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| | ===Impact of Global Warming=== | | ===Impact of Global Warming=== |
| − | 3.13 Vector competence of Culicoides vectors for Orbiviruses is partly influenced by
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| − | temperature. Orbivirus development in Culicoides vectors is unable to occur at
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| − | temperatures below about 10°C to 15°C depending on the Orbivirus species and
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| − | serotype. Furthermore, there needs to be a minimum amount of time at suitable
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| − | temperatures (expressed as “day degrees or hour degrees”) for completion of the
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| − | development cycle in the Culicoides vector before virus transmission can occur. This
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| − | “physiological” time is the cumulative product of virus development time multiplied
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| − | by the temperature in degrees above the threshold for virus replication. Increasing
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| − | environmental temperature (climate change) will also extend the vector season.
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| − | Combined, these conditions may result in Orbivirus development within Culicoides
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| − | being able to take place over a greater proportion of the year and over a wider
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| − | geographical area. In addition, within the range of temperatures over which Orbivirus
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| − | development can occur, the levels of vector competence of a Culicoides vector
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| − | population for some Orbivirus serotypes increases linearly with temperature and so the
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| − | impact of warmer temperatures may be even greater.
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| − | 3.14 Temperature can also affect the competence of ‘non-vector’ Culicoides species.
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| − | For example, C. nubeculosus generally is considered to be incapable of transmitting
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| − | BTV due to a midgut infection barrier. However, exposure of the immatures to rearing
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| − | temperatures close to their upper lethal limit (33-35°C) can result in >10% of adults
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| − | becoming competent to transmit BTV. It is likely that the integrity of the gut wall of
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| − | some adults is damaged by the extreme rearing temperatures, thereby allowing virus
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| − | particles to bypass the midgut barriers, enter the haemocoel and develop as in a normal
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| − | vector. The increase in frequency and intensity of extremely warm days predicted to
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| − | occur with climate change will enhance the chances of this phenomenon occurring in
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| − | non-vector Culicoides species and hence could increase the number of BTV competent
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| − | adults within populations.
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| − | 3.15 The vectorial capacity of a Culicoides population (and hence the potential for
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| − | virus transmission) is affected by (a) the number of adult midges in the population and
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| − | (b) the proportion of adults capable of transmitting the virus, and is greatest when
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| − | these factors are at a peak.
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| − | 3.16 Within favourable limits, the development rate of Culicoides from egg to adult
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| − | is directly related to temperature. Thus increasing temperatures coupled with an
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| − | extension in the developmental season may result in a greater number of generations
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| − | (and therefore adults) per year. In addition, the overwintering ability of adult
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| − | Culicoides is likely to improve, as winters become both warmer and shorter. Improved
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| − | overwintering success is also likely to increase the spring population input, which in
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| − | turn could result in even larger populations during the summer
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| − | Technical Review - Bluetongue: The Virus, Hosts and Vectors
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| − | ___________________________________________________________________________
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| − | 7.
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| − | Version 1.5; 21 November 2002
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| − | 3.17 The proportion of adult Culicoides capable of transmitting virus is dependent
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| − | on (a) vector competence (the capacity for the virus to develop in and be transmitted
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| − | by the vector), (b) adult survival, (c) the blood-feeding interval and (d) the extrinsic
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| − | incubation period (EIP; development time of the virus in the vector). In order to
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| − | transmit virus Culicoides must not only be vector competent, but also survive long
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| − | enough to blood-feed after the completion of the viral EIP. Culicoides vectors are
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| − | more likely to satisfy these criteria at high temperatures (e.g. 27-30°C), because,
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| − | although adult survival is reduced at high temperatures, this is more than compensated
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| − | for by the accompanying decrease in duration of the EIP and blood-feeding interval.
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| − | Consequently, it is likely that warmer temperatures as a result of climate change will
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| − | increase the likelihood that Culicoides will survive long enough to transmit virus.
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| − | 3.18 Changes in weather (temperature, precipitation, humidity and wind) and climate
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| − | from global warming could produce both wider distribution of vectors towards the
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| − | poles or upwards in elevation and increased vectorial capacity (the ability of a vector
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| − | population to transmit virus to a vertebrate population) of Culicoides vector
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| − | populations, resulting in increased prevalence of BTV in Europe. The present BT
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| − | outbreak in the Mediterranean Basin is already the most serious epizootic on record.
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| − | 3.19 An expansion in the range of C. imicola will increase the areas of Europe at risk
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| − | from BTV. Also, the extended distribution of C. imicola could bring BTV into the
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| − | range of C. obsoletus group and C. pulicaris group midges much more frequently and
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| − | this could result in even greater areas of Europe being affected by BTV.
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| − | 3.20 The impact of climate change on the vectorial capacity of Culicoides
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| − | populations will have three main effects on BTV transmission in the Mediterranean
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| − | basin:
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| − | · the greater abundance of adult Culicoides combined with the increased proportion
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| − | of adults capable of transmitting the virus will increase the likelihood and severity
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| − | of an epizootic, following the introduction of BTV into an area. The greatest risk
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| − | will be at times of the year when temperatures reach approximately 25-30°C (i.e.
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| − | when conditions are optimal for Culicoides development and virus transmission
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| − | · as temperatures will be conducive for both viral and Culicoides development for a
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| − | greater proportion of the year, the length of the viral transmission season will
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| − | increase.
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| − | · the enhanced overwintering success of adult Culicoides combined with the
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| − | extension in the Culicoides development season will prolong the seasonal
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| − | occurrence of adult midges and hence improve the overwintering chances of BTV.
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| − | 3.21. Studies are needed to correlate the day degrees required for BTV development
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| − | in the potential vectors against British climate data to establish the risk of
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| − | establishment of a BTV infection under present climatic conditions and with global
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| − | warming.
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| | ==Pathogenesis== | | ==Pathogenesis== |