Emergence of a Novel Pathogenic Poxvirus Infection in the Endangered Green Sea Turtle (Chelonia mydas) Highlights a Key Threatening Process

Abstract

Emerging viral disease is a significant concern, with potential consequences for human, animal and environmental health. Over the past several decades, multiple novel viruses have been found in wildlife species, including reptiles, and often pose a major threat to vulnerable species. However, whilst a large number of viruses have been described in turtles, information on poxvirus in cheloniids remains scarce, with no molecular sequence data available to date. This study characterizes, for the first time, a novel poxvirus, here tentatively designated cheloniid poxvirus 1 (ChePV-1). The affected cutaneous tissue, recovered from a green sea turtle (Chelonia mydas) captured off the Central Queensland coast of Australia, underwent histological examination, transmission electron microscopy (TEM), DNA extraction and genomic sequencing. The novel ChePV-1 was shown to be significantly divergent from other known poxviruses and showed the highest sequence similarity (89.3%) to avipoxviruses (shearwater poxvirus 2 (SWPV2)). This suggests the novel ChePV-1 may have originated from a common ancestor that diverged from an avipoxvirus-like progenitor. The genome contained three predicted unique genes and a further 15 genes being truncated/fragmented compared to SWPV2. This is the first comprehensive study that demonstrates evidence of poxvirus infection in a marine turtle species, as well as a rare example of an avipoxvirus crossing the avian-host barrier. This finding warrants further investigations into poxvirus infections between species in close physical proximity, as well as in vitro and in vivo studies of pathogenesis and disease.

Keywords: green sea turtle; poxvirus; skin lesions; species conservation.

Figure 1

Pathological and transmission electron microscopic analysis of cutaneous tissue collected from a wild green sea turtle. (A) Histological changes are characterized by vacuolation and intracytoplasmic eosinophilic inclusions in cells in the stratum spinosum with frequent displacement of the nucleus to the periphery (blue arrows). H&E stain, scale bar = 40 µm. (B) Transmission electron microscopy (TEM) on tissue section, no viral particles were discerned in these inclusions, which appeared to be composed of proteinaceous material. Scale bar = 2.5 µm. (C) Cheloniid poxvirus particles showing brick-shaped virion with regularly spaced thread-like ridges comprising the exposed surface, measuring approximately 140 nm × 98 nm. Outer envelope of cheloniid poxvirus particles is highlighted with a blue arrow.

Figure 2

Comparative genomic illustration of the novel Cheloniid poxvirus 1 (ChePV-1) with shearwater poxvirus-2 (SWPV2). Sequence alignment using MAFFT in Geneious (version 10.2.2) was performed to compare open reading frames (ORFs) between cheloniid poxvirus (ChePV-1, GenBank accession no. MT799800) and shearwater poxvirus-2 (SWPV2, GenBank accession no. KX857215). The arrows symbolize genes and open reading frames (ORFs), indicating their direction of transcription. Each gene or ORF is color-coded, as indicated by the color key in the legend.

Figure 3

Dot plots of the ChePV-1 genome (x-axis) vs. other selected sequenced poxvirus genomes (y-axis). (A) ChePV-1 vs SWPV2, (B) ChePV-1 vs canarypox virus (CNPV) (C) ChePV-1 vs shearwaterpox virus-1 (SWPV1) and (D) ChePV-1 vs fowlpox virus (FWPV). The Classic color scheme was chosen in Geneious (version 10.2.2) for the dot plot lines according to the length of the match, from blue for short matches, to red for matches over 100 bp long. Window size = 12.

Figure 4

Phylogenetic relationship between novel cheloniid poxvirus 1 (ChePV-1) and other chordopoxviruses. (A) Bootstrap Network based on the complete genome sequences of the selected ChPVs. (B) Maximum likelihood (ML) tree and (C) Bootstrap Network based on the concatenated amino acid sequences. The numbers on the left show bootstrap values as percentages, and the ChePV-1 is highlighted with magenta color. The labels at branch tips refer to original ChPVs GenBank accession number followed by abbreviated species name.