A man complaining of terrible headaches and nausea, slipped into a coma, and required emergency surgery to remove tapeworm larvae.

A US man suffering from violent headaches learned just in time what was making him ill: a tapeworm larvae that had lodged in his brain.

Doctors told local media that the pupal parasite burrowed in the brain of Luis Ortiz, 26, a student in Sacramento, news reports said on Thursday.

The worm was forming in a cyst that blocked the flow of water to chambers in Ortiz's brain, "like a cork in a bottle", his neurosurgeon Dr Soren Singel told the Napa Valley Register newspaper.

Ortiz, who had gone to the hospital emergency room complaining of terrible headaches and nausea, eventually slipped into a coma, and required emergency surgery to remove the larvae.

"I was shocked," he later told CBS television.

"I just couldn't believe something like that would happen to me. I didn't know there was a parasite in my head trying to ruin my life."

"My memory is like a work in progress," Ortiz said, adding that his overall brain function is slowly improving thanks to therapy.