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Epidemiological analysis of the 2006 bluetongue virus serotype 8 epidemic in north-western Europe: Dispersal of BTV8 by wind
| Acronym: | BTV8Wind |
| Project type: | Short term RTD |
| Time frame: | 2006 - 2007 |
| Funding agency: | EFSA (Parma, Italy) | FOD Volksgezondheid (Brussels, Belgium) |
| Geographic keyword: Europe | BENELUX | France | Germany | UK | |
| General keyword: Decision support | Dynamic disease modeling | |
| Specific keyword: Wind dispersal | Bluetongue | Culicoides |
Bluetongue (BT) is an arthropod-borne viral disease of ruminants. All ruminant species - sheep, goats, cattle, buffaloes, antelopes and deer - are susceptible. In general1 of the domestic species, sheep are the most severely affected. Infection in cattle, although of great epidemiological (reservoir) and economical (production losses, movement ban) significance, is generally sub-clinical. Whilst in Europe prior to 2006 the disease was mainly associated with the spread of the invasive tropical midge Culicoides imicola around the Mediterranean, in the BTV8 outbreak under study, though the exact entrance route is as yet not known, it is spread by endemic Culicoides midges.
During the BLUETONGUE project (see separate Avia-GIS project thumbnail) Avia-GIS established for the first time a quantifiable relationship between wind and BT outbreak patterns taking historical data in Greece and Bulgaria as an example. This result paved the way for near real time identification of BT dissemination risk from known foci. In this project this innovative methodology was further developed. Based on modeled forward wind trajectories (ECMWF2 data) originating from infected herds (termed ‘potentially infective wind events’), wind density raster images (1X1km) were computed on a weekly basis. These cumulative maps have shown to be good proxies, with a two to four week time lag, for the risk of spread of the epidemic. Based on the developed wind models the following conclusions could be drawn:
- Density of wind events contributed to explain at least part of the horizontal asymmetrical spread pattern of the epidemic. The developed modeling approach may become an important risk management tool and could enable the implementation of more cost efficient monitoring and prevention (e.g. vaccination) activities.
- Short (<5km), medium (5-31km) and long (>31km) distance spread had a different impact on disease spread. Computed wind densities were linked to the medium/long distance spread.
- Medium/long distance spread outbreak followed an asymmetric pattern and this asymmetry increased with distance whilst short term spread occurred in a symmetrical pattern. This suggests that the latter is mainly driven by insect individual random movement, whilst the former is driven by an external factor.
- Though a positive relationship between wind density and case density was established, more work is needed to further fine tune this and relate disease spread and wind density patterns on a weekly basis.
- Controlling density dependent factors affecting longer range spread may play an important role in preventing the spread of an epidemic at an early stage. Preventing the installation of primary outbreak clusters that are sufficiently dense may inhibit the long range spread of the epidemic in the absence of virus carrier movement.
- UK has been at risk of introduction during the 2006 epidemics, but:
- Wind events with the highest potential risk of introduction, and pointing to East Anglia, occurred mainly at an earlier stage of the epidemic, when infected herd clusters were too distant from the Channel coast line;
- At a later stage infected herd clusters were well established within the 31km medium distance of spread range from the coast, but potentially infective wind events over UK showed a very low density.
- Terrain roughness may be an important factor preventing spread of infected midges.
- Independently modeled spread based on animal movement data (VAR, Uccle, Belgium) showed that whilst some of the spread may be related to animal movement this did not account for explaining the asymmetrical spread of the epidemic.
1 It is important to note that the symptoms caused by the Northern Europe BTV serotype 8 outbreak under study has shown major impact on cattle also.
2 ECWF wind data: European Center for Medium Weather Forecast, U and V wind components every 6 hours at 1000 hPa, 850 hPa and 700 hPa.
Legend to the figures:
Figure 1: Per pixel the infected wind events cumulated from the onset of the epidemic is shown and overlayed with outbreaks occurring four weeks later. All new outbreaks are found in areas with a high density of infected wind events and following a distinct East-West horizontal spread pattern, indicating that the cumulated wind density is a good proxy for disease risk.
Figure 2: The radial distribution pattern of outbreaks can be separated in different distance classes: short, medium and long distance spread. In the first class (<5 km) spread of vectors is mainly due to active movement, which is shown by the symmetrical outbreak pattern. In the medium (5-31km) to long distance (>31 km) classes the spread is caused by passive wind-borne transport, which is characterised by an asymmetrical spread pattern.
Figure 3: The case density, corrected by cattle distribution, has a high correlation to the wind density maps. Higher wind density is reflected in a high case density of BT.