GENERAL INTRODUCTION: BUNYAVIRIDAE

BUNYAVIRIDAE

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

The family Bunyaviridae (from Bunyamwera, a location in Uganda where the type virus was isolated in 1943) is one of the largest virus families with more than 330 animal and plant viruses assigned to five genera: Orthobunyavirus (from the Greek orthos meaning “true or proper”), Phlebovirus (from phlebotomus/sandfly fever), Nairovirus (from Nairobi sheep disease) Hantavirus (from the Hantaan River, Korea, where the type virus was isolated) and Tospovirus (from tomato-spotted wilt, a large group of plant-only pathogens spread by sap-sucking insects) (Table 1).1 The exact number of virus members and organization of this widely divergent and globally spread virus family varies as new members of the family are discovered, or existing known viruses are characterized as members of a particular genus. A further approximately 50 inadequately characterized animal viruses are considered possible members of the family. Most of the known bunyaviruses were discovered in the course of surveys on haematophagous arthropods or wild vertebrates, and have no known medical or veterinary significance (most members of the Orthobunyavirus genus), however a few in the family are highly important pathogens of humans and/or livestock (e.g., Akabane virus, Crimean Congo hemorrhagic fever virus, and Rift Valley fever virus). Of the five genera, four are maintained by alternating replication and transmission cycles in arthropod vectors and mammalian hosts, with the exception of the rodent-borne Hantavirus genus whose members are not known to cause any veterinary disease, but can cause severe renal and respiratory disease in humans (eg., Sin Nombre virus – causing Hantavirus Pulmonary syndrome, and Hantaan virus – leading to Hemorrhagic fever with Renal Syndrome).

Across the Bunyaviridae, key morphological and genomic features unify the family. The viruses are roughly spherical, 80 to 120 nm in diameter, and have a host cell derived bilipid-layer envelope acquired during virion budding and maturation in the cellular golgi compartment.  This lipid envelope is decorated with virus-encoded glycoprotein spikes or peplomers required for virus attachment and infection. Genomes are organized into three-segments, of single-stranded RNA. These segments are named by size and comprise the L (large – encoding the virus polymerase), M (medium – encoding the virus glycoproteins), and S (small – encoding the nucleocapsid protein and other non-structural proteins) all of which are individually encapsidated in ribo-nucleoprotein complexes within the mature virion. Total genome size varies from 11 to 20 kb, and the sizes of the structural proteins and RNA segments vary with the genus.

Typically, the genomic RNA of the Bunyaviridae is encoded in the negative-sense (i.e., complementary to mRNA). However, the S segment of the Phlebovirus genome consists of ambisense RNA, i.e. has bi-directional coding, a property which is shared only with the RNA of viruses of the family Bunyaviridae and the unclassified plant pathogen Tenuiviruses. Genome replication and transcription is regulated by conserved residues located in the first 8 to 13 nucleotide bases at the 3’ ends of the RNA segments that are typically conserved at the virus genus level, with a complementary (palindromic) consensus sequence occurring at the 5’ end allowing for the formation non-covalently linked panhandle structures held in a loosely circular structure by nucleocapsd proteins; allowing for recognition and initiation of replication by the virus polymerase.

The approximately 170 viruses of the Orthobunyavirus genus, fall into 19 serogroups of antigenically related viruses, with weak cross-reactivity existing between some groups. The Phlebovirus genus is further divided into a sandfly fever virus group, the mosquito borne Rift Valley fever virus, and the tick-transmitted Uukuniemi virus group, with at least 32 additional ungrouped viruses. The genus Nairovirus contains at least 33 viruses in seven serogroups, and there are at least five serogroups in the genus Hantavirus, with a large and growing list of ungrouped viruses. As virus discovery efforts continue throughout the world, it is certain that the number of viruses within this broad and diverse virus family will continue to grow.

There is a broad tendency for antigenic grouping within genera to correlate with geographic distribution and with the type of vector involved in transmission. Most of the viruses of the family appear to be transmitted by culicine mosquitoes including aedines, but some are transmitted by anopheline mosquitoes. Simbu serogroup bunyaviruses are associated particularly with ceratopogonid midges (Culicoides spp.), while the sandfly fever serogroup of phleboviruses (apart from Rift Valley fever and a few other mosquito-borne viruses), are associated with phlebotomids (sandflies). The Tete serogroup of bunyaviruses, Uukuniemi serogroup of phleboviruses and the nairoviruses are associated with ixodid and argasid ticks. Some viruses have been isolated from more than one type of vector.

Table 1. Major disease causing viruses of the Bunyaviridae

GENUS SEROGROUP VIRUS DISEASE IN LIVESTOCK
Bunyavirus Simbu Akabane, Schmallenberg, and others Ruminant teratology
Phlebovirus: Sandfly fever group Rift Valley fever Rift Valley fever Rift Valley fever (zoonosis)
Uukuniemi group Uukuniemi Uukuniemi None
Nairovirus Nairobi sheep disease Nairobi sheep disease Nairobi sheep disease
Crimean-Congo haemorrhagic fever Crimean-Congo haemorrhagic fever None (zoonosis)
Hantavirus Hantaan Hantaan, Seoul, and others None (zoonoses: Europe & Asia)
Sin Nombre Sin Nombre, Andes, and others None (zoonoses:N&S America)

References

  1. https://talk.ictvonline.org/ictv-reports/negative-sense-rna-viruses-2011/w/negrna_viruses/205/bunyaviridae