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Pigs and horses do not become infected with BTV, but may act as a food source for the Culicoides midges that transmit bluetongue virus to ruminants. Their habitats may also provide areas suitable for vector breeding.
Pigs and horses do not become infected with BTV, but may act as a food source for the Culicoides midges that transmit bluetongue virus to ruminants. Their habitats may also provide areas suitable for vector breeding.
==Transmission==
==Transmission and Epidemiology==
Vector species of Culicoides biting midge tend to breed in damp or wet soil
enriched with fresh or composted dung and blood-feed opportunistically on large
vertebrate hosts. Since appropriate breeding sites are very common around livestock
holdings Culicoides are particularly abundant at such sites and therefore feed
predominantly upon domestic livestock (cattle, horses, sheep). They rapidly become
much less abundant as distances from livestock holdings increase. Culicoides tend to
be most active from about 1 hour before sunset until 1 hour after sunrise. They are
most active in the evening until about midnight, then ease off with another peak of
activity around sunrise. However, on dull days or in shady areas vectors may be active
during the day. On windy days, they tend to be less active. Measures to protect
susceptible animals from infection should particularly target these active periods. Most
Culicoides species, including the British species, are averse to entering confined
spaces such as buildings or vehicles, although small numbers might be brought inside
while biting a host and/or while host seeking.
3.2 Culicoides species have a normal insect complete metamorphosis life cycle –
egg, four larval instars, pupa and adult. In temperate and cool regions they
“overwinter” at the fourth larval instar stage. Some species enter diapause when the
number of daylight hours declines below a threshold level but others are more
influenced by temperature which affects their activity levels. The adult populations in
Britain tend to fall dramatically from mid to late October. From December adults are
usually either not at all detectable or only in very small numbers, depending upon the
prevailing temperature, until April-May. These periods may be even longer in northern
Britain. The life span of adults is usually about 10 days, but in cooler conditions their
metabolism slows and they may survive for periods of more than a month.
Technical Review - Bluetongue: The Virus, Hosts and Vectors
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Most species require a blood meal before laying eggs, although C. impunctatus (the
Scottish highland biting midge), a member of the C. pulicaris group, does not require a
blood meal for maturation of the first egg batch. This enables this midge to persist in
areas with few mammalian hosts.
3.3 Different Culicoides species have different preferences for breeding sites
ranging from damp dung, damp soil to tree holes, streamsides and the edges of ponds.
The C. obsoletus group breeds in damp soil and composted organic material such as
old manure heaps common around stables and animal housing. The C. pulicaris group
prefers to breed in wet soil, sphagnum marsh and bogs.
3.4 Culicoides imicola is the major vector of BTV in the Old World. It is one of the
most widely distributed of Culicoides species. It occurs throughout most of Africa, the
Middle East, southern Asia, much of Portugal, south-west Spain and the Balearics,
many Greek Islands, substantial parts of the Greek mainland, Corsica, Sardinia, Sicily
and wide areas of southern and central mainland Italy.
C. imicola appears to be expanding its range both northwards and westwards but is
still restricted in Europe to southern parts. C. imicola has not been recorded in Great
Britain.
3.5 Culicoides obsoletus is probably one of the commonest Culicoides species
across the whole of central and northern Europe. Similarly, C. pulicaris is also
common throughout central and northern Europe. Both of these species are widespread
throughout most of the British Isles.
3.6 In practice, the usual reference to C. obsoletus really relates to a complex of
closely related species (C. obsoletus, C. dewulfi, C. scoticus, C. chiopterus, C.
montanus) the females of which are difficult or impossible to separate. In Bulgaria, C.
obsoletus and C. scoticus, at least, co-exist. Since it is the females that are the vectors
of BTV, it is not always possible to determine the identity of the vector when
undertaking virus isolation from midges. In the UK, C. obsoletus, C. dewulfi, C.
scoticus and C. chiopterus occur. These C. obsoletus group species belong to a larger
grouping (subgenus Avaritia) that includes C. imicola (the major European and
African BTV vector) and C. brevitarsis (the major Australian BTV vector). A similar
taxonomic situation exists with C. pulicaris which is a complex of morphologically
similar species, eight of which occur in the UK and with C. nubeculosus, a European
species which is closely related to the North American BTV vector C. sonorensis (= C.
variipennis).
3.7 The distributions of C. obsoletus and C. pulicaris group midges in UK are not
well understood. Observations of both have been made in many parts of the British
Isles. When observations have not been recorded in certain areas it usually means that
efforts have not been made to collect rather than the species is absent there. Generally,
the insects congregate where there are breeding sites and hosts upon which to feed.
Technical Review - Bluetongue: The Virus, Hosts and Vectors
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Thus, the highest concentrations of C. obsoletus and/or C. pulicaris group midges are
found where cattle, horses, pigs and, to a lesser extent, sheep populations are highest.
If domestic animals are removed from a site, over several months the midge
population reduces significantly, by a factor of ten to twenty times, but will usually
persist at the lower level if other ecological factors are favourable, by feeding on wild
hosts and/or humans.Vector numbers are likely to be low in hill sites where sheep are
at low densities and where the climatic conditions are likely to be more extreme.
3.8 Studies of Culicoides spp. in Britain are being expanded under a DEFRAfunded
project out of the Institute of Animal Health, Pirbright. Monitoring is being
expanded to twenty-five or more sites, one of the aims being to determine the species
list, species distribution, seasonal incidence and vector competency of the various
species.
Vector competency
3.9 The C. obsoletus group has long been suspected of being a vector, mainly on
the basis of BTV isolations from this species made in Cyprus, and African horse
sickness virus (AHSV) isolations made from mixed pools of C. obsoletus and C.
pulicaris in Spain. In this context it should be borne in mind that BTV and AHSV tend
to utilise the same Culicoides species as vectors.
3.10 It is strongly suspected that C. obsoletus and/or C. pulicaris group midges acted
as BTV vectors in northern Greece and southern Bulgaria during the 1999 BTV
epizootic, as they were by far the most abundant and most prevalent detected. It is
similarly suspected that these species may also have mediated the BT outbreaks in
Serbia, western and southern Bulgaria, FYR Macedonia, Croatia and Bosnia during the
period 2001-2002. C. imicola has not been recorded in these regions.
3.11 Vector competence studies on a British population of C. obsoletus have
recorded oral susceptibility rates of less than 2% in comparison with a known major
vector C. sonorensis (19.5%). This initially suggested that C. obsoletus is likely to be
only a minor or inefficient vector of BTV. Nevertheless, the high abundance and
survival rates of C. obsoletus as exhibited in Bulgaria in 1999, and as seen on farms
and around stables in South East England, could compensate for its low levels of
vector competence. Observations of cattle exposed to midges have shown up to ten
thousand bites per hour. It should be noted that C. brevitarsis, the major vector of BTV
in Australia, has an experimental competency of only 0.3 percent when feeding on
sheep although it is quite an effective vector in the field.
3.12 Vector competence for a particular virus is a hereditary trait and populations of
a vector species with high, low or intermediate levels of competence can be derived by
selective breeding.
Technical Review - Bluetongue: The Virus, Hosts and Vectors
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Version 1.5; 21 November 2002
Potential impact of global warming
3.13 Vector competence of Culicoides vectors for Orbiviruses is partly influenced by
temperature. Orbivirus development in Culicoides vectors is unable to occur at
temperatures below about 10°C to 15°C depending on the Orbivirus species and
serotype. Furthermore, there needs to be a minimum amount of time at suitable
temperatures (expressed as “day degrees or hour degrees”) for completion of the
development cycle in the Culicoides vector before virus transmission can occur. This
“physiological” time is the cumulative product of virus development time multiplied
by the temperature in degrees above the threshold for virus replication. Increasing
environmental temperature (climate change) will also extend the vector season.
Combined, these conditions may result in Orbivirus development within Culicoides
being able to take place over a greater proportion of the year and over a wider
geographical area. In addition, within the range of temperatures over which Orbivirus
development can occur, the levels of vector competence of a Culicoides vector
population for some Orbivirus serotypes increases linearly with temperature and so the
impact of warmer temperatures may be even greater.
3.14 Temperature can also affect the competence of ‘non-vector’ Culicoides species.
For example, C. nubeculosus generally is considered to be incapable of transmitting
BTV due to a midgut infection barrier. However, exposure of the immatures to rearing
temperatures close to their upper lethal limit (33-35°C) can result in >10% of adults
becoming competent to transmit BTV. It is likely that the integrity of the gut wall of
some adults is damaged by the extreme rearing temperatures, thereby allowing virus
particles to bypass the midgut barriers, enter the haemocoel and develop as in a normal
vector. The increase in frequency and intensity of extremely warm days predicted to
occur with climate change will enhance the chances of this phenomenon occurring in
non-vector Culicoides species and hence could increase the number of BTV competent
adults within populations.
3.15 The vectorial capacity of a Culicoides population (and hence the potential for
virus transmission) is affected by (a) the number of adult midges in the population and
(b) the proportion of adults capable of transmitting the virus, and is greatest when
these factors are at a peak.
3.16 Within favourable limits, the development rate of Culicoides from egg to adult
is directly related to temperature. Thus increasing temperatures coupled with an
extension in the developmental season may result in a greater number of generations
(and therefore adults) per year. In addition, the overwintering ability of adult
Culicoides is likely to improve, as winters become both warmer and shorter. Improved
overwintering success is also likely to increase the spring population input, which in
turn could result in even larger populations during the summer
Technical Review - Bluetongue: The Virus, Hosts and Vectors
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Version 1.5; 21 November 2002
3.17 The proportion of adult Culicoides capable of transmitting virus is dependent
on (a) vector competence (the capacity for the virus to develop in and be transmitted
by the vector), (b) adult survival, (c) the blood-feeding interval and (d) the extrinsic
incubation period (EIP; development time of the virus in the vector). In order to
transmit virus Culicoides must not only be vector competent, but also survive long
enough to blood-feed after the completion of the viral EIP. Culicoides vectors are
more likely to satisfy these criteria at high temperatures (e.g. 27-30°C), because,
although adult survival is reduced at high temperatures, this is more than compensated
for by the accompanying decrease in duration of the EIP and blood-feeding interval.
Consequently, it is likely that warmer temperatures as a result of climate change will
increase the likelihood that Culicoides will survive long enough to transmit virus.
3.18 Changes in weather (temperature, precipitation, humidity and wind) and climate
from global warming could produce both wider distribution of vectors towards the
poles or upwards in elevation and increased vectorial capacity (the ability of a vector
population to transmit virus to a vertebrate population) of Culicoides vector
populations, resulting in increased prevalence of BTV in Europe. The present BT
outbreak in the Mediterranean Basin is already the most serious epizootic on record.
3.19 An expansion in the range of C. imicola will increase the areas of Europe at risk
from BTV. Also, the extended distribution of C. imicola could bring BTV into the
range of C. obsoletus group and C. pulicaris group midges much more frequently and
this could result in even greater areas of Europe being affected by BTV.
3.20 The impact of climate change on the vectorial capacity of Culicoides
populations will have three main effects on BTV transmission in the Mediterranean
basin:
· the greater abundance of adult Culicoides combined with the increased proportion
of adults capable of transmitting the virus will increase the likelihood and severity
of an epizootic, following the introduction of BTV into an area. The greatest risk
will be at times of the year when temperatures reach approximately 25-30°C (i.e.
when conditions are optimal for Culicoides development and virus transmission
· as temperatures will be conducive for both viral and Culicoides development for a
greater proportion of the year, the length of the viral transmission season will
increase.
· the enhanced overwintering success of adult Culicoides combined with the
extension in the Culicoides development season will prolong the seasonal
occurrence of adult midges and hence improve the overwintering chances of BTV.
3.21. Studies are needed to correlate the day degrees required for BTV development
in the potential vectors against British climate data to establish the risk of
establishment of a BTV infection under present climatic conditions and with global
warming.
Technical Review - Bluetongue: The Virus, Hosts and Vectors
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8.
==Pathogenesis==
==Pathogenesis==