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SIR, — Outbreaks of bluetongue virus serotype 8 (btv-8) occurred in Belgium, the Netherlands, Luxembourg, Germany and northern France in the second half of 2006. The virus re-emerged this year in May, resulting in outbreaks reaching epizootic proportions (DEFRA 2007a).
The uk authorities had been monitoring the outbreak in north-west Europe and considered that the uk was at risk. A programme of import controls and postimport checks of ruminants was implemented to minimise the risk of importing an infected animal. Daily estimates of the likelihood of windborne incursions of midges from the Continent were undertaken by the Met Office and the Institute for Animal Health (iah), Pirbright (Gloster and others 2007). In preparation, defra worked alongside a representative group of stakeholders to review the uk bluetongue virus (btv) control strategy and provide advice on bluetongue to the farming industry and veterinarians (DEFRA 2007b).
The uk remained free of btv-8 until September 22 when the first case of bluetongue was diagnosed in a Highland cow in Suffolk. During the following week a further five cattle were confirmed positive for btv in the Ipswich and Lowestoft area. From analysis of the test results from the btv Community Reference Laboratory at iah, Pirbright, together with wind plume modelling, it was concluded that btv was circulating between ruminants and the local population of midges, and bluetongue disease was confirmed in the uk on September 28. Also based on the wind plume modelling, there is a strong hypothesis that the first cases of btv may have arisen from an incursion of infected midges from the Continent around August 4.
A bluetongue control zone around the area where cases had been identified and a larger bluetongue protection zone was established in accordance with the uk bluetongue control strategy and eu rules (DEFRA 2007c).
Outbreak and surveillance
Surveillance commenced immediately following identification of the first case to determine the presence and possible extent of local circulation of disease. In addition to investigations of reports of suspect disease from farmers and veterinary surgeons, Animal Health is carrying out targeted surveillance of premises with cattle within a 10 km radius of the first four farms with infected animals and tracing of animals that have moved out of the area. Monitoring of the midge populations is also being undertaken by iah, Pirbright. At the time of writing (17.00, October 7), btv was confirmed on 30 farms; 58 farms were negative and 15 are under investigation. Of the infected farms, 14 premises were detected by the active surveillance programme.
Bluetongue is traditionally considered a disease of sheep, with most other species rarely showing clinical signs. However, clinical disease arising from infection with btv-8 has been widely reported in cattle in Europe and, as on the Continent, at the beginning of the outbreak, disease has been predominantly confined to cattle in the uk. In addition to sheep and cattle, member states have reported clinical disease in alpacas, goats, red deer and mouflon.
The signs range from inapparent, to mild to severe disease and death. Although there is variation in the severity of the disease between breeds, the clinical picture seen at this stage in cattle and sheep in the uk is similar to that seen on the Continent (Darpel and others 2007, Dercksen and Lewis 2007). Clinical signs included high temperature, inflammation of the skin and mucous membranes, petechiation and ulceration in the oral mucous membranes, ocular and nasal discharge, drooling, crusting of the muzzle, respiratory distress, oedema of the lips, muzzle and head, coronitis and lameness. Pictures of the clinical cases in cattle in East Anglia along with archive photos of the disease in sheep can be viewed on the defra website (DEFRA 2007d).
While there are guides on diagnosis of bluetongue, it can be difficult to recognise and differentiate clinically. Diseases such as foot-and-mouth disease (fmd) and malignant catarrhal fever should be considered in cattle and facial eczema, foot rot, contagious digital dermatitis and severe parasitism in sheep. Photosensitisation is also a differential diagnosis. Bluetongue can only be confirmed by laboratory tests for antibodies or virus (DEFRA 2007e).
Pathology and postmortem findings
While the overriding pathology of bluetongue is multisystemic haemorrhage with oedema and low to moderate grade inflammation, it can still be difficult to diagnose on postmortem examination. While there is no pathognomonic lesion in bluetongue, ecchymotic haemorrhages at the base of the pulmonary artery are very strongly suggestive of disease.
Postmortem pictures and descriptions of lesions in the first three confirmed cases in adult cattle can be found on the Veterinary Laboratories Agency website (VLA 2007). Two of these cattle, both of which were clinically affected, showed lesions consistent with bluetongue with one showing haemorrhages at the base of the pulmonary artery. The other cow was not clinically affected but was pcr positive. The only pathology in this cow suggestive of bluetongue was petechiation of skin on the ventral abdomen and around the bases of the teats.
Bluetongue control policy and awareness
Bluetongue is very different from fmd. It is not a contagious disease but is spread by midges. Thus, it is virtually impossible to stamp out by culling infected animals.
defra is working with a core group of stakeholders and has agreed that all current efforts should be made to confine the outbreak to East Anglia through movement controls of ruminants. Alongside this, it is hoped that local meteorological conditions will also diminish midge activity and limit the spread of btv. At this stage it is difficult to predict whether btv will overwinter; however, the experience on the Continent suggests that bluetongue is likely to recur in 2008.
The disease control strategy will be under constant review, taking account of epidemiological information, veterinary risk assessment and economic analysis during the next few months. This includes developing strategies for an outbreak in 2008, which could include vaccination.
The current situation is dynamic and defra is aware of the hardship that bluetongue is bringing. Together with industry we are working as quickly as possible to minimise the impact of this disease as far as possible. We encourage veterinarians to assist us in maintaining vigilance and to encourage farmers, particularly in the restricted zones, to report all suspected cases so that we can monitor whether spread is occurring.
Veterinarians also have an important role in alleviating the adverse welfare consequences for individual animals, including euthanasia if necessary.
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