Treatments for body and head lice which are "simple and effective" could be developed using clinically "obsolete" antibiotics, say university reseachers.
Scientists at Bangor and Reading universities have analysed bacterium that lives within the human body louse, as part of a larger project.
They said antiobiotics that had completely lost their therapeutic value could now be tested.
The researchers said they had found the "achilles heel" of the human louse.
It follows the completion of genetic sequencing of the human body louse.
The sequencing of the louse and bacteria that live within the louse are reported in a paper in the American online journal, Proceedings of the National Academy of Sciences.
The paper said the body louse transmitted "endemic typhus, relapsing fever and trench fever as well as leaving the host, if not infected with a more serious disease, feeling 'lousy'."
Head lice do not transmit disease, but can also lead to unexpected conditions, and are said to be "increasingly resistant to current insecticidal treatments".
The parasitologists at Bangor and Reading were responsible for sequencing a bacterium called Candidatus Riesa pediculicola that lives within the louse, and without which, it cannot survive.
They worked with 25 other institutions in the international research team on the louse genome project.
Dr Henk Braig, a molecular parasitologist at Bangor University's School of Biological Sciences said: "We have established that these bacteria have no immunity to antibiotics as they have been isolated within the body of the lice and transmitted down the generations for millennia with no outside contact.
Solid scientific basis
"Having had no other contact with external bacteria, they have had no need to develop resistance to antibiotics, as they would in the wider environment, in their struggle for survival and dominance."
Reseach collaborator Dr Alejandra Perotti at the University of Reading's School of Biological Sciences added: "All human lice depend on this bacterium for reproduction and survival.
"This bacterium represents a new target for the development of novel anti-louse agents especially for cases were classical treatment fails.
"Antibiotics that have completely lost their therapeutic value could now be tested for this target. The potential of antibiotics to fight human lice is not new but we now have a solid scientific basis for this.
"Several antibiotics might be efficient and should be systematically tested. "