(written from a Production point of view)
According to the final draft script of "Sleeping Dogs", Klingon lice was a type of miniscule lifeform that was common in Klingon living environments and could remain on a person's body after they had left that area.
Klingon lice were mentioned once only in a speech Doctor Phlox was scripted to make to the members of a landing party – a Vulcan and two Humans – who were in Enterprise's decontamination chamber after returning from the IKS Somraw. Phlox told the newly returned officers, "Aside from a predictable complement of Klingon lice, you have little to show for your stay on the Klingon ship." In canon Star Trek, however, Klingon lice have never been referenced nor shown.