Quantifying BSE control by calculating the basic reproduction ratio R 0 for the infection among cattle

Abstract

Abstract. The safety of using meat and bone meal (MBM) in mammal feed was studied in view of BSE, by quantifying the risk of BSE transmission through different infection routes. This risk is embodied in the basic reproduction ratio R 0 of the infection, i.e. the average number of new infections induced by one initial infection. Only when R 0 is below 1, will the disease die out with certainty and the population will become free from BSE. Unfortunately this is a slow process due to the slow progression of the disease.We calculate R 0 explicitly from basic ingredients taking several different transmission routes into account. Several of the basic ingredients are functions of age or of infection-age. We also calculate the exponential growth rate r in terms of the same basic ingredients.Next we quantify the ingredients from available data and compute the effects on R 0 of various scenario's for controlling BSE, with examples for the UK and the Netherlands.

Modelling BSE trend over time in Europe, a risk assessment perspective

“…The surveillance system is based on clinical surveillance as well as on systematic rapid testing of risk animals (emergency slaughter and animals that die on farm) over 24 months of age and healthy slaughtered cattle over 30 months of age. Data generated by this surveillance programme has been analyzed in the past and has provided insight into the trend of the epidemic in different countries (Netherlands [5], UK [6], Italy [7], France [8,9], Switzerland [10]. The length and variability of the incubation period for BSE [11] means that a large percentage of the infected animals are already dead or slaughtered before infection can be detected.…”

Review on the epidemiology and dynamics of BSE epidemics

“…And finally, to substantially reduce transmission for eradication of a present BSE epidemic, a combination of these control measures is needed, but to prevent a new epidemic when import risks are restricted, one could even suffice with only one of these measures, when they are fully enforced, or possibly a combination of two 2 [24]. This is based on a quantitative analysis that showed that a well enforced control measure should be able to reduce transmission levels or risk by 99%.…”

“…However it is very difficult to estimate precisely the age distribution of the atypical cases, because there are very few cases and the search for these cases has been focused retrospectively on the oldest BSE cases, for example in Germany and partly in France. Anyway, when properly investigated, it appears that the atypical cases are significantly more frequent in cattle over eight years 24 . Eighteen atypical cases out of the 35 detected have been found through the abattoir stream of surveillance, and therefore in animals assumed to be healthy, even if it has been demonstrated in France that part of the classical BSE cases detected at the slaughterhouse had shown clinical signs compatible with BSE [18].…”

“…Each set of control measures leads to a specific transmission level of the infection. The number of new cases resulting from an initial infection can be expressed in the basic reproduction ratio, R 0 , [24]. A R 0 of 1 tells us that one infected animal gives rise to (on average) one new infected animal.…”

“…However, they provide estimates of the effect of missing data (e.g., cattle still alive), prevalence and past transmission risk, and can therefore quantify the effect of control measures. The R 0 approach [24] was applied to British, Dutch, and Swiss data [55], and showed the effect of the ban of MBM to ruminants, and more interestingly provided evidence for an external infection source (imports of MBM) that was not considered in the model, and which is therefore to be investigated separately.…”