Sci Fantasy story published 1950s - 1970s about newts living inside people. Looking for title / author please

I am trying to hunt down this story for my boss. He can remember the plot but has no idea re: author or title.

Apparently in the story newts live inside people. The only way to remove them is for somebody else to declare "I don't believe you have a newt inside you".

Any help or suggestions greatly appreciated thanks.