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unpublished evidence reported by Dr Oura on 20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