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Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich<sup>8</sup>. 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.
Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich<sup>8</sup>. 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.
BTV is non-contagious. BTV is transmitted biologically by Culicoides insects
(biting midges), but only a limited number of species are efficient vectors. Cattle are
the main amplifying hosts for BTV. They are also probably important maintenance
hosts. The competent Culicoides vector species feed more abundantly on cattle.
5.2 BTV is limited to those geographical areas where competent Culicoides vectors
are present. Its transmission is limited to those times of the year when the climatic
conditions are favourable for adult vector activity. Peak populations of vector
Culicoides occur in the late summer and autumn and therefore this is the time when
BT is most commonly seen.
5.3 Persistence of BTV within a particular geographical area does not mean
“static”. Once a vertebrate host is infected with BTV it either dies or mounts an
enduring antibody response and so becomes resistant to further infection. This means
that within any small geographical area (a farm or village) most or all of the initially
susceptible hosts are likely to be infected and thus become “unavailable” to the virus
within a fairly short space of time. BTV can only survive under such constraints by
continually moving to new locations occupied by naïve vertebrate hosts. These
movements are via the agency of viraemic hosts or infected vectors. BTV is therefore
a peripatetic virus and even within its enzootic zones its activity may be envisaged as a
pattern of endlessly shifting viral “hot spots”.
5.4 The incidence and geographical distribution of BTV infections are determined
largely by the distribution of insect vectors and this can vary from year to year.
Infection in sheep will usually be preceded by widespread infection of cattle and an
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
there is a source of food (large mammals) and breeding sites.
Since BTV is not transmitted transovarially through its vectors (and is rarely
transmitted directly from vertebrate to vertebrate) long term persistence (i.e. an
enzootic zone) is currently thought to be possible only in areas where active adult
vectors are present throughout the year. In such situations if vector-free periods do
occur then they must be of shorter duration than the maximum period of viraemia in
the local susceptible vertebrate population (up to about 50 days in sheep and 100 days
in cattle). Otherwise the last infected vertebrate host will have died or recovered before
new vectors arrive on the scene.
===Impact of Climate Change===
===Impact of Climate Change===