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Foot-and-Mouth Disease: serological survey of sheep flocks. Owen Denny. Northern Ireland: Location of outbreaks. Sheep involvement. The first outbreak (Newry) was associated with sheep imported from GB. The second (Dungannon) had disease in cattle, but the farm also had a sheep flock.
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Foot-and-Mouth Disease: serological survey of sheep flocks Owen Denny
Sheep involvement • The first outbreak (Newry) was associated with sheep imported from GB. • The second (Dungannon) had disease in cattle, but the farm also had a sheep flock. • The third (Ballymena) showed clinical evidence of FMD in both cattle and sheep, but epidemiological evidence suggests that the cattle were infected by the sheep. • Outbreak number 4 appears to be an extension of the second.
Sheep • There is currently no evidence of infection in pigs in Northern Ireland. • There is currently no evidence of airborne spread of virus from GB, or within NI. • No direct epidemiological links have been found between the outbreaks, e.g. personnel, equipment, animal movements, etc. • Sheep appear to be playing their usual role in the outbreaks here and in GB, i.e. as an inapparent reservoir of disease.
Reasons for a serological survey in sheep • The main reason is that we are unsure of the true level of infection in the national flock. Therefore difficult to predict the epidemic or adequately plan control measures. • Sheep and their movements appear have played a central role in the GB epidemic. • Investigations associated with our first outbreak suggest that unrecorded and illegal movements of sheep may take place into and within NI and also to the ROI It is extremely difficult to accurately trace these movements.
Reasons for a serological survey in sheep • It has been suggested that illegally imported sheep from Scotland may have been distributed to the Glens of Antrim and also to the Sperrins. • There is extensive contact between flocks of hill sheep. This may extend to common grazing arrangements. Potential for FMD to spread largely undetected. • With cattle moving out of winter housing there is increased contact between sheep and cattle.
Objectives of the Survey • Add to our epidemiological investigations • Identify and remove infected flocks
Location of sheep flocks - June 2000 census Glens - 1776 Sperrins - 3237 Mournes 2409 Other areas - 3305 Average flock size - approx 125 adults
What will we sample ? 1. All Risk Flocks. - tracing, contacts, suspect illegal imports, recently imported sheep, 3 and 10 Km zones, the periphery of zones, PVP identified etc. 2. Flocks in the Glens of Antrim and the Sperrins using a systematic random sample.(to detect a minimum of 1% prevalence with 95% confidence). The plan is then to extend this to all flocks. 3. Flocks in the Mournes. Initially survey and then extend as above 4. The rest of NI flocks not covered in I-III
Sampling • Target - 10,000 samples per day. Our staff and PVPs • All flocks are to be sampled at levels to detect 5% prevalence with 95% confidence • For flocks of less than 60 sheep, all sheep are to be samples. For flocks of 60 or more, 60 samples are to be taken. (this is actually more than required) • Each management unit held by a flock owner will be treated as a separate flock and sampled accordingly.
Sampling (2) • Numbers of samples - approx 90,000 “risk” samples, 71,000 random survey. Should be complete in 3 weeks. Then extension to other flocks - approx 0.5 million samples • Tested at 1/90. Positives titrated ( 1/90, 1/180, 1/360, 1/720, 1/1440,) • Positive samples to Pirbright for VNT testing
Serological Survey: distribution of flocks sampled < 24 May 2001 KEY: (1) Meigh SZ (2) Ardboe SZ (3) Cushendun SZ (4) Sperrins sampling area (5) Mournes sampling area (3) (4) (2) (1) (5)
Where are we currently ? • Flocks submitted to VSD - 9,195 • Approx. 80% of all flocks have been tested • Individual Samples submitted to VSD - 436,510 • NI Cases with negative validated blocking ELISA titres from VSD - 8,499 (92.4%) • NI Cases with positive validated blocking ELISA titres from VSD - 696 (7.6%) • Flocks outstanding on the FMD database - 2,594
Pirbright results • Cases sent to Pirbright - 1,014 • Negative VNT cases reported - 985 (97.1%) • Cases with “non-negative” results 29 (2.9%)
Factors that suggest the ELISA titres are not significant regarding FMDV: • The low titres obtained from the survey samples. (The majority of titres do not exceed 1/90) • The large number of “repeats” • The VSD ELISA is used to screen for FMD by initially using a single-point dilution of 1/90 and then titrating any samples that test positive to this. • Biological implausibility • 86% of flocks with VSD ELISA titres had only single samples positive to the test. • Lack of correlation between tests, and low titres from the Pirbright VNT • Recognised problems in the specificity of the VSD ELISA
Conclusions • The serological survey has been a considerable success and is one of the most extensive responses to FMD surveillance anywhere in the world. • Approximately 80% of all sheep flocks in NI have been sampled. • VSD have responded remarkably to the request to sample 10,000 sheep per day. • All non-negative tests generated a detailed epidemiological flock investigation. • The survey has not detected FMD in any NI flocks.
