Hepatitis D Viruses form the genus Deltavirus that is unassigned to any virus family. So, it's a very lonely group of viruses. A Hepatitis D Virus is a so-called satellite virus and needs a Hepatitis B Virus to make infectious particles. Deltaviruses are thought to have evolved in humans, since for a long time, they had not been identified elsewhere.
Mother Nature always manages to surprise us, because recently a Hepatitis D Virus was discovered in birds
[1]. Spurred on by this discovery scientists recently identified a novel Hepatitis D Virus in boa contrictors (
Boa constrictor) and named it snake Hepatitis D Virus (shortened to SHDV)
[2]. The scientists also detected snake Hepatitis D Virus RNA in a water python (
Liasis mackloti savuensis) in the same snake colony, potentially indicating vertical and horizontal transmission.
Sequence comparison showed the snake Hepatitis D Virus antigen (sHDAg) to be 55% and 37% identical to its human and avian counterparts. However, the scientists were unable to detect accompanying Hepatitis B viruses, suggesting that snake Hepatitis D Virus could be a satellite virus of an (at the moment) unknown enveloped virus.
The identification of Hepatitis D Viruses in distantly related species other than humans indicates that the previously suggested hypotheses on the origins of Hepatitis D Viruses need to be updated.
[1] Wille et al: A divergent hepatitis D-like agent in birds in bioRxiv – 2018
[2] Hetzel et al: Identification of a Novel Deltavirus in Boa Constrictors in mBio - 2019
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