Quantitative assessment of the probability of bluetongue virus overwintering by horizontal transmission: application to Germany

Abstract

Even though bluetongue virus (BTV) transmission is apparently interrupted during winter, bluetongue outbreaks often reappear in the next season (overwintering). Several mechanisms for BTV overwintering have been proposed, but to date, their relative importance remain unclear. In order to assess the probability of BTV overwintering by persistence in adult vectors, ruminants (through prolonged viraemia) or a combination of both, a quantitative risk assessment model was developed. Furthermore, the model allowed the role played by the residual number of vectors present during winter to be examined, and the effect of a proportion of Culicoides living inside buildings (endophilic behaviour) to be explored. The model was then applied to a real scenario: overwintering in Germany between 2006 and 2007. The results showed that the limited number of vectors active during winter seemed to allow the transmission of BTV during this period, and that while transmission was favoured by the endophilic behaviour of some Culicoides, its effect was limited. Even though transmission was possible, the likelihood of BTV overwintering by the mechanisms studied seemed too low to explain the observed re-emergence of the disease. Therefore, other overwintering mechanisms not considered in the model are likely to have played a significant role in BTV overwintering in Germany between 2006 and 2007.

Figure 1

Pathways for overwintering considered in the model: (I) horizontal transmission in the insect vectors, (II) horizontal transmission in the ruminant hosts and (III) horizontal transmission in the insect vector plus the ruminant population. [a] represents infection of vectors before the PLVA and [b] infection of vectors during the PLVA. In pathways Ia and IIIa, the vectors need to have emerged before the PLVA, while in pathways Ib and IIIb, the vectors may have emerged before the PLVA, but also during the PLVA.

Figure 2

Steps for overwintering for pathway I and pathways II and III.

Figure 3

Mean daily temperatures (red line) for November to May in North Rhine-Westphalia. Virogenesis rate limit (blue line) and biting rate limit (green line). Source: Bundesministerium für Verkher, Bau und Stadtentwicklung. Klimadaten Deutschland. http://www.dwd.de/bvbw/appmanager/bvbw/dwdwwwDesktop?