Importance

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases caused by prions. Although these infections usually remain asymptomatic for years, the disease is always progressive and fatal once the clinical signs develop. TSEs affecting animals include scrapie (tremblante de mouton, rida), bovine spongiform encephalopathy (BSE, “mad cow disease”), feline spongiform encephalopathy (FSE), transmissible mink encephalopathy (TME, mink scrapie) and chronic wasting disease (CWD). Although some prion diseases usually occur in one or a few closely related species, other prions can cross species barriers. BSE has a particularly wide host range. Cattle are the most important hosts for this disease, but some other ruminants, cats, lemurs and humans can also be affected; in cats, the disease is known as feline spongiform encephalopathy, and in humans, it is called variant Creutzfeldt-Jakob disease (vCJD). Recent evidence suggests TME might also be a caused by a variant of the BSE agent. The discovery that BSE can cross species barriers and is zoonotic has raised concerns about all TSEs. Prion diseases in livestock can also result in trade sanctions. For these reasons, many countries are conducting control or eradication programs for these diseases. The U.S. has eradication and flock certification programs for scrapie, a certification program for chronic wasting disease in captive cervids, and a surveillance program for BSE. Etiology Transmissible spongiform encephalopathies are caused by unconventional disease agents. These agents are resistant to the treatments that ordinarily destroy bacteria, spores, viruses and fungi. They are generally thought to be prions, although a minority opinion suggests that TSEs may be caused by virinos or retroviruses. Prions are infectious proteins that appear to replicate by converting a normal cellular protein into copies of the prion. The cellular protein, which is called PrPc, is found on the surface of neurons. Pathogenic isoforms of PrPc are designated PrPres; Pr