The prevalence of Escherichia coli O157 displays striking variability across the Scottish cattle population. On 78% of farms, in a cross-sectional survey of 952, no shedding of E. coli O157 was detected, but on a small proportion, approximately 2%, very high prevalences of infection were found (with 90-100% of pats sampled being positive). We ask whether this variation arises from the inherent stochasticity in transmission dynamics or whether it is a signature of underlying heterogeneities in the cattle population. A novel approach is taken whereby the cross-sectional data are viewed as providing independent snapshots of a dynamic process. Using maximum-likelihood methods to fit time-dependent epidemiological models to the data we obtain estimates for the rates of immigration and transmission of E. coli O157 infection - parameters which have not been previously quantified in the literature. A comparison of alternative model fits reveals that the variation in the prevalence data is best explained when a proportion of the cattle are assumed to transmit infection at much higher levels than the rest - the so-called super-shedders. Analysis of a second dataset, comprising samples taken from 32 farms at monthly intervals over a period of 1 year, additionally yields an estimate for the rate of recovery from infection. The pattern of prevalence displayed in the second dataset also strongly supports the super-shedder hypothesis.

Animal health surveillance programmes may change in response to altering requirements or perceived weaknesses but are seldom subjected to any formal evaluation to ensure that they provide valuable information in an efficient manner. The literature on the evaluation of animal health surveillance systems is sparse, and those that are published may be unstructured and therefore incomplete. To address this gap, we have developed SERVAL, a SuRveillance EVALuation framework, which is novel and aims to be generic and therefore suitable for the evaluation of any animal health surveillance system. The inclusion of socio-economic criteria ensures that economic evaluation is an integral part of this framework. SERVAL was developed with input from a technical workshop of international experts followed by a consultation process involving providers and users of surveillance and evaluation data. It has been applied to a range of case studies encompassing different surveillance and evaluation objectives. Here, we describe the development, structure and application of the SERVAL framework. We discuss users' experiences in applying SERVAL to evaluate animal health surveillance systems in Great Britain.

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The strategies used and the results obtained in Orkney's bovine viral diarrhoea virus (BVDV) eradication programme over eight years (2001 to 2008) are presented and discussed. The venture was undertaken by local veterinary practices and the Orkney Livestock Association (OLA) with the financial support of the Orkney Islands Council. Participation is voluntary; the programme comprises screening of youngstock, a whole-herd test if required, elimination of persistently infected animals and strict biosecurity measures and/or vaccination. BVDV-free herds are certified, and certification is updated annually by retesting the youngstock. The programme aims to minimise economic losses, thereby increasing the competitiveness of the Orcadian cattle industry and to improve animal health and welfare by eliminating virus circulation. Information from databases of the Scottish Agricultural College, Biobest Laboratories and OLA show that despite a significant reduction in the overall prevalence of BVDV on Orkney during the initial stages of the eradication programme, there has been little progress made since 2006 and that some difficulties have been encountered, with herd BVDV breakdowns following initial eradication. These results highlight the need for continued motivation of farmers, strict application of biosecurity measures and/or systematic vaccination of all seronegative breeding animals.