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 / WALES, TO SLAUGHTER IN SCOTLAND, AFTER THE "VECTOR-FREE" PERIOD ENDS ON 15 MARCH.

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 and on mainland Europe. 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, including infertility and abortion, with significant levels of suffering, compromised animal welfare and production losses.

• Bans on exports of live ruminants and germplasm can be long term and economically damaging.

• Movement restrictions severely stifle economic activity within and between restricted regions

2. Outbreaks can be very difficult to control and eradicate because little can be done about the vector, and 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 as this species can produce its first batch of eggs without taking a blood meal). 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 last 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 detectable 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.

Vector competence

4. Critical in the understanding of the epidemiology of Bluetongue is a knowledge of the competence of the Culicoides species as a virus vector 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 in the host, 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 "Scottish" 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, or moved illegally. 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). A "proper" viraemia is usually detectable by day three when clinical signs become apparent and an infected animal could be shown to be PCR positive for the first time if tested.

• The animal is viraemic but not demonstrating clinical signs: "Silent" disease in cattle is a feature of BTV infections. Usually clinical signs, such as fever, correspond with the onset and peak of viraemia, but chronic clinical signs can outlast the viraemia. 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 140 days (by PCR which detects viral RNA and does not necessarily reflect a true infectious state).

• Clinical signs of disease in susceptible animals are not recognised by the animal keeper. Almost half of the early confirmed cases in England have been detected by surveillance testing rather than by report case investigations and around twenty more cases have been confirmed to date following disclosure of ELISA positive animals at pre-movement testing during the vector free period from 20 December 2007 to 15 March 2008

• Clinical signs are recognised but the animals are transported illegally.

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. This exercise is almost completed

• Active serosurveillance

Risk Assessment

8. The last confirmed case of bluetongue in the English protection zone, prior to the vector free period starting on 20 December, was on 3 December 2007. The protection zone and the surveillance zone (together the restricted zone) have been extended over the vector free period on a number of occasions as recovered animals have been detected retrospectively, but despite testing some 35,000 pre-movement samples for BTV, the current zones do appear to have been in the right places to have contained the disease over winter. Scotland has had no incursions of positive animals or disease from the English surveillance zone - so far as can be determined. However Defra continues to disclose previously undetected, recovered livestock at pre-movement tests in their surveillance zone.

9. An extensive awareness campaign has been initiated throughout Great Britain designed to mitigate 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. However, as the Spring progresses and the "vector free period" ends, there will be an increasing likelihood, during the coming year, that the number of viraemic animals in the surveillance zone will increase as disease overwinters and midge activity resumes. Consequently, there will be a increasing risk that one of these viraemic animals will be transported to Scotland as the year progresses. The European experience last year supports that view.

11. 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.

12. The risk from viraemic livestock can be reduced by not transporting them when midge activity is high, i.e. they should not be transported between one hour before sunset and one hour after sunrise.

13. The previous English movement rules mean that no cattle for sale at surveillance zone markets were allowed to have come from the protection zone.

14. The risk of transporting viraemic slaughter stock to Scotland, from the Bluetongue Surveillance Zone in England, immediately after the "vector-free period" is considered to be very low (very rare but cannot be excluded). This risk will increase throughout the year inline with an increase in seasonal temperatures and a consequent resumption of midge activity. Midges will become more dangerous with time as the extrinsic incubation period shortens. By summer 2008 the risk of infected competent midges being mechanically transported to Scotland in livestock vehicles is considered to be "medium" (occurs regularly).

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

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, based on our knowledge of the current protection and surveillance zones, 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 bite and transmit disease to Scottish ruminants.

Risk Assessment

16. The risk of disease spread from midges that are transported in livestock vehicles was reduced by restricting movements to within the "vector free period", however this risk increases as temperatures rise throughout the year. The risk from any midges that are transported in livestock vehicles could be mitigated by the diligent use of insecticides inside the vehicle before transit.

17. The risk of infected competent midges being mechanically transported to Scotland in livestock vehicles after the vector free period is considered to be very low (very rare but cannot be excluded) immediately after March 15 (when the vector free period ends) 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 residual 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. This risk will increase throughout the year inline with an increase in seasonal temperatures and a consequent resumption of midge activity. These midges will become more dangerous with time as the extrinsic incubation period shortens. By summer 2008 the risk of infected competent midges being mechanically transported to Scotland in livestock vehicles is considered to be "medium" (occurs regularly).

Conclusion

21. Bringing slaughter stock from the BTV surveillance zone in England/Wales to Scotland, after the vector free period ends on 15 March, creates a risk of introducing either viraemic livestock or infected midges. This risk is considered to begin as "very low" (very rare but cannot be excluded - immediately after 15 March) and rise to "medium (occurs regularly) by summer 2008. This risk could be eliminated by not bringing slaughter stock to Scotland from BTV surveillance zones.

Dr Martyn Blissitt

Veterinary Adviser

Veterinary Division , 29 February 2008