In the 1950s, ALS on Guam peaked at a prevalence of about 140:100,000. PDC had a similar high peak. The prevalence rate of ALS is over 100 times that of ALS in the rest of the world.
From 1950-2000, the prevalence of both ALS and PDC fell. ALS on Guam currently occurs at rates similar to those in the rest of the world. PDC has declined by over 50%
The age at onset of both disorders has risen. The average age of onset of PDC was about 50 in the 1950s and is currently over 70.
These rapid declines in rates of ALS-PDC on Guam suggest that an environmental factor contributed to the disease