Discovery and Taxonomy
The first member of the family was discovered in 1997 by a group of researchers led by Nishizawa. The virus was isolated from the blood serum of a patient with the initials T.T. The clinical context of the discovery was concerning: the patient presented with post-transfusion hepatitis of unknown etiology, initially prompting researchers to suspect a novel infectious agent as the cause of the disease.
According to current classification, the Anelloviridae family comprises 9 genera. The taxon name derives from the Latin word anellus, meaning "ring," which directly reflects the specific structure of the viral genome.
The type species of the family is TTV (Torque Teno virus, meaning "thin necklace virus"), which belongs to the genus Alphatorquevirus. In addition to classical TTV, the family includes other variants differing in their characteristics, such as mini-TTV (Torque Teno mini virus) and midi-TTV (Torque Teno midi virus).
Virion Morphology and Genome Structure
Anelloviruses are simple viruses—they are non-enveloped, meaning they lack an outer lipid supercapsid. Virions are extremely small, with a diameter of only 30–32 nm.
Key structural features:
- Capsid: Possesses an icosahedral symmetry and consists of a structural protein designated as VP1.
- Genome: Composed of a circular single-stranded minus-sense DNA.
- Genetic diversity: The family is characterized by high variability, with researchers currently identifying over 20 different anellovirus genotypes.
Reproduction Features
The life cycle of anelloviruses is closely linked to host cells, though many details of their replication are still under investigation.
- Localization: Viral DNA replication is hypothesized to occur within the nuclei of hematopoietic (blood-forming) cells.
- Virion assembly: The final formation of new viral particles takes place in the cytoplasm of the infected cell.
- Cell egress: Formed virions leave the host cell via lysis (cell destruction).
- Reservoir: Hepatocytes (liver cells) are presumed to serve as the primary viral reservoir in the human body.
Epidemiology and Distribution
Despite their history of discovery associated with hepatitis, the epidemiological profile of anelloviruses is viewed quite differently today. They are detected in 90% of the healthy global population. In modern microbiology, anelloviruses are not regarded as classical pathogens, but rather as normal members of the human microflora—the so-called healthy virome.
Transmission occurs via several routes:
- Blood-borne route — transmission of the pathogen through infected blood and blood products.
- Sexual route — transmission via unprotected sexual contact.
Notably, anellovirus circulation is not restricted to the human population; viruses with similar characteristics circulate widely in nature among non-human primates as well.