Bluetongue Serotype 8 Epidemic Bulletin by EFSA Epidemiology Working Group
Last updated: 30 October 2006
Publication Date: 23 October 2006
Bulletin of 30 October 2006. Outbreak Period until 25 October, 2006
Based on date of laboratory confirmation of diagnosis, the current bluetongue serotype 8 epidemic is now into its 10th week, with the case total having risen to 1196 herds. The vast majority of cases having been reported from Belgium, Germany and the Netherlands, and over half of them within the last 3 weeks in October. The spatial spread has continued, but visual analysis suggests the appearance of three distinct clusters, which include the original focus in the area where Belgium, Germany and The Netherlands share borders, the west of Belgium, and an area around the Bergisches Land in Germany. It needs to be emphasized that this observation has to be interpreted cautiously as this observed clustering may be the result of spatial variation of the population at risk.
Until very recently, bluetongue virus (BTV) had not been reported from the more northern parts of Europe, and serotype 8 had not previously been reported in the EU. The epidemiological mechanisms underlying the current bluetongue epidemic are unclear, including its origin and the geographic, climatic, and husbandry factors influencing the establishment and the spread of this disease. EFSA has been requested by the European Commission to investigate these issues through an epidemiological analysis of the surveillance data for the region affected by the epidemic. One aspect of the epidemiological analysis is a descriptive analysis of the spatial and temporal disease pattern during the progression of the epidemic. The findings are reported in this Bulletin, which is to be updated at weekly intervals in the light of the epidemic.
Results and Discussion
It should be noted that at this stage the analyses are of a descriptive nature, and it is not yet possible to draw causal inferences, i.e. to determine which factors affect for example the spread of the disease. Due to the impact of several biases in the data, which are further discussed below, caution has to be applied when using these findings for developing causal hypotheses.
The results indicate that the disease was first confirmed in mid August 2006 in an area where Belgium, Germany, and the Netherlands are sharing borders. Since the disease was previously absent in this part of Europe, farmers’ and veterinarians’ awareness of its clinical symptoms was likely to be low in this initial period, resulting in an unknown level of underreporting bias.
Since the beginning of the epidemic (with the earliest confirmation date being August 17, 2006), a total of 1196 cases has been reported until October 24, 2006. Of these 440 were reported from Belgium, 5 from France, 454 from Germany, and 297 from the Netherlands. The cases mainly include herds with either only cattle or only sheep.
The data summaries in the current bulletin consist of:
Bias and interpretation
It needs to be emphasized that any apparent spread or lack thereof deduced from these data will be affected by an unknown level of underreporting bias, as well as varying temporal lags between occurrences of infection, its detection, and its subsequent confirmation. It is also to be noted that observed clusters of high or low numbers of cases may well be a reflection of variation in the population at risk for the animal species affected.
The following specific sources of underreporting bias have to be considered:
Sub-clinical cases can be detected through serological screening programmes. This is illustrated by the reports from France where 3 of the 5 case herds were identified through serology at a time when none of the animals in these herds showed clinical symptoms. Similarly, animals that were exposed to the virus can also be detected when tested serologically for certification prior to trade or export.
The Working Group is currently organising access to additional data sources such as livestock population denominator data, so that more detailed epidemiological analyses can be conducted and therefore the effect of some of the aforementioned biases can be reduced.
The bluetongue disease data used in this analysis are obtained as follows:
From the ADNS database the following data fields were used for each case: the country name, the date of confirmation, the geographical coordinates, and the animal species. The data for these four fields were available for this bulletin up until 24 October, 2006 (extracted 25 October, 2006).
The geographical references were verified prior to their use by checking for obvious inaccuracies. Missing or implausible geo-references were excluded from the analysis, except for data missing only at the highest level of precision (seconds). Cases with incomplete or implausible information are brought to the attention of the European Commission which is seeking clarification from the Member States for these data.
Justification for using confirmation date for this analysis
Different dates2 can be recorded in the ADNS system:
Whereas the actual date of first infection is unknown, the date of first clinical signs observed by the farmer could potentially be used, if obtained as part of a formal outbreak investigation. However, these data are not always available when the notification to the ADNS is made. Since the date of confirmation is always provided, it was therefore decided to use it for the purpose of the weekly reporting in this Bulletin.
The data utilised by the EFSA Bluetongue Epidemiology Working Group are the result of the work of a large number of dedicated individuals, including animal owners, veterinary practitioners, experts in diagnostic and other aspects of bluetongue disease, and national and European-level veterinary services.
Working Group and its Subgroup
The EFSA Bluetongue Epidemiology Working Group consists of a group of experts from all Member States which have been invited to join. It is chaired by D. Pfeiffer (RVC, UK).Specific analytical tasks are being carried out by a smaller Subgroup which currently includes the following experts: A. Brouw (European Commission), A. Conté (OIE Ref Lab., Italy), H. Deluyker (EFSA), A. Elbers (CIDC-Lelystad, the Netherlands), G. Hendrickx (Avia-GIS), P. Mellor (IAH, UK), K. Mintiens (VAR, Belgium), D. Pfeiffer (RVC, U.K.), C. Staubach (FLI, Germany), G. Gerbier (CIRAD, France), and S. Zientara (AFSSA, France). The membership of this Subgroup has not yet been finalised, since additional experts with specific expertise in the disease from around Europe and elsewhere are in the process of being identified.