26 August 2006

When bluetongue first appeared in northern Europe in 2006, the outbreak was widely expected to be stamped out by the arrival of winter and the cold conditions that prevent normal bluetongue transmission. In the event, the disease re-emerged months later to devastating effect. In fact, the capacity of bluetongue to re-appear after months of absence has been recognised for decades, but how it does so remains a mystery. So where does bluetongue virus sleep in the winter? Anthony Wilson, Karin Darpel and Philip Mellor of the Institute for Animal Health consider the possibilities in an article published in PLoS Biology* this month.

The question is not merely one of academic interest. The answer could dictate restrictions to the international trade of ruminants, and determine how to deal with bluetongue outbreaks in the future. In addition, it could help us to assess our risk from other midge-transmitted viruses related to bluetongue, such as epizootic haemorrhagic disease virus (which also affects ruminants) and African horse sickness virus (which affects equine species).

 

Bluetongue virus is normally spread by Culicoides midges and infects ruminants, but evidence for a number of other possible transmission routes has surfaced over the years. Such routes may be critical to the survival of bluetongue virus during cool conditions. Wilson and colleagues consider hypotheses that have been around for nearly as long as bluetongue epidemiology itself, alongside others that have been suggested for the first time during the current outbreak in northern Europe. The authors summarise the current evidence for each and compare their ability to explain confirmed examples of overwintering in the field, as well as considering how lessons learned from bluetongue and other insect-borne pathogens might inform research on other diseases in the future.

 

Professor Mellor said: "Although the major route of bluetongue virus transmission is undoubtedly that between Culicoides midges and ruminants, experiments have revealed a toolbox of possible additional mechanisms, with the potential to interact with and complement one another. These may assume greater importance during the cooler months at northern latitudes, when normal transmission is interrupted."

 

The most likely routes involved in the persistence of bluetongue in northern Europe are the longer survival of adult midges during the winter (aided by mild conditions and a possible inclination to seek shelter in animal housing), the persistence of latent virus infections in apparently recovered livestock, or the spread of virus from pregnant infected livestock to the foetus. Mechanisms thought to be less likely include extremely prolonged infections in a currently unknown reservoir such as deer, virus spread from infected midges to the next generation via the eggs, or transmission by ticks or biting flies. However, it will only be possible to shed further light on complex epidemiological problems of this kind through collaborative projects involving disciplines as diverse as entomology, climatology and veterinary science, as well as by improving the collection and sharing of basic data.

 

*Where does bluetongue virus sleep in the winter?
Anthony Wilson, Karin Darpel and Philip Mellor.
Public Library of Science Biology (2008) volume 6, issue number 8: page e210.
This paper is freely available online:
http://dx.doi.org/10.1371/journal.pbio.0060210

 

For more about bluetongue in the UK and elsewhere, start here:
http://www.iah.ac.uk/bluetongue/bt.shtml