Orthonairovirus

Orthonairovirus is a genus in the family Nairoviridae of the order Bunyavirales that include viruses with circular, negative-sense single stranded RNA.[1] It got its name from the Nairobi sheep disease that affects the gastrointestinal tracts of sheep and goats.[1] The vast majority, and perhaps all viruses in this genus are tick-borne viruses that can have human or other vertebrate hosts.[2]

Orthonairovirus
Crimean-Congo hemorrhagic fever orthonairovirus virion and genome
Virus classification
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Negarnaviricota
Class: Ellioviricetes
Order: Bunyavirales
Family: Nairoviridae
Genus: Orthonairovirus
Type species
Dugbe orthonairovirus

Structure

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.

Genome
Crimean-Congo hemorrhagic fever virus (CCHFV) genome

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.[2] The large segment is about 11000–14400 nucleotides long (11–14.4 kb), and it encodes the viral polymerase.[1][3] 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][3] The small segment is about 1,700–2,100 nucleotides long (1.7–2.1 kb), and it encodes the nucleocapsid protein.[2]<[1][3]

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]

Replication
Nairovirus life cycle

Nairoviruses attach to the host receptor by their Gn-Gc glycoprotein dimer.[3] The virus is then endocytosed into the host cell via a vesicle. The ribonucleocapsid segments are released into the cytoplasm, commencing transcription.[3] Transcription and replication occur within the cell, and the newly synthesized virions are released by budding.

Transmission and distribution

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.[3]

Clinical importance

Only four viruses in this genus have, to date, been recognised as human pathogens:

  • Crimean-Congo hemorrhagic fever virus
  • Dugbe virus
  • Nairobi sheep disease virus
  • Kasokero virus.

A fifth— Erve virus —may also be pathogenic for humans.

Evolution

Phylogenetic analysis has shown that these viruses fall into two major monophyletic groups, the hard (Ixodidae) and soft (Argasidae) tick-vectored groups.[4] Fossil and phylogenetic data places the hard tick-soft tick divergence between 120 million years ago and 92 million years ago. 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

  • Crimean-Congo hemorrhagic fever
  • Nairobi sheep disease
  • Sakhalin
  • Tamdy

The soft bodied tick serogroups are

  • Hughes
  • Dera Ghazi Khan
  • Qalyub

The tick vectors for the Kasokero and Thiafora serogroups are not currently known.

Taxonomy

Nairobi sheep disease orthonairovirus (NSD) is the prototype virus of the genus.[2][5] NSD is found in East and Central Africa, and causes acute hemorrhagic gastroenteritis in sheep and goats. The genus includes 15 species:[1]

Serogroups

The genus is divided into at least nine serogroups. The Hughes and Sakhalin serogroups appear to be sister groups. Within each serogroup are a number of related viruses:

Crimean-Congo hemorrhagic fever serogroup

Crimean-Congo hemorrhagic fever virus
Hazara virus
Tofla virus

Dera Ghazi Khan serogroup

Abu Hammad virus
Abu Mina virus

Hughes serogroup:

Caspiy virus
Farallon virus
Hughes virus
Puffin Island virus
Punte Salinas virus
Raza virus
Soldado virus
Zirqa virus

Kasokero serogroup

Kasokero virus
Leopards Hill virus
Yogue virus

Nairobi sheep disease serogroup

Dugbe virus
Ganjam virus
Kupe virus
Nairobi sheep disease virus

Qalyub serogroup

Bandia virus
Chim virus
Geran virus
Qalyub virus

Sakhalin serogroup

Avalon virus
Paramushir virus
Sakhalin virus
Tillamook virus

Tamdy serogroup

Burana virus
Tamdy virus

Thiafora serogroup

Erve virus
Thiafora virus

Unclassified

Ahun virus
Artashat virus
Burana virus
Gossas virus
Issyk-Kul virus
Keterah virus
South Bay virus
Tamdy virus
Uzun Agach virus

References
  1. "ICTV Report Nairoviridae".
  2. Crabtree, Mary B., Rosemary Sang, and Barry R. Miller. "Kupe Virus, a New Virus in the Family Bunyaviridae, Genus Nairovirus, Kenya." Emerging Infectious Diseases 15 (2009): 147–54.
  3. "Nairovirus." Nairovirus. Viral Zone. <http://www.expasy.ch/viralzone/all_by_species/251.html>.
  4. Honig JE, Osborne JC, Nichol ST (2004) The high genetic variation of viruses of the genus Nairovirus reflects the diversity of their predominant tick hosts. Virology 318(1):10-16
  5. Clerx, John PM, Jordi Casals, and David HL Bishop. "Structural Characteristics of Nairoviruses (Genus Nairovirus, Bunyaviridae)." Journal of General Virology 55 (1981): 165–78.
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