vulpecula are infected, and transferring it to previously uninfected possums when their carcasses are scavenged elsewhere. The white-tailed deer Odocoileus virginianus has been confirmed as the sole maintenance host in the Michigan outbreak of bovine tuberculosis which remains a significant barrier to the US nationwide eradication of the disease in livestock. Moose and deer can carry rabies. Docile moose may suffer from brain worm, a helminth which drills holes through the brain in its search for a suitable place to lay its eggs. A government biologist states that "They move around looking for the right spot and never really find it." Deer appear to be immune to this parasite; it passes through the digestive system and is excreted in the feces. The parasite is not screened by the moose intestine, and passes into the brain where damage is done that is externally apparent, both in behaviour and in gait. Deer, elk and moose in North America may suffer from chronic wasting disease, which was identified at a Colorado laboratory in the 1960s and is believed to be a prion disease. Out of an abundance of caution hunters are advised to avoid contact with specified risk material (SRM) such as the brain, spinal column or lymph nodes. Deboning the meat when butchering and sanitizing the knives and other tools used to butcher are amongst other government recommendations. Evolution Deer are believed to have evolved from antlerless, tusked ancestors that resembled modern duikers and diminutive deer in the early Eocene, and gradually developed into the first antlered cervoids (the superfamily of cervids and related extinct families) in the Miocene. Eventually, with the development of antlers, the tusks as well as the upper incisors disappeared. Thus, evolution of deer took nearly 30 million years. Biologist Valerius Geist suggests evolution to have occurred in stages. There are not many prominent fossils to trace this evolution, but only fragments of skeletons and antlers that might be easily confused with false antlers of non-cervid species. Eocene The ruminants, ancestors of the Cervidae, are believed to have evolved from Diacodexis, the earliest known artiodactyl (even-toed ungulate), 50–55 Mya in the Eocene. Diacodexis, nearly the size of a rabbit, featured the talus bone characteristic of all modern even-toed ungulates. This ancestor and its relatives occurred throughout North America and Eurasia, but were on the decline by at least 46 Mya. Analysis of a nearly complete skeleton of Diacodexis