Scientists have been unable to diagnose the reason why livestock near oil- and gas-drilling zones across the country are sick and dying.

NBC News reports scientists believe chemicals used in drilling and hydrofracking operations are poisoning animals nearby, but a direct cause has not been determined.

In 2009, cattle from a North Dakota ranch adjacent to a drilling site experienced weight loss from 60 to 80 pounds in a week. The animals were unable to produce milk for their calves and were limping on swollen legs.

Cattle from the ranch were infected and their tails mysteriously dropped off. The owner of the ranch, Jacki Schilke, lost five of the cattle. The remaining livestock have recovered, but she’s not selling them because she’s concerned with the safety of the meat they’d produce.