Contact tracing offers route to manage bovine TB
Genetic analysis of infections can be applied to trace cattle disease outbreaks over space and time.
The coronavirus pandemic has highlighted the challenges posed by keeping track of the myriad interactions between people, and the associated risks of infection, as we go about our daily lives.
The complexity of this problem lies in the potential of one person to infect many others, who in turn could quickly infect many more, over a wide area, in a short space of time.
Genetic contact tracing provides a solution to make sense of the web of infection that results.
Determining the genetic make-up of infection from patient to patient, place to place, and over time, can enable data models to trace the progress of disease, establishing where and when infection has been passed on.
This approach could be applied to infectious disease in many scenarios, and has been put to use in tracing the source of a novel outbreak of bovine TB, an infectious respiratory disease in cattle that can affect other animals.
Researchers studied disease in cattle across a previously uninfected area of northern England. Their findings showed that bovine TB had been imported to the region in livestock imported from Northern Ireland.
Professor Rowland Kao, Chair of Veterinary Epidemiology and Data Science at the Roslin Institute, led the analysis.
This traced TB spreading among cattle on a farm, before being passed to local badger populations, and back again to cattle. The research did not establish the first cow to carry infection – patient zero – but it came near.
This was an unusual outbreak in that we were able to trace the infection to a single source – as close to a smoking gun as you can get.
Researchers say their approach could be applied, with regular testing of cattle, to improve the understanding of bovine TB movements, forensically distinguishing between the roles that cattle and badgers have in the circulation of disease with a resolution that would otherwise be impossible to achieve.
This could lead to improved disease management, potentially limiting the impact on livestock and wildlife of TB, for which control measures cost £100 million each year in England alone.
“We were seeing a TB outbreak in a way it hadn’t happened before,” Professor Kao explains. The incidence of disease in a previously unaffected area afforded an opportunity to trace the origin of the infection.
Genetic contact tracing as a forensic tool is not novel, but we are the first to use it in this way, to precisely trace a TB outbreak – it was a good opportunity to apply it to a geographical area where outbreaks had not previously taken place.
Researchers examined the DNA of bacteria samples recovered from infected cows and badgers in the area and combined the genetic data with spatial locations and contact tracing. This allowed them to compare small changes in the DNA as it spread from one animal to the next.
All bacteria were found to be closely related to each other, pointing to a single importation of cattle as the most likely source of infection.
Experts from the Universities of Edinburgh and York, University College Dublin, and the UK Animal and Plant Health Agency collaborated on the study, published in the Journal of Applied Ecology.
The research was funded by the Biotechnology and Biological Sciences Research Council, part of UKRI, and the Department for Environment, Food and Rural Affairs (Defra).
Movement of animals on large farms, which are becoming more common, means livestock are more likely to interact with one another and with wildlife such as badgers, raising the risk of disease spread.
As changes of land use take place, with increased rewilding and an accent on biodiversity, we may expect livestock to come more into contact with wildlife, making outbreaks more likely.
Genetic contact tracing, which has become more affordable in recent times, could emerge as a timely tool to manage disease.
It could be valuable in responding to outbreaks, Professor Kao says. "If you can understand sooner how infection is spreading, you can act sooner, and make decisions more easily, tailored to the specific situation, to manage disease."