The Scottish Government

VETERINARY RISK ASSESSMENT - WHAT IS THE RISK OF CAUSING BLUETONGUE (BTV8) OUTBREAKS IN SCOTLAND BY ALLOWING MOVEMENTS OF ANIMALS TO SLAUGHTER IN SCOTLAND, FROM THE BLUETONGUE PROTECTION ZONE IN ENGLAND/WALES, FROM AUGUST 2008 ONWARDS.

Introduction

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 2007 BTV8 outbreak in England, and on mainland Europe in 2006 and 2007. The disease impacts on animal populations in four ways:

• Direct losses (mainly) 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 restrict economic activity within and between restricted regions.

2. Outbreaks can be very difficult to control and eradicate because little can be done about the midge vector, and non-symptomatic cattle, goats and sheep 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 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 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 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) 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 four to twenty days. 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.

The Protection / Surveillance (Restricted Zone) in England / Wales - 2007/2008 and BTV8 Cases in England

5. 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) were extended over the 2007/2008 vector free period as recovered animals were detected retrospectively by pre-movement testing. However, despite testing some 35,000 pre-movement samples for BTV and disclosing more locations of infection, the various Protection and Surveillance zones do appear to have been correctly positioned to contain any dissemination of disease. So far as can be determined, Scotland had no incursions of infected animals (or midges) from the English Restricted zones. At the time of writing, England has had 136 confirmed cases of BTV8 - all attributed to circulating virus in 2007 and all largely contained in the East Anglia and the South East, with a solitary outlying case in Lincolnshire and two outlying cases in Dorset. There have been no cases to date confirmed in England in 2008 which are considered to be due to virus circulating this year.

Vaccination

6. Defra began to make BTV8 (Intervet) vaccine available to the English Protection Zone in May 2008. The vaccine is administered as one dose for sheep, and two doses for cattle - given three weeks apart (Intervet vaccine), or four weeks apart (Merial vaccine) with "full protection" three weeks later. Since May, Defra have followed a vaccine roll-out delivery plan by progressively extending the Protection Zone to previously Free Areas of England in the absence of any more disease - to facilitate the wider delivery of vaccine on a county basis (because no vaccination is allowed in Surveillance Zones or Free-areas). At the time of writing only Cumbria and Northumberland remain outside the Protection Zone in England and Defra's intention is to extend the English Protection Zone to the whole of England over the coming weeks. We are advised by Defra that there has been an average uptake of BTV8 vaccine of around 70% across the Protection zone as it stands - in terms of doses sold through pharmaceutical wholesalers when compared to the eligible numbers of cattle and sheep per county. However, it is acknowledged that some counties (especially outlying counties) have significantly lower uptake when measured this way. There is no available measure of how many cattle or sheep have actually been vaccinated and no available measure of how many cattle and sheep that have been vaccinated are actually protected. It appears that some vaccinated sheep, for example, are ELISA negative but may still be protected as having been "primed" with one dose of vaccine.

Rules for Movement of Livestock for slaughter from BTV8 Protection Zones

7. Vaccinated cattle and sheep can move freely to Scotland regardless of whether the movement is to slaughter (subject of course to meeting the Defra licence conditions, and the Scottish Government veterinary vaccination certification and post movement notification requirements). Unvaccinated cattle and sheep can also move from a Protection Zone to a Free Area for slaughter (Council Regulation 1266/2007) providing that:

· There have been no BTV8 cases in the last 30 days on the source holding - Article 8 (4) (a)

· Animals are transported under official supervision, directly to a slaughterhouse for slaughter within 24 hrs of arrival - Article 8 (4) (b)

· 48 hrs prior notice of the move is given- Article 8 (4) (c)

8. Article 8 (5) allows Scottish Government to designate slaughterhouses to take animals out of Protection zones, under certain conditions designed to reduce the risks of spreading disease to new areas. Designated slaughterhouses are identified on the basis of a risk assessment that takes account of, for example, the time of day at which transport takes place, biosecurity measures in place at slaughterhouse etc

Estimating the Number of movements to slaughter in Scotland from England.

9. Figures supplied by QMS indicate that 37,000 cattle of non-Scottish origin were slaughtered in Scotland in 2007 and 48,000 cattle in 2006. Figures from Scottish Association of meat Wholesalers ( SAMW) members indicate that, between June and November 2006, 17,084 cattle and 25,124 sheep were moved from England to Scotland for slaughter. It is clear that the numbers of livestock involved are significant in terms of risk.

Risk analysis

10. Allowing movements of cattle to slaughter in Scotland from the bluetongue Protection Zone in England/Wales 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 / Midges to Scotland

Animals

11. For this to occur, animals on the premises of origin in the Protection 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. Over half of the confirmed cases in England have been detected by surveillance testing (including pre-movement testing during the vector free period from 20 December 2007 to 15 March 2008) rather than by report case investigations.

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

12. In the Protection 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 has been completed for virus circulating in 2007.

• Active serosurveillance - completed for the 2007 season.

13. An extensive awareness campaign has been initiated throughout Great Britain designed to mitigate the risks of unreported disease.

14. 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, a previous risk assessment recognised that, as the year progressed after the 2007/2008 "vector free period", there was an increasing likelihood, as the summer approached, that the number of viraemic animals in the Protection Zone would increase as (a) disease overwinters (b) midge activity resumes and (c) the zone increases in size allowing the free movement of infected animals over a wider area. Consequently it was concluded that there was an increasing risk that a viraemic animals would be transported to Scotland as the year progressed. The European experience last year supported that view.

Midges

15. The risk of disease spread from midges that are transported in livestock vehicles was reduced previously by restricting movements to within the "vector free period", and the risk mitigated further by the diligent use of insecticides inside the vehicle before transit. The risk of infected competent midges being mechanically transported to Scotland in livestock vehicles after the vector free period ended was considered to be very low (very rare but cannot be excluded) immediately after March 15. However, it was also thought that this risk would rise towards the summer and fall again after September 2008.

16. The previous risk assessment, in March 2007, concluded that:

• 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.

17. It is now necessary to reassess the risk as determined previously, in the light of developments in 2008.

Factors tending to Increase/Maintain Risk

• There have been over two thousand outbreaks of BTV8 in France and unrestricted livestock from affected areas can travel freely to the English Protection zone - albeit with the risk mitigated by current import control requirements. Other Member states have also reported circulating BTV8 virus in 2008;

• It is the height of summer currently, with peak temperatures and peak conditions for rapid virus development in midges and the shortest extrinsic incubation periods;

• There is still the second peak of midge numbers/activity to come in Scotland - traditionally seen in September / October;

• There is now the free movement of animals from known infected areas in 2007 right up to the counties bordering Scotland;

Factors tending to reduce risk

• There have been no new confirmed cases of BTV8 identified in the English Protection zone in 2008 as a result of virus circulating in 2008;

• There is now a significant number of vaccinated animals (circa 70%) of the eligible population on an average county basis) in the English Protection zone that provide a buffer against the livestock in East Anglia/the South East where farms are known to have been infected n 200;

• England and Wales have experienced no spread of BTV8 having allowed movements of livestock from Protection zones to slaughter in Free areas in England for almost a year;

• The Institute of Animal Health at Pirbright have revised down their estimate of the number of new confirmed outbreaks of BTV8 in England in 2008 (in the light of the vaccination campaign and cooler weather generally) from 3,000 to around/under 100;

• There are now widespread vaccination campaigns across the European Union.

Conclusion

18. It is now concluded that:

Bringing slaughter stock from the BTV Protection zone in England/Wales to Scotland from August 2008 creates a risk of introducing either viraemic livestock or infected midges. However, this risk is now considered to have changed from "medium (occurs regularly) to "low (rare but does occur) in the light of the absence of evidence of circulating BTV8 virus so far this year in the UK, the vaccination campaign in the Protection zone, and significantly lower estimates (from IAH Pirbright) of the possible number of new BTV8 outbreaks in England in 2008. Furthermore this risk, and the impact of any disease incursion, should progressively decrease after September and be "very low" or "negligible" as we enter the next vector free period. The risk could be eliminated by not bringing slaughter stock to Scotland from BTV surveillance zones, and can be mitigated by allowing licensed entry under certain conditions.

Dr Martyn Blissitt

Veterinary Adviser (Notifiable Diseases)

Veterinary Division

Rural Directorate

Scottish Government,

15 August 2008