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Wellcome Trust Animal Health Research: Recent and Future Developments. Modelling the impact of cattle movement and pre-movement testing on Bovine TB in mainland Great Britain. William Wint 1 , Neil Alexander 1 , Joshua Mawdsley 2 , Richard Clifton-Hadley 3 and Andy Mitchell 2
Animal Health Research: Recent and Future Developments
Modelling the impact of cattle movement and pre-movement testing
on Bovine TB in mainland Great Britain.
William Wint1, Neil Alexander1, Joshua Mawdsley2, Richard Clifton-Hadley3 and Andy Mitchell2
1 Environmental Research Group, Oxford. Department of Zoology, South Parks Road, Oxford OX1 3PS. William.Wint@zoo.ox.ac.uk
2 Centre for Epidemiology and Risk Analysis, The Veterinary Laboratories Agency, New Haw, Addlestone, Surrey, KT15 3NB
3 Department of Statutory and Exotic Bacteria, The Veterinary Laboratories Agency, New Haw, Addlestone, Surrey, KT15 3NB
If you are in a hurry
It has been shown that the movement of cattle from previously infected areas is consistently the most effective predictor of the distribution of Bovine TB (BTB) in mainland Great Britain. The models used were based on logistic regression analyses which established the statistical relationships between indicators of environmental, climatic and animal movement parameters and the distribution of the disease, defined as presence or absence of confirmed herd breakdowns within 5 by 5 kilometre grids. The models provided estimates of the probability that the disease would be present in each grid. The recent introduction of pre-movement testing (PrMT) throughout mainland Great Britain has been one of the major planks of the strategy to combat BTB. Currently, only the farm to farm movements (whether direct or via a market) of animals over 15 months, from herds subject to BTB testing at 1 or 2 year intervals, are liable to the new measures. Simulating the envisaged reduction in movements within the statistical models suggests a reduction in herd incidence of about 200 cases in the first year – a figure matched by the preliminary evidence available to August 2006. The models also suggest a similar impact should PrMT be extended to younger animals.
1: The BTB situation
4: Impact of PrMT
Bovine TB has been increasing in England and Wales since the mid-eighties and is now entrenched and widespread in the South West, the Midlands, and much of Wales. The disease is also found sporadically elsewhere, through much of mainland Great Britain. The number of herds affected continues to rise inexorably, and there are now in the region of 2000 confirmed herd breakdowns per year. .
A reduction of 211 – 274 cases
in a year
A series of models have tested the potential impact of reduced movements from animals that originate from 1 and 2 yearly tested areas for the different age groups. In these, a tuberculin test sensitivity of 75% has been assumed i.e. that only 75% of the potentially infective movements will have been successfully stopped by pre movement testing.
The map shows the estimated reduction in probability of BTB presence resulting from reducing movements of animals over 15 months (top), 6 weeks to 15 months (middle), and all animals over 6 weeks (lower), respectively.
These translate into a predicted reduction in confirmed herd breakdowns in the first yearof testing of a little over 200 for tests on animals over 15 months, some 250 for those on animals between 6 weeks and 15 months, and so approximately a quarter of all cases if movements of infected animals from both groups were successfully stopped.
Table 1: Predicted drop in BTB herd cases from movement restrictions, data from 2004
Number of confirmed herd breakdowns in mainland GB. 2004 to mid 2006 years shown on map.
?? ● = 2006
● = 2005
6000 confirmed herd cases are predicted by 2020 if the trend continues. However, the first few months of 2006 have seen a reduction in the predicted numbers.
2: BTB distribution modelling
These estimates are likely to be somewhat conservative, when applied to 2005/6, because they are based on reductions in potentially infective movements from areas subject to 1 or 2 yearly testing in 2004. This area has expanded by over a third over the last two years: approximately 36% more movements would have been subject to pre-movement testing in 2005 than was the case in 2004.
Analyses carried out in 2003/4 for DEFRA project SE 3034 showed that cattle movement was consistently the most effective predictor of the distribution of Bovine TB in mainland Great Britain. The models used were based on logistic regression analyses which established the statistical relationships between indicators of environmental, climatic and animal movement parameters and the distribution of the disease, defined as presence or absence of confirmed herd breakdowns within 5 by 5 kilometre grids. The models provided estimates of the probability that the disease would be present in each grid.
113 cases detected by PrMT to end of November
= 192 – 239 cases in a year
BTB in 2005: Probability of Occurrence predicted in 2003.
Actual Distribution inset
Table 2: Cattle movements in areas of England
where PrMT is in effect, April to July 2005/6
After PrMT was introduced, movements of affected animals dropped by 25%, whilst those of ineligible animals remained more or less constant in comparison to 2005.
Cases found by PrMT 2006, and herd breakdown number in 2005/6
3: PreMovement Testing
BTB surveillance testing is routinely carried out on the majority of herds in GB, at an interval determined by the frequency of TB recorded. High incidence areas are tested every year. After much deliberation and consultation, and in the light of evidence from, amongst other sources, the models summarised here, a phased regime of pre-movement testing was introduced in England and Wales in the late spring of 2006. In Scotland both pre and post movement testing have been required since September 2005
The regime required PrMT on all animals over 15 months old originating from herds in high incidence areas (defined as those that were normally subject to one or two-yearly testing) being moved from farm to farm, including movements via markets. Phase two was intended to extend the pre-movement tests to animals between 6 weeks and 15 months.
One of the first objectives of this project has been to adapt the disease distribution models that incorporated animal movements in order to assess the likely impact of the reduction in movements of infected animals resulting from the introduction of PrMT.
This has been achieved by redefining the models described above, using separate cattle age groups, re-running them with the number of movements reduced by three quarters (thereby assuming the tests to be 75% effective at detecting BTB), and calculating the difference in the predicted probability of BTB occurrence.
Between January and November, there were 2105 herd breakdowns in 2005, and 2034 in 2006. Approximately 6% of the 2006 breakdowns were first detected from PrMT tests, shown in blue on the map. In 2005/6 this period accounted for 59% of the cases from March to march, which suggests that there may be 192 detections per year. Given that overall movements of animals eligible for testing (>15 months) have declined by 25%, the figure for comparison with the predicted reduction in A above is 192*1.25-239 which is very similar to the reduction of the number of herd breakdowns per year predicted by the models presented here.
Funded by DEFRA, Project SE3229