Age-dependent patterns of bovine tuberculosis in cattle

Abstract

Bovine tuberculosis (BTB) is an important livestock disease, seriously impacting cattle industries in both industrialised and pre-industrialised countries. Like TB in other mammals, infection is life long and, if undiagnosed, may progress to disease years after exposure. The risk of disease in humans is highly age-dependent, however in cattle, age-dependent risks have yet to be quantified, largely due to insufficient data and limited diagnostics. Here, we estimate age-specific reactor rates in Great Britain by combining herd-level testing data with spatial movement data from the Cattle Tracing System (CTS). Using a catalytic model, we find strong age dependencies in infection risk and that the probability of detecting infection increases with age. Between 2004 and 2009, infection incidence in cattle fluctuated around 1%. Age-specific incidence increased monotonically until 24-36 months, with cattle aged between 12 and 36 months experiencing the highest rates of infection. Beef and dairy cattle under 24 months experienced similar infection risks, however major differences occurred in older ages. The average reproductive number in cattle was greater than 1 for the years 2004-2009. These methods reveal a consistent pattern of BTB rates with age, across different population structures and testing patterns. The results provide practical insights into BTB epidemiology and control, suggesting that targeting a mass control programme at cattle between 12 and 36 months could be beneficial.

Figure 1

Demographics of the cattle population in Great Britain. a) Population pyramids for dairy and beef cattle in Great Britain. The distributions were calculated using the Cattle Tracing System on 01-01-2009. b) Unadjusted age-specific reactor numbers for 2009 for dairy and beef cattle in Great Britain, calculated from the VetNet database.

Figure 2

Age specific reactor rates and estimated force of infection. Panels a) and b) show data and fits for the age and year model for 2005; panels c) and d) for 2008 for dairy (panels a and c) and beef (panels b and d) cattle. In each panel, age-specific reactor rates are given by the points; the joined points with error bars show the mean estimated age-specific force of infection with 95% credible intervals and the step function shows the model fit using the mean force of infection for each cohort.

Figure 3

Results and parameter estimates from the age and year-dependent model. Panels a) and d) show the year-dependent infection rates for dairy (red) and beef cattle (blue). The box represents the central 50% of the posterior distribution with the median marked by the central line. The outer whiskers represent the 95% CI and the outliers are shown as points. Panels b) and e) show the age-dependent infection risks for dairy and beef cattle. Panels c) and f) give the posterior density for the removal rate for dairy and beef.

Figure 4

The relationship between reactor age and probability of finding visible lesions (VL) at slaughter. The different coloured lines show the results from different test types. The data were extracted from VetNet.