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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. |
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| − | ===Impact of Global Warming=== | + | ===Impact of Climate Change=== |
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| − | Although BTV has been the subject of intense molecular
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| − | and structural studies, the epidemiology and geographic
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| − | dispersal of BTV have also been a major subject of interest
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| − | to virologists and entomologists, because this virus
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| − | is pathogenic for a range of domestic and wild ruminants.
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| − | Seasonal incursions of the virus from Africa into
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| − | more temperate latitudes, sometimes accompanied by disease,
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| − | have occurred under favourable climatic conditions,
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| − | but the recent introduction of serotype BTV-8, and the
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| − | establishment of a transmission cycle that has resulted
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| − | in its spread into northern Europe including the UK (see
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| − | below), is of significant economic importance. BTV is a
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| − | member of the genus Orbivirus in the family Reoviridae
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| − | but, unlike many other arboviruses, does not infect humans
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| − | and therefore is not zoonotic. There are 24 recognised
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| − | serotypes of the virus, which contain between 10 and 12
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| − | segments of double-stranded RNA. Until recently BTV was
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| − | considered to be almost exclusively a disease of some European
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| − | breeds of sheep that, for commercial purposes, have
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| − | been distributed widely in Africa, Asia and Australasia.
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| − | In cattle and goats, clinical disease has been considered
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| − | rare, and much milder than in sheep.76 However, recent
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| − | observations suggest that cattle frequently show disease
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| − | symptoms resulting from infection by the BTV-8 serotype
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| − | that is currently circulating in northern Europe (see below).
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| − | There is evidence that infected midges are carried on
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| − | the wind for long distances,77,78 and it has been postulated
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| − | that the major epidemics of bluetongue, in regions
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| − | where disease occurs only sporadically, result from windborne
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| − | carriage of infected Culicoides from distant endemic
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| − | areas.79 Competent midges may be infected when biting
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| − | viraemic vertebrates. The probability of infection depends
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| − | in part on the genotype of the midge, the strain of virus,
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| − | the level of viraemia and environmental factors.80 The
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| − | extrinsic incubation period (the period between feeding on
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| − | infected blood and the appearance of virus in the saliva
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| − | of the arthropod vector) is 1—2 weeks. Contrary to the
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| − | BTV strains referred to above, the recent appearance of
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| − | BTV-8 in northern Europe, including the UK, has unexpectedly
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| − | been accompanied by the appearance of overt disease
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| − | and mortality in cattle. Moreover, as the result of currently
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| − | unpublished evidence reported by Dr Oura on 20
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| − | March 2008,81 it is now recognised that healthy infected
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| − | animals may remain ELISA- and RT-PCR-positive for at least
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| − | 4 months.82 This observation helps to explain how BTVpositive
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| − | animals may be detected in mid-winter in the UK
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| − | when midge transmission activity is presumed to be minimal.
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| − | Symptoms of BTV infection in sheep are variable but
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| − | typically include fever. Facial oedema results in swelling
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| − | and soreness of the lips and nose with mucopurulent discharge,
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| − | which is exacerbated by champing to produce
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| − | frothy saliva. The term ‘bluetongue’ is derived from the
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| − | cyanosis of the tongue that is observed in some cases.
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| − | Erosion of the coronal band above the hooves and musculoskeletal
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| − | damage cause pain and lameness, inducing
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| − | the sheep to adopt a posture similar to that shown in
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| − | Figure 5.
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| − | BTV circulates widely throughout tropical and subtropical
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| − | regions, but until relatively recently the disease had
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| − | been observed only infrequently in some areas of southern
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| − | Europe. However, during the past decade, six strains of BTV
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| − | are known to have spread across 12 European countries, and
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| − | significantly the virus has gradually dispersed further north
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| − | in central and western Europe. This dispersal has probably
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| − | been driven by the northward expansion of the range of
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| − | Cu. imicola, the main BTV vector, and by climate change,
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| − | which has probably contributed to increased persistence
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| − | Figure 5 Posture often observed in cases of bluetongue
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| − | infection in sheep (source: http://129.186.78.52/DiseaseInfo/
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| − | ppt/bluetongue.ppt#17).
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| − | during winter, consequently increasing the subsequent risk
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| − | of transmission over larger geographical regions83 and an
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| − | extended period of time. To the north of the Cu. imicola
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| − | range, other species (Cu. obsoletus, Cu. pulicaris, Cu.
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| − | chiopterus and Cu. dewulfi) with distributions extending
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| − | across central and northwestern Europe84 were probably
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| − | involved in the appearance of BTV-8 in Belgium, France,
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| − | Luxembourg, Germany and the Netherlands in August 2006,
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| − | and subsequently in the UK in September 2007.85 This presence
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| − | of multiple vectors of BTV-8 appears to apply to large
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| − | parts of northern Europe and has almost certainly contributed
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| − | to the dramatic spread of this arbovirus across this
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| − | area. In addition to the impact of climate change on vector
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| − | range expansion and the northerly establishment of BTV-8,
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| − | the commercial transportation of asymptomatic infectious
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| − | ruminants and the wind-borne dispersal of infected midges
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| − | are believed to be highly significant contributory factors
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| − | to the rapid dispersal of the virus. Understanding this
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| − | sequence of events may aid predictions of the emergence
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| − | of other vector-borne pathogens, such as the more devastating
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| − | African horse sickness virus, another animal pathogen
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| − | in the genus Orbivirus that may be transmitted by several
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| − | of the same vectors as BTV.
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| − | Another important observation has appeared as the
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| − | result of the incursion of BTV into northern Europe. Conventional
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| − | opinion has previously considered it extremely
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| − | unlikely that BTV could be transmitted vertically to newborn
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| − | offspring. New evidence suggests that this virus may
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| − | be transmitted across the bovine placenta to infect the
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| − | fetus, causing an unusually high rate of malformed, stillborn
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| − | and weak calves born on holdings with a known history
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| − | of BTV infection.86 At the time of writing, this observation
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| − | has not been confirmed through systematic investigation.
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| − | Nevertheless, whether or not this represents an acquired
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| − | new characteristic of BTV-8 clearly needs close attention.
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| − | Transplacental infection has only previously been associated
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| − | with attenuated BTV vaccine viruses. In further
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| − | support of these reports, the recent unpublished finding
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| − | of imported heifers in Northern Ireland, leading to the
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| − | suspicion that newborn calves infected in utero can act
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| − | as virus reservoirs for the Culicoides vector, is another
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| − | worrying development that needs immediate investigation Methods for controlling BTV include reducing exposure
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| − | of the animals to the competent midges, the use of insecticides
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| − | to dissuade the insects from biting the animals,
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| − | and the use of vaccines. While the strategies of reducing
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| − | exposure and using insect repellents might reduce the
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| − | levels of BTV transmission, clearly these measures cannot
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| − | be expected to eradicate BTV from northern Europe.
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| − | Vaccination is associated with several practical difficulties.
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| − | Firstly, there are 24 serotypes of BTV, and while
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| − | there is some antigenic cross-reactivity between different
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| − | serotypes, the preparation of a single live attenuated virus
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| − | multivalent vaccine to protect against all 24 is impractical,
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| − | partly because different serotypes may outcompete
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| − | each other in the vaccine, partly because at the moment
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| − | only BTV-8 is circulating in northwestern Europe and partly
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| − | because of the costs and time involved in producing a
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| − | multivalent vaccine. Moreover, the use of live attenuated
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| − | vaccines presents a low but potential risk of reversion
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| − | to virulence, or in some circumstances the possibility of
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| − | reassortment of the RNA gene segments between different
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| − | serotypes of BTV. However, for reasons beyond the
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| − | control of the manufacturers, the production of a vaccine
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| − | in time to prevent the reemergence of BTV-8 in northern
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| − | Europe during 2008 is proving to be seriously problematic.
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| − | It will be interesting to see whether or not BTV-8 is brought
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| − | under control in the UK and northern Europe during 2008.
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| − | Non-infectious vaccines based on engineered recombinant
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| − | proteins are also under development, but in addition to
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| − | the requirement for multiple dosing, these vaccines are
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| − | likely to be expensive and therefore not favoured by farmers.
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| | ==Pathogenesis== | | ==Pathogenesis== |