Bird flu can infect horses without causing any symptoms, according to new research, raising fears that the virus could be spreading undetected. It’s another twist in the emerging threat of the H5N1 virus, widely seen as the most likely cause of the next pandemic.

Scientists at the University of Glasgow found antibodies to the virus in blood samples taken from horses living in Mongolia. Their results have been published in the journal Emerging Infectious Diseases. Professor Pablo Murcia, who led the research, told Sky News that the finding suggests horses worldwide could be vulnerable in areas where bird flu is present, and they could pass on the virus to humans.