Changes

Althbough bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts and are most abundantly fed on by ''Culicoides'' vectors. Infection of sheep with BTV is therefore usually preceded by widespread infection of cattle and an increase in vector density¹.  

Althbough bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts and are most abundantly fed on by ''Culicoides'' vectors. Infection of sheep with BTV is therefore usually preceded by widespread infection of cattle and an increase in vector density¹.  

Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV². This is illustrated by the fact that bluetongue virus is limited to geographical areas where competent vectors are present and that transmission only occurs at times of the year when conditions are favourably for vector activity¹. In Britain, transmission occurs mainly in late summer and autumn. Once bluetongue virus is transmitted to a vertebrate host, there are two possible outcome: either the host may die, or an immune response is mounted against the virus and renders the host resistant to re-infection. Either way, animals quickly become "unavailable" for BTV infection as the virus spreads, particularly where livestock populations are small. This presents a hurdle that must be surmounted if bluetongue virus is to persist in an area. By movement of infected vectors or viraemic animals, BTV can become established in new locations with naive hosts in order to overcome this obtascle. This means that even in zones where bluetongue virus is endemic, persistence dynamic and comprises perpetually shifting "hot spot" of infection¹.

Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV². This is illustrated by the fact that bluetongue virus is limited to geographical areas where competent vectors are present and that transmission only occurs at times of the year when conditions are favourably for vector activity¹. In Britain, transmission occurs mainly in late summer and autumn. Once bluetongue virus is transmitted to a vertebrate host, there are two possible outcome: either the host may die, or an immune response is mounted against the virus and renders the host resistant to re-infection. Either way, animals quickly become "unavailable" for BTV infection as the virus spreads, particularly where livestock populations are small. This presents a hurdle that must be surmounted if bluetongue virus is to persist in an area. By movement of infected vectors or viraemic animals, BTV can become established in new locations with naive hosts in order to overcome this obtascle. This means that even in zones where bluetongue virus is endemic, persistence dynamic and comprises perpetually shifting "hot spot" of infection¹. Creation of an enzootic zone is only possible in locations where adult midges are present throughout the year since bluetongue cannot be maintained through vertebrate-vertebrate or vector transovarial transmission. Any points where vectores are absent from the system must not exceed the maximum duration of viraemia in the ruminant host, otherwise the last infected vertebrate will have died or recovered by the time new vectors are availble for onwards transmission.

In some areas, bluetongue can occur in annual bouts. This may be due to new introduction of virus each year from adjacent areas where the disease is endemic, via the transportation of ''Culicoides'' on the wind for up to 100 kilometres. Alternatively, this could be the manifestation of low-level persistence.  

largely by the distribution of insect vectors and this can vary from year to year.

increase in vector density.

5.5 Where annual bouts of BT occur, they may represent new introductions (from

adjacent infected areas) or may be the visible evidence of low-level persistence from

year to year. Annual re-introduction is possible if enzootic foci of the virus are

geographically close by, as infected Culicoides can reputedly be transported on the

wind over distances of 100 kilometres or more. Culicoides do not normally fly far if

occur then they must be of shorter duration than the maximum period of viraemia in

new vectors arrive on the scene.

The vector species varies with location and is poorly characterised in some areas, including northern Europe. However, as described above, it has been demonstratated that ambient temperature has profound effects on vector survival and feeding activity, and the replication of BTV in the insect⁶. Therefore, the relationship between temperature and effective transmission of BTV is likely to be a limiting factor to the spread of BTV, which may potentially be overcome by global warming.

Introduction of bluetongue virus to a new area has the potential to occur in several ways. Firstly, infected animals may be transported to the region, and local insect vectors could spread and perpetuate BTV infection within naive animals. It is also possible that local vectors could acquire BTV from infected animals in neighbouring areas, where there is a cross-over in the distribution of ''Culicoides'' species. Finally, infected vectors can be acquired from areas where bluetongue infection exists. ''Culicoides'' can be transported considerable distances on the wind, and it is also conceivable that the distribution of competent vectors may expand to colonise previously unpopulated areas under the influence of climate change.

 

Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich⁸. It is believed that BTV-laden vectors were dispersed to the UK on the wind, since meteorological conditions on 4th August 2007 were capable of carrying midges from northern Europe to East Anglia. This would be expected to produce disease at a point that would coincide with the first case, given the time necessary for clinical detection to occur.

Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich⁸. There are several possibilities for how the virus spread to the British Isles: a) through transport of infected animals; b) by local vectors acquring virus from infected animals (unlikely given the island geography of the UK), or c) via movement of vectors carrying BTV. It is believed that BTV-laden vectors were dispersed to the UK on the wind, since meteorological conditions on 4th August 2007 were capable of carrying midges from northern Europe to East Anglia. This would be expected to produce disease at a point that would coincide with the first case, given the time necessary for clinical detection to occur.

==Pathogenesis==

==Pathogenesis==