tortoises (T. kleinmanni) (Marschang and Ruemenapf, 2002). The animals from which these agents have been isolated have presented various pathologies, including diphtheroid-necrotizing stomatitis and pharyngitis, conjunctivitis, rhinitis, pneumonia, enteritis and ascites. However, the virus has also been isolated from healthy animals (Marschang and Ruemenapf, 2002) and correlation with any specific disease remains to be proven. A virus, TGT1A/96, isolated from a spur-thighed tortoise was cultivated in Terrapene heart cells (TH-1) (Clark and Karzon, 1967) causing a lytic infection with an observable CPE. Hauser et al. (2010) have sequenced 7077 nt of the genome of TGT1A/96 (although an unknown length of the 5’ UTR remains to be determined). The predicted polyprotein was 2218 aa (6654 nt) long and had a typical picornavirus L-4-3-4 layout. The leader polypeptide (52 aa) contained seven cysteine residues and had some similarity to metallothionein-like proteins. The capsid region (784 aa) was most closely related to the erbo- (61% aa similarity). The 2B polypeptide was distantly related to the Theiler’s virus 2B (40% aa similarity), while 2C, 3C and 3D were most closely related to human cosavirus A (56% aa similarity), foot-and-mouth disease virus (62% aa similarity) and aichi virus (67% aa similarity), respectively. The 2A (145 aa), 3B (19 aa) and 3A (85 aa) polypeptides did not match any picornavirus proteins, however, 3B was short and contained a tyrosine a position 3, consistent with it functioning as the genome-linked protein, VPg. The predicted junction between 2A and 2B was NPG↓P, as it is in the most closely related picornaviruses. The 3’ UTR was 232 nt long and extremely A+T rich (87%). Within the 115 nt preceding the translation initiation codon, a 65 nt stretch had 73.8% identity with Aichi virus (genus Kobuvirus) suggesting some similarities with the type V IRES.

The near-complete genomes of seven tortoise picornaviruses have recently been described by Farkas et al. (2015). They have suggested the name topivirus from tortoise picornavirus.

Genome organisation:

VPg+5'UTR[L/1A-1B-1C-1D-2Anpgp/2B-2C/3A-3BVPg-3Cpro-3Dpol]3'UTR-poly(A)


References

Clark, H. F., and D. T. Karzon. 1967. Terrapene heart (TH-1), a continuous cell line from the heart of the box turtle, Terrapene carolina. Experimental Cell Research 48:263-268.

Farkas, S.L., Ihász, K., Fehér, E., Bartha, D., Jakab, F., Gál, J., Bányai, K. and Marschang, R.E. (2015). Sequencing and phylogenetic analysis identifies candidate members of a new picornavirus genus in terrestrial tortoise species. Arch Virol. 160: 811–816 [Epub ahead of print 2014 Nov 29].

Heuser, W., Kaleta, E., Giesow, K., Keil, G.M. and Knowles, N.J. (2010). Genome sequence of virus “X”, a picornavirus isolated from a spur-thighed tortoise (Testudo graeca). EUROPIC 2010: XVI Meeting of the European Study Group on the Molecular Biology of Picornaviruses, St. Andrews, Scotland, 11-16 September 2010. Abstract H15, p. 147.

Heuser, W., Pendl, H., Knowles, N.J., Keil, G., Herbst, W., Lierz, M. and Kaleta, E.F. (2014). Soft plastron, soft carapace with skeletal abnormality in juvenile tortoises. Histopathology and isolation of a novel picornavirus from Testudo graeca and Geochelone elegans. Tierarztl Prax Ausg K Kleintiere Heimtiere 42: 310-320.

Marschang, R.E., and T.H. Ruemenapf. 2002. Virus ‘‘X’’: characterizing a new viral pathogen in tortoises. In: Proceedings of the Association of Reptilian and Amphibian Veterinarians. Reno, 101–102.

Paries S, Funcke S, Lierz M. Investigations on the prevalence of tortoise picorna-virus in captive tortoises in Germany. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2018 Oct;46(5):304-308. doi: 10.15654/TPK-180156. Epub 2018 Dec 12. PubMed PMID: 30541170.
 
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