Beyond hepatitis D virus: the discovery of animal deltaviruses, their diversity, helpers and their position in an expanding world of circular RNA replicons.

Hepatitis D virus (HDV) was long considered a singular human satellite virus that depends on hepatitis B virus (HBV) for envelopment and transmission. Recent metatranscriptomic mining has uncovered a growing diversity of HDV-like viruses, we term deltaviruses, across vertebrates, invertebrates, and environmental samples, overturning assumptions about strict hepatic tropism and obligatory hepadnavirus helpers. Here, we synthesize shared structural and replication principles of deltaviruses: rolling-circle amplification by host polymerases and ribozyme-driven RNA processing. We then discuss emerging models of helper-virus association, including flexible glycoprotein usage and 'Trojan Horse' co-packaging within unrelated virions, and consider implications for tissue tropism, host shifting, and zoonotic potential. Finally, we highlight key priorities for linking sequence discovery to mechanism, ecology, and disease.