Epidemiological analysis of the 2006 bluetongue virus serotype 8 epidemic in north-western Europe

By the European Food Safety Authority. This report describes the results of the different tasks that were performed through 31 January 2007 to determine the conditions for introduction, establishment and spread of BTV-8 in northern Europe.
calendar icon 24 April 2007
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Executive Summary

Bluetongue (BT) is an arthropod-borne non-contagious viral disease of domestic and wild ruminants, affecting particularly certain breeds of sheep with severe clinical disease, including mortality. On 14 August 2006, a private veterinary practitioner in the southern province of Limburg, in The Netherlands, notified the veterinary authorities of BT-suspect cases on four different holdings in that Member State. These were the first suspicions of a rapidly-spreading BT virus (BTV)-epidemic in northern Europe, which has since affected cattle and sheep holdings in Belgium, Germany, France, Luxembourg, and The Netherlands. On 28 August 2006, the CRL in Pirbright announced that BTV-serotype 8 (BTV-8) was causing the outbreaks.

The veterinary authorities of the three initially-affected countries (Belgium, Germany, and the Netherlands) decided at the onset of the BTV-epidemic in northern Europe to form a research group with epidemiologists from these countries in order to provide science-based decision support for future BTV monitoring and surveillance. The European Food Safety Authority (EFSA) was requested by the European Commission (EC) to carry out a global epidemiological analysis of the ongoing outbreak. Therefore a BTV-8 epidemiology working group (BTV-8WG) was established by EFSA on 6th of October 2006.

This report describes the results of the different tasks that were performed through 31 January 2007 to determine the conditions for introduction, establishment and spread of BTV-8 in northern Europe. They concern the following aspects of the outbreak investigation:
  • The introduction of the BTV-8 serotype; which focused on the place, time and possible routes for this introduction;
  • A section on clinical aspects in which the nature and severity of the disease caused by this strain are described;
  • The characterisation of within-herd spread which is of relevance to assess the ratio of sub clinical versus clinical cases. This has implications for detection of BTV-8-infected herds;
  • The information on factors favouring virus establishment includes the results of vector surveillance in the affected countries and factors that can affect virus persistence such as animal densities, environmental factors, and meteorological conditions;
  • Elements that may influence short-distance spread that were studied include observed speed of local spread and the characteristics and pertinence of the 20 km zones;
  • Finally, in the section on factors affecting long-distance spread into new areas the potential for vector spread through wind and restrictions on animal movements were considered.

These provisional results, to be delivered by 31 January, were presented to the BTV-8WG on the 6th of February and to the Chief Veterinary Officers (CVOs) at an EC meeting on the 7th of February in Brussels. Using the feedback from the meetings on the 6th and the7th of February the reports were revised and finalised.

The final outcome of a full epidemiological analysis on the identification of associations or causality between disease incidence and possible risk factors requires a multiple variable analysis assessing all possible risk factors simultaneously. Such an analysis was not achievable in the time frame given to generate this first report. Thus, the focus of this report is more of a descriptive and exploratory nature. The initial findings reported by the EFSA bluetongue working group are reported below.

1. Statistical modelling showed that the initial infection occurred in the area close to Maastricht. Difficulties in the initial diagnosis were due to the fact that the disease was thus far not known (exotic) in the area. An obvious source for the introduction of BTV-8, such as import of infected ruminants, could not be identified and the exact origin and route of the introduction of BTV-8 thus far remains unknown. However,
  • the absence of legal import of ruminants from outside the EU into the AFI; and
  • the absence of BTV-8 from southern Europe;

suggest that, the introduction of the BTV-8 infection into the more northern part of Europe took place via another route.

Hence, the potential for introduction via mechanisms other than those that have been previously incriminated also need to be considered. Specifically, the potential for Culicoides to be imported along with or independently of the import of animals, plants or other ‘materials’, and the effectiveness of measures to reduce such a possibility, merit further study.

2. Clinical signs in BTV-8 infected herds were expressed differently in cattle herds and sheep flocks. BTV-8 associated clinical signs were much more prominent in sheep than in cattle.

3. Based on the (sparse) data from whole herd sampling there was a trend suggesting a high proportion of cattle to be PCR and seropositive in infected cattle herds and a small proportion of sheep to be PCR and seropositive in infected sheep herds.

Taken together with the results on clinical signs, these findings therefore suggest that
  • for sheep flocks a monitoring system based on clinical signs could be considered; and
  • for cattle a monitoring system based on serological surveillance appears to be the more effective approach .

4. The BTV-8 virus was found to be present in vectors (Culicoides species) which are endemic to north-western Europe. C. imicola, which is thought to be responsible for at least 90% of BTV transmission in the Mediterranean Basin, was not found once amongst a total of 100,000 Culicoides collected in the infected MS. This demonstrates that species endemic to the Palaearctic region are quite capable of transmitting BTV and – judging from the rapid spread of the virus – no pre-adaptive phase was required in the indigenous Culicoides. One of the species found to be PCR-positive was C. dewulfi a species which breeds exclusively in the dung of cattle and sheep.

Vectors of BTV in other parts of the world have been shown to transmit a range of other viral pathogens of livestock (African horse sickness virus, Akabane virus, epizootic haemorrhagic virus, equine encephalitis virus), this suggests that such pathogens may be transmitted if they were to be introduced into northern Europe during climatically favourable periods. However, overall, there is a paucity of information on the behavioural activities of vector species of Culicoides, especially in relation to their interactions with host animals and their biting activities. Detailed data are urgently required before clear and reliable recommendations can be provided to the veterinary authorities on the subject.

The discovery that larger numbers of Culicoides may be found indoors than outdoors, and especially towards the end of the season, demands that the earlier attempt to declare the vector-free period to be when “…<10 Culicoides are found in a light trap suspended outdoors for one night” be refined.

The daily variation of number of Culicoides captured in The Netherlands were found to be correlated with prevailing temperatures, for all species. The prevailing high temperatures in the summer of 2006 resulted in high numbers of various Culicoides species. Thus, the warm(ing) climatic conditions may favour the establishment of these viruses after they have been adventitiously introduced in a new part of Europe. The number of new bluetongue cases over time was equally influenced by changes in temperatures. The lag time between a change in temperature and a correlated change in number of outbreaks was estimated to be about 4 weeks. This resulted in an initial peak and then a second peak of new cases which were separated by a cooler period.

Besides temperature, the number of observed bluetongue cases was also related to other environmental factors such as altitude and animal density.

5. Local spread was modelled and found to occur at a rate of about 15 km per week. The timing of implementation of the first restriction measures and the timing of the implementation of subsequent changes to these restriction measures did not coincide with subsequent changes in the pattern of the epidemic curve.

6. It was demonstrated that wind may affect spread over long distances. In particular, the density of the observed wind events contributed to explain, at least in part,
  • the predominant horizontal (east-west) spread the epidemic,
  • the more limited spread north and south spread, and
  • the absence of recorded outbreaks in the U.K.

In conclusion, changes in climatic conditions coupled with frequent travel might increase the risk in the appearance and the establishment of diseases in parts of Europe that were thus far exotic for those regions.

A final report based on data obtained until 1 February 2007 and on analyses conducted through 31 March 2007 is under review.

April 2007
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