At least 180 children have developed severe hepatitis from an unknown cause in the last seven months, the CDC said May 18.
The cases occurred in 36 states and territories since October. The new tally of 180 cases is up from 109 cases reported May 5.
"While this may appear to be a large increase in patients under investigation over the last two weeks, it's important to understand that the vast majority of these are what we consider 'retrospective' patients," the CDC said. "Not all are recent, and some may ultimately wind up not being linked to this current investigation."
In total, five children have died, though no reported deaths have occurred since February, the CDC said. The proportion of children requiring liver transplants also decreased from 15 percent May 5 to 9 percent May 18.
Globally, there have been 429 probable cases reported in 22 countries as of May 15. The CDC and other researchers are still investigating the cause of the hepatitis outbreak.