The majority of outbreaks of bovine TB within cattle herds are caused by multiple transmissions routes – including failed cattle infection tests, cattle movement and reinfection from environmental reservoirs such as infected pastures and wildlife – according to the first national model of bovine TB spread, published today.
It's most likely that both cattle movements and the local environment are driving the bovine TB epidemic
The model, developed by researchers at the University of Warwick and University of Cambridge, suggests that improved testing, vaccination of cattle and culling of all cattle on infected farms would be the most effective strategies for controlling the disease. It found that whilst badgers – the subject of controversial culling plans to stem the spread of the disease – form part of the environmental reservoir, they only play a relatively minor role in the transmission of infection.
Based on a study of cattle and the causes of bovine TB in Great Britain, the model, published in the journal Nature, sought to ascertain how and why the epidemic has grown over the past 15 years. Using data from the Animal Health and Veterinary Laboratories Agency and the Department for the Environment, Food and Rural Affairs (Defra), the researchers developed a mathematical model that incorporated both within- and between-farm bovine TB transmission.
“Our model offers a dispassionate, unbiased view of the spread of bovine TB through the cattle industry of Great Britain,” says Professor Matthew Keeling, from the University of Warwick’s School of Life Sciences and Department of Mathematics. “The model is based on the recorded pattern of positive and negative tests and uses the known movement of cattle around the country. We aim for it to provide policy-makers with the best evidence possible from which to make decisions relating to bovine TB and to contribute to the ongoing discussions on this sensitive issue.”
The model allowed the researchers to tease apart how different routes involved in transmission interact and overlap.
Dr Ellen Brooks-Pollock from the Department of Veterinary Medicine at the University of Cambridge adds: “By using the most recent data, our model predicts that it is most likely that both cattle movements and the local environment are driving the front of the epidemic. Imperfect cattle skin tests contribute to the spread by delaying the time until infected herds are detected for the first time and incorrectly identifying herds as clear of infection.”
One of the key results from the model is the large variation in what happens to farms once they are infected.
“We found that the vast majority of infected farms don’t spread the infection to any other farms before they clear infection themselves. Only a small number of farms spread the infection, and they can cause the majority of new cases”, says Dr Brooks-Pollock.
The researchers argue that the findings are essential for improving the targeting of control measures. If infected farms can be identified and caught early then it might be possible to make substantial progress in tackling the epidemic.
“The model we are putting forward can be used to address several potential control methods – but there is no single panacea,” says Professor Keeling. “All controls have advantages and disadvantages. However, we find only three controls have the power to reverse the current increase in cases: more frequent or more accurate testing, vaccination of cattle and culling all cattle on infected farms.”
The control measures the researchers investigated were designed to be ‘idealised’ control options to understand what measures in theory could stop the increasing epidemic. The researchers did not consider the practicalities or economics of implementing control measures.
The research was funded by the Biotechnology and Biological Sciences Research Council, the Wellcome Trust and the Engineering and Physical Sciences Research Council.
Adapted from a press release from the University of Warwick.
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