More than 300 viruses, mostly arthropod-transmitted, are classified into the family Bunyaviridae, making it one of the largest groupings of animal viruses (Karabatsos, I985). Until relatively recently these viruses were somewhat the "Cinderellas" of animal virology, but with the increased recognition of their role in human diseases together with the results generated by the application of molecular techniques, the Bunyaviridae have achieved greater respectability. Rift Valley fever, Crimean- Congo haemorrhagic fever and California encephalitis vIruses are serious human pathogens that are classified in the family Bunyaviridae. In the tropics febrile illnesses are often diagnosed under the "great umbrella" (Downs, I975) of malaria and treated as such; in fact many cases are probably caused by members of the Bunyaviridae, although true diagnosis is rarely achieved (Shope, I985). Hantaan and related viruses, the causative agents of haemorrhagic fever with renal syndrome, are now recognized as belonging to the Bunyaviridae (Schmaljohn & Dalrymple, 1983) and cause a severe haemorrhagic disease with significant mortality throughout Asia, especially in China. As will be discussed later the Bunyaviridae have the capacity for sudden dramatic variation comparable to the antigenic shift associated with influenza viruses and thus warrant continual surveillance. At an academic level the Bunyaviridae display novel features in their gene coding arrangements and in their replication to maintain scientific interest. On a lighter note the Bunyaviridae also offer the opportunity to work with viruses having such glorious names as Main Drain, Pongola, Mahogany Hammock, Wongal etc. ; the reader is referred to the International Catalogue of Arboviruses (Karabatsos, 1985) for other gems.
The criteria for inclusion in the family Bunyaviridae are the following : virus particles are spherical, 99-100 nm in diameter and enveloped with glycoprotein surface projections ; the virions contain three unique segments of negative-sense single-stranded RNA in the form of cIrcular ribonucleoprotein complexes (nucleocapsids) and a transcriptase enzyme. Viruses have the ability to lnteract genetically with certain other closely related vIruses by genome segment reassortment. The viruses replicate in the cytoplasm of the infected cell and mature by budding into smooth-surface vesicles in or near the Golgi region (Bishop er al., I980).
In this review recent data concerning the molecular characteristics of the Bunyaviridae will be discussed to provide an update on previous accounts of this area (e.g. Obijeski & Murphy, I977; Bishop & Shope, I979; Bishop. I979, I985 a). Important aspects at the biology and pathogenesis of the Bunyaviridae are afforded only cursory treatment, but have been covered elsewhere (e.g. Parsonson & McPhee, I985; Shope, 1985 ; Canonico & Pifat, 1987; Beaty & Bishop I988 ; Gonzalez-Scarano et al., 1988; Monath, 1988; Tesh, 1988).
Outlook
Our knowledge of the molecular biology of the Bunyaviridae has expanded rapidly over the last few vears revealing the different coding strategies used by individual genome segments and providing some clues about the evolutionary relationships between the viruses, but these data are far from complete. As might be expected from such a large taxonomic grouping of viruses considerable diversity exists within the family. At the very least we need to know the entire genome sequences of a representative of each of the five genera (and putative members of the Bunvaviridae, such as tomato spotted wilt virus) to assess the extent of this diversity in terms of coding and replicative strategies. This informatlon Is required before re-evaluation of the classification wlthin the family can be undertaken. More extensive sequencing projects are needed to provide data for comparative purposes to further our understanding of the phylogenetic relationships within the family. Of particular interest would be analyses of the polymerase proteins, since the bunyavirus polymerase appears unrelated to other negative-strand virus polymerases, whereas the homologous proteins of rhabdoviruses and paramyxoviruses show a clear evolutionary relationship (Tordo et al., I988). Data on the Bunyaviridae polymerase may supply evidence to suggest whether there was a single progenitor, or whether there were multiple ancestral viruses.
Nucleotide sequence data provide the foundation to elucidate gene function and this must be a major avenue for future research. In order to design effective strategies to control the important pathogens in the family we need an understanding of the functions of the viral proteins in replication, the role played by these proteins in biological aspects such as virulence or interaction with vectors and the recognition of these proteins by the immune system. A serious obstacle to some of this work is our inability to produce viruses containing specific genetic alterations - the development of such a system presents a great challenge.