1 - Introduction
2 - Rift Valley Fever Virus
3 - Bluetongue Virus
4 - Vesicular Stomatitis
5 - Vocabulary
6 - Web Links
7 - Outside Reports
Introduction. Bluetongue virus (BTV) is an orbivirus that infects both domestic and wild ruminants and is transmitted by Culicoides spp. biting midges (Price and Hardy 1954, Borden et al. 1971, Matthews 1982). Among the arboviruses transmitted by Culicoides, BTV has the greatest economic impact (Bath 1989), with losses attributed to effects on animal health and productivity (Bowne 1971, Kvasnicka 1985, Luedke 1985, MacLachlan et al. 1985, MacLachlan et al. 1994, Bonneau et al. 2002), as well as non-tariff trade restrictions that affect the sale and movement of animals and germplasm (Roberts et al. 1993). The primary vector species for specific geographic regions vary. In the US the primary vector is C. sonorensis, whereas in Europe primary vectors include C. imicola, C. obsoletus, and C. dewulfi.
Disease distribution is dependent upon climate, competent vectors, pathogenic serotypes, and susceptible host animals capable of amplifying the virus; all of which must be considered in determining the risk of domestic animal populations to foreign animal diseases. Because of environmental restrictions on the distribution of competent vectors around the world, it had been assumed that BTV would only occur within certain latitudes (40•N to 35•S). However, the most recent BTV outbreaks in Europe have spread much further north (53•N). This may be associated with a warming climate, providing favorable conditions for potential vectors, and may result in higher infection rates, faster rates of virogenesis, earlier transmission (Mullens et al. 2004) and an overall increase in vector competence (Purse et al. 2005). Serotypes enzootic to the US are BTV-2, 10, 11, 13, and 17; however, exotic serotypes have been isolated recently in Louisiana (BTV-1) (Johnson et al. 2006) and Florida (BTV-3, 5, 6, 14, 19, and 22) (Johnson et al. 2007, Mertens et al. 2007, Mertens et al. 2008). From where these exotic serotypes originated has not been determined and raises the questions of whether the epidemiology of BTV in the US is changing and if these changes could result in more extensive livestock and wildlife disease (Gibbs et al. 2008).
Recent BTV outbreaks. Since 1998, invasion of six BTV serotypes in Europe have resulted in the largest outbreak of the disease on record. Among the numerous recent outbreaks (Appendix Table 1), the EU-BTV-8 outbreak has been unprecedented, with rapid spread and devastating morbidity and mortality. Sequence analysis has shown that the isolate originated in sub-Saharan Africa (Saegerman et al. 2008). Distinct serotypes of BTV are vectored by different species of Culicoides in specific regions of the world (Gibbs and Greiner 1994, Tabachnick 1996, 2004). Prior to the EU-BTV-8 outbreak, C. imicola was thought to be responsible for 90% of transmission and three other species were believed to play only a minor role in BTV epidemiology (Mellor 2004). However, field collections have implicated other endemic species as vital to local BTV transmission (Meiswinkel et al. 2007, Dijkstra et al. 2008).
The recent incursion of BTV throughout much of the Mediterranean Basin and into northern Europe and the UK has highlighted the risks that many countries in Europe face with regard to BTV, and the risk that the US faces for accidental or intentional introductions of serotypes with unknown virulence to na•ve US livestock. It is unknown whether the most predominant vector of enzootic strains of BTV in the US, C. sonorensis, is a competent vector for EU-BTV-8 or other exotic serotypes which may be introduced. However, the rapid spread in Europe, in the absence of what was thought to be its primary vector species, indicates that there is no required 'pre-adaptive' phase in indigenous Culicoides (Meiswinkel et al. 2007). This is of particular concern, as there are several endemic Culicoides species throughout the US considered to be refractory or nominal vectors and there is no knowledge of their potential as vectors of exotic BTV strains.
Another important epidemiological unknown is whether EU-BTV-8, if introduced, would cause devastating disease similar to that seen in Europe. It is not clear whether the high morbidity and mortality is due to the naivet• of the European livestock breeds, or whether EU-BTV-8 is truly a more virulent serotype. Sheep are the most susceptible livestock species in the US and although most of the common breeds are of similar European origin, it is possible that they may be partially protected due to the immune boosting effect from our circulating domestic serotypes.
In addition to livestock, BTV also infects wildlife. Most North American deer species appear to be much more susceptible to BTV than Eurasian deer species. Thus, it is not known whether an EU-BTV-8 outbreak would result in lower disease levels as those seen in Europe, or whether it would be more similar to outbreak levels seen with our enzootic serotypes. Infection by exotic BTV strains is of concern not only for the direct loss of wild ruminants, but also because they serve as virus reservoirs for the spread and perpetuation of disease outbreaks in livestock (Drolet et al. 1990, Gaydos et al. 2002). Circulation of virus among susceptible domestic and wildlife species is necessary for a complete and relevant epidemiological assessment and risk analyses for the US. White-tailed deer are the wildlife species most often affected by enzootic BTV, the most likely wildlife reservoir, and therefore, the most likely wildlife species to play a role in the ability of EU-BTV-8 to become established.
Mechanisms of infection, virulence and pathogenesis. The initial steps in infection involve binding of the virus, via the outer coat protein VP2 (Huismans et al. 1983, Cowley and Gorman 1987, Roy et al. 1990, Hassan and Roy 1999), to specific ligands on the cell surface and penetration of the cellular membrane. Studies have implicated cellular glycoprotein and carbohydrate moieties in this process; however, the cellular receptor(s) have not been identified. Preliminary data from our laboratory suggest that glycosaminoglycan carbohydrate moieties on the cell surface interact with virus, which leads to more efficient binding to a secondary receptor, necessary for internalization and subsequent replication.
Diagnosis. Initial diagnosis of BT may be indicated by the clinical disease signs (Elbers et al. 2008); however, diagnostic tests are necessary for accurate disease diagnosis and for trade regulations. Current diagnostic procedures rely on virus, antigen (Mecham et al. 1990, Mecham 1993), nucleic acid (Dangler et al. 1990, Akita et al. 1992, Wilson et al. 1992, Akita et al. 1993, Katz et al. 1993a, Katz et al. 1993b, Wilson and Chase 1993, MacLachlan 1994, Shad et al. 1997, Orru et al. 2004, Wilson et al. 2004, Shaw et al. 2007) or antibody detection (Drolet et al. 1990, Pearson et al. 1992, Mecham and Wilson 1994, Mecham 1997, Zhou et al. 2001) by various by enzyme-linked immunosorbent assays (ELISA) and polymerase chain reaction (PCR) assays. These are particularly valuable for virus detection in field collected insects and in studies of vector competency, and are recommended by the Office International des Epizooties (OIE).
Vaccines. In the US there is only one nationally licensed commercial vaccine (Colorado Serum Company, Denver CO). This vaccine is an attenuated BT-10 that may provide limited cross-protection to other serotypes, but no data are available. Two additional attenuated vaccines; 10, 11, and 17 for sheep, 10, 11, 13, and 17 for wildlife, are available from the California Wool Growers Association, through a state license. However, because these vaccines are not marked, infected and vaccinated animals cannot be differentiated, therefore, cattle are not routinely vaccinated. Both WY and MT are provisionally allowing the use of an inactivated BTV-17 vaccine from MO, in response to the recent outbreaks.
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