Bunyaviridae

The family Bunyavirales comprises viruses with a single-stranded RNA genome. The associated species are characterized by an enveloped, spherical virion. They infect arthropods, plants, protozoa, and vertebrates. Certain viruses of this order are human pathogenic. The name Bunyavirales is derived from the place Bunyamwera (Uganda), where the first representative "Bunyamwera orthobunyavirus" was discovered.

The majority of bunyaviruses are transmitted by vectors. With the exception of hantaviruses and arenaviruses, all viruses of the order Bunyavirales are transmitted by arthropods (mosquitoes, ticks or sand flies). Hantaviruses are transmitted by contact with rodent droppings. The occurrence of infections is closely linked to vector activity, e.g. viral infections transmitted by mosquitoes are more frequent in summer.

The diversity of the more than 200 Bunyavirus species is reflected in their clinical importance. For example, human infections caused by Nairovirus, the causative agent of Crimean-Congo hemorrhagic fever, are associated with high morbidity and mortality. Therefore, handling of these viruses is done in biosafety level laboratories.

Hantaviruses represent a medically important member of the order Bunyavirales. They occur worldwide, being relatively common in Korea, Scandinavia (including Finland), Russia, western North America, and parts of South America. Hantavirus infections are associated with high fever, pulmonary edema, and respiratory failure. The mortality rate varies widely depending on the form, being as high as 50% for New World hantaviruses (the Americas), up to 15% for Old World hantaviruses (Asia and Europe), and about 0.1% for Puumala virus (primarily in Scandinavia). Antibody response plays an important role in reduction.

Bunyavirales form enveloped, spherical virions with diameters of 80-120 nm. The viruses do not contain matrix proteins. Bunyaviruses have two- or three-part genomes consisting of a large (L) and small (S) or large (L), medium (M), and small (S) RNA segments. The total genome size ranges from 10.5 to 22.7 kbp. The RNA segments are single-stranded and present in a helical formation within the virion. They also have a pseudo-circular structure due to the complementary ends of the individual segments. The L segment encodes RNA-dependent RNA polymerase, which is required for viral RNA replication and mRNA synthesis. The M segment encodes the viral glycoproteins that protrude from the viral surface and help the virus attach to and invade the host cell. The S segment encodes the nucleocapsid protein (N). Integrins have been described as possible attachment receptors for the humapathogenetically important hantavirus.

Life cycle: The ambisense genome requires two rounds of transcription. First, the negative-sense RNA is transcribed to generate mRNA and a full-length replicative intermediate. From this intermediate, a subgenomic mRNA is produced that encodes the small segment nonstructural protein. While the polymerase produced after the first round of transcription can now replicate the full length RNA. Bunyaviruses replicate in the cytoplasm as the viral proteins move through the ER and Golgi apparatus. Mature virions bud from the Golgi apparatus into vesicles that are transported to the cell surface and shed from the cell.

Human pathogenic species of the Bunyaviridae involve the following genera:

Orthobunyavirus (>150 species or subtypes; they are transmitted by mosquitoes). Significant for Europe are:

• Tahyna virus (pathogen reservoir: hedgehogs and rabbits)
• Inkoo virus (reservoir: elk and reindeer).

In the Midwest of the USA (1/§ of all arthropod-borne viral encephalitides are due to this orthobunyavirus), endemic are:

• California encephalitis virus
• La Crosse virus
• Jamnestown canyon virus
• Snowshoe-hare virus

Phlebovirus (3 serogroup types, more than 45 species) carriers are butterfly mosquitoes- Phlebotomus species)

• Phlebotomus fever virus: In Europe, the Phlebotomus fever virus plays a role (Tuscany type). It causes Pappataci fever (sandfly fever) with photophobia, neck stiffness and arthralgia.
• Rift Valley fever virus: 1977 epidemic in Egypt with 20,000 cases, complicating encephalitis.

Nairovirus (6serogroups, 34 species) transmitted by ticks. Clinically significant is the

• Crimean-Congo haemorrhagic fever syndrome, which can be associated with high lethality.

Hantavirus (infection does not occur through arthropods but through smear infections and dust injections with excrements of rodents, e.g. mice). Of importance for Europe are:

• Hantaan virus
• Puumula virus
• Dobrava-Belgrade virus

1. Hof H (2019) Special Virology. In: Hof H, Schlüter D, Dörries R, eds Duale Reihe Medizinische Mikrobiologie. 7th, completely revised and expanded edition. Stuttgart: Thieme p 240-241