Ania Lichtarowicz BBC science reporter

A form of Mad Cow Disease, which effects Elk and Deer is spreading westwards across North America.

Officials have reported cases in captive herds, which will now have to be culled.

Chronic Wasting Disease (CWD), or as it is more commonly known, Mad Elk Disease, has been found before in wild deer as well as wild and captive elk. There are also reports of the first case of the disease in mules.

Cases of Mad Elk Disease have been found for the first time on the west side of the Rocky Mountains, indicating that CWD had crossed the US continental divide.

The first case of the disease has also been reported on a ranch in the western Canadian province of Alberta.

Economic impact

CWD is similar to BSE in cattle and scrapie in sheep - fatal conditions caused by mutated proteins called prions in the brain.

The disease could have a significant economic impact - deer are bred for venison and elk are also farmed for their massive velvet-covered antlers, which are exported to Asia.

Along with deer, elk are also part of the multimillion dollar hunting industry in the US which is now being threatened by CWD.

As there is no live test for the disease, animals in the same herds will be slaughtered, and wild animals living close by will also be culled.

They will then be tested to try and determine how far CWD has spread.