Like other transmissible spongiform encephalopathies (TSEs), variant Creutzfeldt-Jakob disease (vCJD), the human form of mad cow disease, has a long incubation period before irreversible brain and nerve damage occurs. These diseases always result in progressive dementia and death. There are no signs or symptoms in the early course of the diseases and the first signs or symptoms may not appear for years or even decades after the infection occurred.

After symptoms appear, the diseases tend to progress very quickly, often leading to death within a single year. According to the Food and Drug Administration, vCJD patients only live for an average of 13 months after symptoms appear.

The first symptoms of vCJD are psychiatric problems (e.g., depression, anxiety, insomnia). Personality changes may also occur. Eventually, neurologic signs develop. Patients may experience unpleasant sensations, problems walking and a lack of muscle coordination. They may have involuntary jerking motions (myoclonus). Vision problems, which may lead to blindness, are also common. Patients may feel confused, forgetful, or have difficulty thinking and speaking.

In later courses of vCJD, patients may become unable to speak or move, and enter a coma. Infections (e.g., pneumonia) are common and may be the cause of death for many patients. The presence of many tiny holes in the brain (giving it a spongy appearance) at autopsy indicates TSEs such as vCJD. Flower-shaped areas of damage (clumps of prions called plaques) are also generally present.