VETERINARIANS in Alaska are at a loss to explain a disease that has killed almost 200 Arctic ringed seals since July. The animals’ hair falls out, they develop ulcers on their flippers and skin, and they have lesions in their lungs, livers, hearts and brains. The disease may have spread to other species including spotted seals, bearded seals and walruses.

“We don’t know what’s causing it,” says Julie Speegle, speaking for the US National Oceanic and Atmospheric Administration’s office in Alaska. Tests have ruled out viruses including poxvirus, papillomavirus and morbillivirus, which killed hundreds of seals in Europe in 1988. Vets continue to screen for other viruses, bacteria and fungi, and for algal toxins and others that may be responsible.

Similar seal deaths have been reported in Canada and Russia, where scientists have joined the investigation. NOAA has advised hunters not to eat any sick seals in case the agent can also infect humans.