Orthonairovirus is a genus of viruses in the family Nairoviridae of the order Bunyavirales which includes viruses with circular, negative-sense single stranded RNA.[1] The name is derived from the Nairobi sheep disease which affects the gastrointestinal tracts of sheep and goats.[1] All viruses in this genus are tick-borne viruses with human or other vertebrate hosts.[2]
The virions for viruses in this genus have a spherical shape.[3] They range in size from about 80–120 nm in diameter, with 50% of their weight attributed to proteins and 20–30% of their weight attributed to lipids.[1] The ribonucleocapsid is filamentous, having a length of about 200-300 nm and a width of about 2–2.5 nm.[1] These nucleocapsids are surrounded by a single envelope that has projections made of glycoproteins protruding from its surface. These projections evenly cover the surface of the virion, and are about 5–10 nm long.[1] They aid in attachment to the host receptor in replication.
Nairovirus genomes are negative sense, single-stranded RNA. The complete genome is about 17,100–22,800 nucleotides long, and is divided into three segments: large, medium, and small. The large segment is about 11000–14400 nucleotides long (11–14.4 kb), and it encodes the viral polymerase.[1] The medium segment is about 4,400–6,300 nucleotides long (4.4–6.3 kb), and it encodes for glycoproteins G¬n and Gc.[1] The small segment is about 1,700–2,100 nucleotides long (1.7–2.1 kb), and it encodes the nucleocapsid protein.<[1]
The genome has terminally redundant sequences, with the sequences being repeated at both ends. The terminal nucleotides are base-paired forming, non-covalently closed, circular RNA.[1] Both the 5’ and 3’ ends have conserved regions, 9 nucleotides in length. The sequences are, 5’end: UCUCAAAGA, and 3’end: AGAGUUUCU.[1]
Nairoviruses attach to the host receptor by their Gn-Gc glycoprotein dimer. The virus is then endocytosed into the host cell via a vesicle. The ribonucleocapsid segments are released into the cytoplasm, commencing transcription. Transcription and replication occur within the cell, and the newly synthesized virions are released by budding.
Members of this viral genus infect many different vertebrate hosts, and are transmitted via ticks.[1]
Members of the genus Nairovirus may be found the world over, wherever their arthropod vectors and vertebrate hosts are found together.
As of 2020 the genus included 41 species:[4]
In 2021, Yezo virus was described in Japan,[5] Beji nairovirus was detected,[6] and Tacheng tick virus in China.[7] In 2024, Songling virus[8] and Wetland virus were described.
Members of this viral genus which infect humans include Crimean–Congo hemorrhagic fever, Dugbe virus, Nairobi sheep disease virus, Songling virus, Yezo virus, Tacheng tick virus, Beiji orthonairovirus and Wetland virus. Except for the first they cause relatively mild disease.[9]
Kasokero virus[10] and Erve virus are likely also pathogenic for humans.[8]
Phylogenetic analysis has shown that these viruses fall into two major monophyletic groups, the hard (Ixodidae) and soft (Argasidae) tick-vectored groups.[11] Fossil and phylogenetic data places the hard tick-soft tick divergence between and . This suggests that the Nairoviruses have been associated with these ticks for over 100 million years.
Additionally, nairoviruses vectored by ticks of the genera Argas, Carios and Ornithodoros form three separate monophyletic lineages, again supporting the suggestion of host-virus cospeciation.
The hard bodied tick serogroups are
The soft bodied tick serogroups are
The tick vectors for the Kasokero and Thiafora serogroups are not currently known.