VETERINARY RISK ASSESSMENT - WHAT IS THE RISK OF CAUSING BLUETONGUE (BTV8) OUTBREAKS IN SCOTLAND BY ALLOWING MOVEMENTS OF ANIMALS FROM THE BLUETONGUE SURVEILLANCE ZONE IN ENGLAND, TO SLAUGHTER IN SCOTLAND, DURING THE "VECTOR-FREE" PERIOD
Hazard identification
1. Bluetongue virus ( BTV) naturally infects domestic and wild ruminants and camelids and is transmitted by several species of Culicoides (biting midges). Bluetongue is primarily a disease of sheep where cattle exist as reservoirs of virus; however cattle have also shown clinical signs of disease in the current BTV8 outbreak in England. The disease impacts on animal populations in four ways:
- Direct losses to the sheep farming industry - with potentially high levels of sheep mortality.
- Severe illness in infected sheep (and cattle) with significant levels of suffering, compromised animal welfare and production losses.
- Bans on exports of live ruminants and germ plasm can be long term and economically damaging.
- Movement restrictions severely stifle usual economic activity within restricted regions?
2. Outbreaks can be very difficult to control and eradicate because little can be done about the vector. Non-symptomatic cattle and goats can harbour high levels of undetected infection.
Culicoides vectors
3. Vector species of Culicoides biting midges tend to breed in damp or wet soil, enriched with fresh or composted dung, and they blood-feed opportunistically on large vertebrate hosts. Culicoides midges tend to be most active from about an hour before sunset until an hour after sunrise. They are most active in the evening, until about midnight, when activity eases off, with another peak of activity around sunrise. However, on dull days or in shady areas vectors may be active during the day. Most Culicoides species are averse to entering confined spaces, such as buildings or vehicles, although recent and ongoing research work suggests that there may be more midge activity indoors in autumn/winter than was originally thought to be the case. Female midges take a blood meal prior to egg laying ( Culicoides impunctatus, the Highland midge, being the exception). They feed at roughly four day intervals and live typically for two to three weeks. The eggs hatch in two to three days and depending on the temperature the larval stages lasts 12 to 16 days. The life cycle is egg, four larval instars, pupa and adult. In temperate cool regions midges "over winter" at the fourth level instar stage. Adults emerge two to three days after pupation and take a blood meal roughly one day later. They also mate during this time. Over the year, Culicoides populations in Britain fluctuate but there is generally a peak in May/June and a second peak in September/October. Adult midge populations in Britain tend to fall dramatically from mid to late October and from December adults are usually either not detectable at all, or only in very small numbers, depending on the prevailing temperature. In April to May populations rise again. In cooler conditions their metabolism slows and they survive for periods of more than a month. When infected with BTV midges remain infected for life.
Top of page
Vector competence
4. Critical in the understanding of the epidemiology of Bluetongue is knowledge of the virus competence of the Culicoides species in different ecosystems. Competent midges may become infected when biting viraemic vertebrates and the probability of infection is dependent on (a) the genotype of the midge, (b) the strain of the virus, (c) the level of viraemia, and (d) other environmental factors, including climate. The extrinsic incubation period ( EIP-the period between feeding on infected blood and the appearance of virus in the saliva of the midge) is one to two weeks. The proportion of adult Culicoides capable of transmitting virus is dependent on (a) the vector competence (the capacity for the virus to develop in, and be transmitted by, the vector - temperature dependent); (b) adult midge survival; (c) the blood feeding interval and (d) the EIP. In order to transmit virus Culicoides midges must not only be vector competent, but also survive long enough to blood feed after the completion of the viral EIP. They are most likely to satisfy these criteria at high temperatures (e.g. 27 to 300C) because, although adult survival is reduced at high temperatures, this is more than compensated for by the accompanying decrease in the duration of the EIP and blood-feeding interval. Consequently, it is likely that warmer temperatures will increase probability that Culicoides will survive long enough to transmit virus.
Risk analysis
5. Allowing movements of cattle to slaughter in Scotland from the bluetongue Surveillance Zone in England presents two risks:-
- The transport of viraemic animals into Scotland where they might be bitten by midges which then become infected and capable of infecting Scottish livestock.
- Transport of infected competent midges to Scotland in livestock vehicles, with their subsequent release in Scotland, where they might bite and infect Scottish livestock.
Transport of Infected/Viraemic Animals to Scotland
6. For this to occur, animals on the premises of origin in the Surveillance Zone would have to be infected with Bluetongue but not showing clinical signs, or have gone undetected. There are four mechanisms by which an infected animal could be transported into Scotland:-
- The animal is incubating the disease and is not yet showing clinical signs. The incubation period following infection from a biting midge is three to 20 days in sheep and cattle ( OIE reference).
- The animal is viraemic but not demonstrating clinical signs. Viraemia has been detected in infected sheep for up to 54 days (although it has been recovered from sheep skin for up to 63 days post infection), in goats up to 38 days, and in cattle for up to approximately 1440 days (by PCR which may not reflect a true infectious state).
- Clinical signs of disease in susceptible animals are not recognised by the animal keeper. Almost half of the confirmed cases in England have been detected by surveillance testing rather than by report case investigations.
- Clinical signs are recognised but the animals are transported illegally.
Top of page
7. In the Surveillance Zone, the absence of infection in animals is determined by three methods:-
- The investigation and negation of report cases. Report cases are dependent on the animal keeper recognising signs of BTV and reporting it to Animal Health.
- The tracing and investigation of movements from premises known to have been infected.
Risk Mitigation
8. The last confirmed case of bluetongue in the English protection zone was on 3 December 2007. There have been no confirmed cases of bluetongue in the Bluetongue Surveillance Zone.
9. An extensive awareness campaign has been initiated throughout Great Britain mitigating against the possibility of unreported disease.
10. It is rare to be able to detect viraemia in sheep after day 14, post infection and the OIE considers the "true" viraemia of cattle as lasting less than 4 weeks, with less than one per cent exceeding eight weeks. As the winter progresses into the "vector free period" there will be a decreasing livelihood that viraemic animals remain.
11. BTV8 infections of cattle and sheep have been shown to present with clinical signs in both species and any clinical signs would be indicative of viraemia.
12. The risk from viraemic livestock can be reduced by slaughtering them as soon as possible after arrival at Scottish abattoirs, to minimise any contact time with native midges.
13 The risk from viraemic livestock can be reduced by not transporting them when midge activity is high ie between one hour before sunset and one hour after sunrise; however during the vector free period this is considerably less important.
14. The current English movement rules mean that no cattle for sale at Surveillance Zone markets are allowed to have come from the protection zone.
Transport of Competent Infected Midges from the Surveillance Zone in England in Livestock Vehicles, and their subsequent release in Scotland where they could bite and infect susceptible Scottish livestock
Top of page
15. The flight range of Culicoides is generally short (one to two kilometres per day range) and the Protection Zone is likely to be large enough to limit the movement of infected vectors by flight. The risk from wind dissemination of infected midges is considered to be negligible from England and very low from continental Europe. However, depending on climatic temperature in a Surveillance Zone, infected midges could be transported in livestock vehicles to Scotland and may (a) bite and transmit disease to Scottish ruminants or (b) over winter in Scotland and emerge in spring 2008 to infect Scottish ruminants.
Risk Mitigation
16. The risk from any midges that are transported in livestock vehicles can be mitigated by the diligent use of insecticides inside the vehicle before transit.
17. The risk of disease spread from midges that are transported in livestock vehicles is reduced by restricting movements to within the "vector free period".
18. The risk is further minimised by restricting movements to within the "transmission free period". Within the "transmission free period", even if a few midges are able to fly, all infected midges from 2007 should have already died naturally.
Conclusion
19. The risk of infected competent midges being mechanically transported to Scotland in livestock vehicles during the vector free period is considered to be very low (very rare but cannot be excluded). The possibility cannot be excluded because:
(a) Midge activity can resume in temporarily warmer periods,
(b) the criteria used by IAH Pirbright to determine the "vector free" and "transmission free" periods are dependent on incomplete midge trapping and climatic data;
(c) There is the possibility that some infected midges may survive over winter, either inside buildings, or by some other, as yet uncharacterised mechanism;
(d) There will be some midge activity, even during the "vector-free period" as defined in the Regulation (fewer than five parous females caught by weekly trapping), although this population may not be capable of transmitting infection.
20. The risk of transporting viraemic slaughter stock to Scotland, from the Bluetongue Surveillance Zone in England, during the "vector-free period" is considered to be very low (very rare but cannot be excluded) and decreasing.
Veterinary Division
Rural Directorate
Scottish Executive
Room 336, Pentland House
47 Robb's Loan, Edinburgh , EH14 1TY
19 December 2007
Top of page