How common is prion disease?

What is the rate?

About 1 to 2 people in a million develop a prion disease each year, and that figure is reasonably consistent across countries that count carefully. Reported numbers for the United States range from about 350 to 600 cases a year, with CDC giving 500 to 600 reported annually for CJD.

To put it another way, a city of a million people would expect one or two cases a year, and a family doctor may never see one.

How does that split between the types?

Most of it is sporadic CJD, which happens with no known cause and accounts for about 85 in every 100 cases. Inherited forms are 10 to 15 in every 100. Cases that followed contact with prions during medical care, called iatrogenic, are about 1 to 2 in every 100, and those routes have largely been closed.

Variant CJD, the form linked to beef from cattle with BSE, is now very rare. In total 233 people worldwide have been reported with it since it was first described in the UK in 1996.

What causes prion disease sets out the three groups, and prion diseases lists the individual conditions.

Who does it affect?

Classic CJD occurs in older adults, with the average age in the late 60s. Very few cases happen in people under 30. A French study of 2,510 sporadic CJD deaths recorded between 1992 and 2016 found rates highest at 75 to 79 years and lower after that.

The inherited forms often begin earlier, in midlife, and variant CJD affected a much younger group, with an average age of 28.

Why do the numbers vary between sources?

Because counting a rare disease is difficult. Diagnosis in life was less certain before the current tests existed, some cases are only confirmed after death, and countries differ in whether they run a surveillance system that reviews every suspected case.

Those differences show up in the data. In the French study above, the sharpest rise in reported rates came in the earliest years, and the authors attributed it to the surveillance network itself finding cases that would previously have been missed, rather than to any change in the disease.

What this means in practice is that published rates are best read as close estimates. Rates in the same range, reported by different countries in different years, are not usually evidence of a real difference.