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SUMMARYRelevance of epidemiological models depends on assumptions on the population structure and dynamics, on the biology of the host–parasite interaction, and on mathematical modelling. In this paper we reviewed published models of the bovine viral diarrhoea virus (BVDV) spread within a herd. Modelling options and assumptions on herd dynamics and BVDV transmission were discussed. A cattle herd is a population with a controlled size. Animals are separated into subgroups according to their age or their physiological status inducing heterogeneity of horizontal transmission. Complexity of models results from: (1) horizontal and vertical virus transmission, (2) birth of persistently infected animals, (3) excretion by transiently and persistently infected animals. Areas where there was a lack of knowledge were identified. Assumptions on the force of infection used to model the horizontal virus transmission were presented and discussed. We proposed possible ways of improving models (e.g. force of infection, validation) and essential model features for further BVDV models.

Economic decisions on animal health strategies address the cost-benefit aspect along with animal welfare and public health concerns. Decision tree analysis at an individual cow level highlighted that there is little economic difference between the use of either dry cow antibiotic or an internal teat sealant in preventing a new intramammary infection in a cow free of infection in all quarters of the mammary gland at drying off. However, a potential net loss of over £20 per cow might occur if the uninfected cow was left untreated. The only economically viable option, for a cow with one or more quarters infected at drying off, is antibiotic treatment, although a loss might still be incurred depending on the pathogen concerned and the cure rates achievable. There was a net loss for cows with quarters infected with Corynebacterium spp. at drying off, for both the teat sealant and untreated groups (£22 and £48, respectively) with only antibiotic-treated cows showing a gain.

As with any measurement procedure, the performance of a subjective classification procedure must be evaluated. Observers have to be trained and their performance has to be assessed, preferably on a regular basis, to guarantee sufficient consistency and accuracy of classification results. The current paper is a study of observer performance where observers were asked to classify the gait of cows from video recordings. Gait was classified in nine ordered categories (ranging from 1=normal gait to 9=severely abnormal gait) and also as a continuous fraction by putting a mark on a paper strip (the left end corresponding to 0=normal gait and the right end to 1=severely abnormal gait). The use of statistical models and methodology for analysis of these visual scores is demonstrated and discussed. Observers were assessed by comparing their classification results with the results of an expert. Models and methodology take proper account of typical features of the data, i.e. the fact that data are discrete scores or continuous scores with an upper and lower bound, the variance heterogeneity and non-linearity of model terms that arises from this, and the dependence between repeated classifications of videos of the same cow. Results of the analyses are summarized in simple tables and plots. These are useful tools to indicate possible flaws in judgement of an observer, that may be corrected by further training. When a high standard is developed, which usually takes the form of the opinion of one or more experts, this methodology can be applied prior to any experiment where responses are ordered subjective scores.