ICTV Virus Taxonomy Profile: Anelloviridae 2026

Abstract

The family Anelloviridae includes viruses with circular, negative-sense, ssDNA genomes of 1.7–3.9 kb. Anelloviruses form non-enveloped icosahedral virions constructed of the ORF1 protein, which adopts the single jelly-roll structural fold. Anelloviruses have been isolated from a broad range of birds and mammals, including humans, but are not associated with pathogenicity, except for members of the genus Gyrovirus. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Anelloviridae, which is available at www.ictv.global/report/anelloviridae.

Virion

Anellovirids form non-enveloped, icosahedral (T=1) capsids, 30–32 nm in diameter (Table 1). The capsid is built from 60 copies of the capsid protein (encoded by orf1), which adopts the single jelly-roll structural fold [1]. In some anellovirids, ORF1 protein contains a large insertion within the jelly-roll domain corresponding to a (spike) domain, which projects away from the capsid surface, forming crown-like structures around the fivefold symmetry axes (Fig. 1) [2].

Table 1. Characteristics of viruses in the family Anelloviridae.

Example	torque teno virus 1 (AB041007), species Alphatorquevirus homin1
Virion	Non-enveloped, icosahedral (T=1) capsid, 30–32 nm in diameter
Genome	Circular molecule of negative-sense ssDNA (1.7–3.9 kb)
Replication	Nuclear
Translation	From multiple, spliced mRNAs
Host range	Mammals, birds
Taxonomy	Realm Floreoviria, kingdom Shotokuvirae, phylum Commensaviricota, class Cardeaviricetes, order Sanitavirales: >35 genera, >240 species

Fig. 1. Anellovirus particles. (a) Negative-stained electron microscopy image of anellovirus-like particles generated from TTMV-LY1 constructs. (b) Cryo-EM structures of anellovirus-like particles (Images from Ring Therapeutics, Inc., [2]).

Genome

The genomes of anelloviruses are non-segmented, negative-sense circular ssDNA molecules of 1.7–3.9 kb [3,4]. The putative non-coding region generally has a high G+C content and is postulated to form secondary structures composed of stem loops. Most anelloviruses have three genes, known as orf1 (vp1 in gyroviruses), orf2 (vp2 in gyroviruses) and orf3 (vp3 in gyroviruses), although certain anelloviruses may carry additional ORFs (Fig. 2). The largest gene is orf1, which codes for the capsid protein. The proportion of the genome occupied by orf1 varies widely between species [4]. The N-terminal region of ORF1 protein is largely unstructured and rich in arginine residues; it is postulated to be important for nuclear localization, signalling and binding to the viral genome [5]. This region is followed by a jelly-roll domain, which plays a key role in capsid formation, and the spike domain, predicted to be important for host recognition and/or immune evasion [1,2]. The other ORFs typically overlap orf1 at both 5′- and 3′-termini.

Fig. 2. Organization of representative anellovirid genomes.

Replication

Knowledge of genome expression and replication mechanisms remains limited, mainly owing to the lack of an efficient cell culture propagation system. At least three mRNAs of different sizes are transcribed from the putative circular double-stranded replicative form of the anellovirus genome. Recent evidence suggests that the ORF2/3 product plays a role in initiating viral genome replication through a recombination-dependent mechanism with the aid of host-encoded replication factors [6].

Pathogenicity

Infection is highly prevalent and often chronic. No direct links to disease have been established for anelloviruses other than chicken anaemia virus (genus Gyrovirus), which is associated with immunosuppressive disease in chickens, resulting in a range of clinical outcomes including weight loss, anaemia and intramuscular haemorrhaging [7]. By contrast, in mammalian hosts, the nearly ubiquitous presence of anelloviruses without apparent pathogenesis has led to the hypothesis that these viruses are commensals [8].

Taxonomy

Current taxonomy: ictv.global/taxonomy. The family Anelloviridae includes >35 genera and >240 species. Members of all but one genus are associated with mammals (or blood-feeding insects); members of the remaining genus, Gyrovirus, include members identified in birds or faecal samples from other animals.

Resources

Full ICTV Report on the family Anelloviridae: www.ictv.global/report/anelloviridae.