The 2001 Epidemic
Predictions from 2001 of the spatial distribution of the outbreak based on the early location of cases.
Predictions of the time-course of the outbreak (red) compared to the observed cases (black).
Foot and Mouth Disease is one of the most highly transmissible of any livestock disease and therefore the outbreak in 2001 had the potential to catastrophically impact on both the farming community but also the national economy. It was therefore national policy (and EU law) that infected farms should have all their animals destroyed (culled) as should all "Dangerous Contacts" where there is suspicion that the infection may have been transmitted.
It was abundantly clear from preliminary analysis that early control measures were not sufficiently effective in reducing the spread infection and therefore action was required. This took two forms: firstly, culls of infected premises and dangerous contacts, therefore removing potential sources of infection as early as possible; secondly, it was decided that due to the high risk of local transmission of the virus all farms that were contiguous to an infected farm should also have their livestock culled.
By the end of October 2001, nearly five million animals on ten thousand farms had been culled either as a direct result of infection or in an attempt to control the spread. Our models show that the situation could have been far worse; if only livestock on infected premises were culled then more than three times as many farms would have been affected. However, the situation could have been far better, if culls had been preformed rapidly from the start and the CP culling policy introduced earlier then we predict that the impact would only have been reduce to a quarter of that observed.
Since 2001 our modelling and statistical analysis has continued with three main themes: better statistical inference and analysis; optimal control by vaccination and spatially specific controls; and extending the approach to other countries.
Figure 2: Graphs showing the effect of vaccination had it been implemented during the 2001 outbreak. The success of any vaccination campaign is dependent upon the resources available to implement the campaign and the speed at which any vaccination policy is implemented. Graphs from Tildesley et al. 2006 (Nature).
In 2001, in the heat of the epidemic, the parameter inference was somewhat ad-hoc -- the main aim was to generate useful predictions, not to be too fussy about the statistical methodology. Since 2001, we have re-analysed the methods that were used during the epidemic and compared the accuracy of our models to the data on a farm-by-farm basis. In addition, we have been developing Bayesian MCMC methodology which provides a more rigorous assessment of model parameter values, but also allow us to assess the risk (on a day-by-day basis) of each farm being infected but not yet detected (so-called occult infections). Knowledge of occult infections is a vital step in efficient targeting of surveillance -- this methodology was formulated to aid with the 2007 outbreak in Surrey.
Vaccination obviously has the potential to be a powerful tool in the control of any infection. It was not used in 2001 due to difficulties of distinguishing between infected carriers of foot-and-mouth disease and animals that had been vaccinated, but modern vaccinology tools have overcome this issue. However, one of the major challenges is the delay between injecting the animal and them becoming protected against infection. In 2003 & 2006, we therefore focused in detail on the advantages and disadvantages of vaccinating cattle (vaccinating sheep is not considered cost effective); we showed that a national rapidly implemented prophylactic vaccination campaign could be a viable control measure, and that vaccinating farms in the vacinity of recently detected infections was also highly effective (figure 2).
Foot-and-Mouth outside the UK
Figure 3: Results of simulations for FMD spreading through US counties. Showing the number of subsequent counties infected when the outbreak starts in a particular location.
More recent work has focused on extending this work to Denmark and the USA. Denmark presents interesting challenges and opportunities as they have comparible quality data to the UK (recording farm locations and animal numbers in each farm), but their livestock industry is dominated by pigs.
In contrast, the data in the USA is of far poorer quality, often with just the number of farms and animals in each of the 3000 counties being recorded. Despite this sparsity of data, we have shown that mathematical modelling can still be a useful tool in assessing the scale of control measures that are optimal. We have shown that county-level demographic information is sufficient to characterize disease spread and inform policy at epidemiologically and policy relevant spatial scales in the US.
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Funded by: Wellcome Trust, BBSRC, DEFRA, NIH, Dept of Homeland Security
SBIDER people involved
Colleen Webb (Colorado)
Matt Ferrari (Penn State)