Such was the shock caused by bluetongue in Western Europe that initial reaction was blue what? Temperate midge species like Culicoides obsoletus, already spreading the Mediterranean BTV (bluetongue virus) serotypes, picked up BTV8 with relish and began to spread it with speed. Scientists instinctively knew this serotype was different, even before laboratory identification as BTV8, because the 'Mediterranean' strains had only seriously affected sheep, whereas this serotype was already killing both cattle and sheep in large numbers.
The disease hit sheep farming in Belgium hard, affecting 15% of the national flock and killing 30,000 animals, half of those infected. Germany with 20,000 farms infected lost 10,000 sheep as well as 1,500 cattle, 8% of the 17,000 infected. Comparable losses occurred in the Netherlands and France with 6,400 and 12,700 farms infected. BTV8 is lethal to cattle and sheep whereas other serotypes have reserved their most deadly consequences for sheep.
BTV8 predictably appeared in the United Kingdom (UK) in late summer 2007. Meteorologists were able to pin-point arrival on UK soil down to the day and the hour by plotting offshore winds from the continent, that carried midges into East Anglia. There were 138 infected premises up to 19 August 2008. All future hope for bluetongue management in the UK is pinned on dedicated BTV8 vaccine, now being administered to livestock in bluetongue protection zones that continue to be extended westwards and northwards across the country.
Reports from France of new cases starting July 2008 show bluetongue survived another northern European winter and French farmers are vaccinating in a race against time. By third week of August 2008 new cases of BTV8 were accumulating at the rate of 1000 per week piling up to 5000 since July 2008. Most were occurring along the front line of the 2007 outbreak, suggesting the epidemic was spreading into new areas despite a compulsory vaccination policy in France.
Voluntary vaccination was rolled out in May 2008 in the UK from East Anglia where the disease arrived in 2007. The 'vaccination front' has moved steadily westwards and northwards reaching Wales in the west and Yorkshire in the north and now alarmingly close to the UK's main sheep producing areas. Experts are concerned because sheep just outside the vaccinated zone are more at risk but will probably not be vaccinated until the autumn. All depends they say on whether sufficient livestock south and east of the vaccination front line have been vaccinated to slow down spread of disease.
Modelling research by Camile Szmaragd and others at IAH (Institute of Animal Health) Pirbright predicted the extent of bluetongue spread [in the UK] by October 2008 in the absence of a vaccine. There was an 80-100% chance of the virus covering most of southern, central and eastern England and 10-20% chance of western England, Wales and parts of lowland Scotland becoming infected. The question still on everyone's lips concerns the native Scottish midge (C. impunctatus), and will it start to carry and transmit BTV8, thus putting huge highland sheep flocks and prime beef cattle in Scotland at risk.
Midge pest monitoring is urgently required to pin-point all potential midge-species vectors and any pattern of spread. The European Reference Laboratory for Bluetongue at IAH Pirbright has so far only recommended use of light traps, but these clearly have limitations. They will catch hundreds of midges but are they the ones responsible for vectoring blue tongue virus?
Experiments carried out at Pirbright have shown up potential limitations of light traps and deficiencies in their catch profile for midges associated with sheep. Catches from penned sheep covered with nets to secure all midges on animals were compared with catches in light traps sited nearby. Comparative trap catches did not match up so scientists are now viewing with some urgency midge monitoring using dedicated traps loaded with animal host odour lures proven for target midge attraction.
Sentinel trap with livestock odour lure
Most prominent and appropriate for midge monitoring is the BG Sentinel Trap patented by BioGents (Germany), and loaded with the lure from AgriSense-BCS (UK) containing carbon dioxide (CO2) and key synergistic components identified from animal host odours.
The 'BG Sentinel' trap mimics convection currents created and generated during heat exchange by the animal body, as well as displaying attractive visual (black and white) cues, and releasing attractant chemicals from the lure over an extensive surface area. Traps used for general midge monitoring are supplied with an integral light at the top-located orifice to complement midge catches. As the chemical attractants become more finely tuned for the BTV vectors this light requirement should become redundant, thus eliminating catch of non BTV vectors.
An integral fan blows volatile chemicals from the lure upwards through the fabric gauze on top of the trap in a convection-like current, thus mimicking scents continually emitted by sweat on the surface of the animal body. Any midges following the scent trail are visually attracted to the edge of the black suction column centrally placed in the top of the trap and surrounded by the white fabric gauze. Insects are subsequently forced into a net inside the trap by the downward suction forces generated by the trap's fan.
Key component and synergist in the AgriSense lure is the R (-) isomer of 1-octen-3-ol, an optically-active isomer (enantiomer) with proven midge attraction. The lure is already well established for monitoring and control of Scottish midge in its current role as a nuisance biting insect.
Research has shown how light or CO2 as stand-alone factors are too much of a blunt instrument, attracting all sorts of other insects as well as the target midges. The addition of 1-octen-3-ol synergises (heightens and clarifies) the main CO2 plume, allowing target midges to distinguish between CO2 plumes from the trap lure and others such as those from rotting fruit. Target midges are therefore more likely to follow this plume from the BG Sentinel Trap, which consequently provides more selective catches giving a clearer picture of target midge profile in the surrounding environment.
Re-evaluation of C. obsoletus and other closely related midge species as potential vectors of the BTV8 serotype is required as a matter of urgency. The situation is complicated by 24 different BTV serotypes, varying widely in virulence and mortality, and the 'complex classification' of C. obsoletus. This 'species' comprises a number of different biotypes morphologically similar but genetically different. The complex includes C. chiopterus, C. scoticus and C. dewulfi as well as C. obsoletus. Highly specific midge monitoring only achieved using animal host odour lures is required.
AgriSense-BCS is willing to collaborate with anyone over their semio-chemical lures which they believe are the most appropriate, pulling in those species of midge that specifically respond to animal cues. This willingness extends to any appropriate trap testing procedure although their lures should ideally be used in the BG Sentinel trap for which they are custom-designed.
