ICTV Virus Taxonomy Profile: Nairoviridae.

Members of the family Nairoviridae produce enveloped virions with three single-stranded RNA segments comprising 17.1 to 22.8 kb in total. These viruses are maintained in arthropods and transmitted by ticks to mammals or birds. Crimean-Congo hemorrhagic fever virus is tick-borne and is endemic in most of Asia, Africa, Southern and Eastern Europe whereas Nairobi sheep disease virus, which is also tick-borne, causes lethal haemorrhagic gastroenteritis in small ruminants in Africa and India. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Nairoviridae, which is available at ictv.global/report/nairoviridae.

Where known, virions are spherical in shape, 80–120 nm in diameter with a membrane envelope decorated with glycoprotein (GP) spikes composed of GN and GC (Table 1, Fig. 1). Isolated ribonucleoprotein (RNP) complexes are composed of individual segments of genomic RNA encapsidated in nucleocapsid (N) protein. The RNPs are associated with large (L) protein.

Table 1.

Characteristics of members of the family Nairoviridae

Typical member:	Dugbe virus [S segment: AF434161; M segment: M94133; L segment: U15018], species Dugbe orthonairovirus, genus Orthonairovirus.
Virion	Enveloped, spherical virions 80–120 nm in diameter with heterodimer surface spikes
Genome	Three single-stranded, negative-sense RNA molecules, S, M, and L of about 2 kb, about 5 kb, and about 12 kb, respectively
Replication	Cytoplasmic. The nucleocapsid protein (N) encapsidates the genomic RNA forming ribonucleoprotein (RNP) complexes with the viral RNA-directed RNA polymerase (RdRP)-containing large protein (L). Anti-genomic RNAs are generated and serve as templates for synthesis of nascent RNP complexes containing genomic RNA
Translation	From capped mRNAs that lack poly(A) termini. The 5′-cap structure is derived from cellular mRNAs via cap-snatching
Host range	Birds, humans, rodents, hares, shrews, ruminants, bats, ticks (Orthonairovirus); spider vector (Shaspivirus) or water strider vector (Striwavirus) with unknown host range
Taxonomy	Realm Riboviria, phylum Negarnaviricota, class Ellioviricetes, order Bunyavirales; several genera and >15 species

Fig. 1.

(a) Transmission electron micrograph of a Crimean-Congo hemorrhagic fever virus particle. (b) Schematic illustration of a nairovirus particle.

The nairovirus genome (Fig. 2) consists of two to three single-stranded, negative-sense RNA molecules, termed S (small), M (medium; if present), and L (large). These RNAs encode respectively, in the virus-complementary sense, N, the GP precursor (GPC), and L (containing RdRP, helicase, and endonuclease domains).

Fig. 2.

Schematic representation of nairovirus genome organization.

Virions attach to unknown cell-surface receptors and enter via the endosomal route [1]. Viral fusion with the host cell results in early or late endosomal release of the virion RNP complex into the cytoplasm. This pH-dependent fusion event likely requires the previous participation of an intracellular receptor [1, 2]. During primary transcription the virion-associated L protein generates antigenomic RNAs, which are capped using host-cell-derived capped primers [3]. Translation is by free (L and S segment mRNAs) or membrane-bound (M segment mRNA) ribosomes. Based on evidence from Crimean-Congo hemorrhagic fever virus, GPC is co-translationally cleaved to yield glycosylated GN and GC and non-structural glycoproteins [4]. Antigenome RNA synthesized by the RdRP domain of the L protein serves as a template for genomic RNA replication. Secondary transcription amplifies the synthesis of mRNAs and genome replication. During morphogenesis, GN and GC accumulate in the Golgi, are terminally glycosylated, modified host membranes are acquired, and the virions bud into the Golgi cisternae [5, 6].

Current taxonomy: ictv.global/report/nairoviridae. Nairoviruses form a family in the polyploviricotine order Bunyavirales, and are most closely related to members of the family Wupedeviridae. Like most other bunyaviruses, nairoviruses (i) have multisegmented, negative-sense single-stranded RNA genomes; (ii) encode proteins with high sequence identity; (iii) have five conserved motifs (A–E) in their RdRP domain; and (iv) produce enveloped virions.

Resources

Current ICTV Report on the family Nairoviridae: ictv.global/report/nairoviridae