About the Authors
Department of Veterinary Medicine, University of Cambridge, Cambridge, United Kingdom
The authors have declared that no competing interests exist.
Conceived and designed the experiments: AJG DJM PM. Performed the experiments: AJG FJEM GLF PM. Analyzed the data: AJG TJM DJM PM. Wrote the paper: AJG DJM PM. Obtained the funding: AJG DJM PM.