The human body louse -- Pediculus humanus -- can transmit dangerous bacterial infections to humans, while the human head louse -- also Pediculus humanus -- does not.
"Body louse-transmitted diseases include trench fever, relapsing fever and epidemic typhus," U of I entomology professor Barry Pittendrigh said in a release Friday.
Pittendrigh, working with John M. Clark of the University of Massachusetts and others, previously compared the sequences of protein-coding genes in head and body lice, finding the two belonged to the same species, despite body lice being bigger than head lice, clinging to clothing instead of hair and transmitting disease.
In the new study, Clark's group infected head and body lice with the bacterium that causes trench fever. Pittendrigh's laboratory then looked at the parasites' gene expression to see how the insects responded to the infection.
The researchers found several immune genes were regulated differently in head and body lice and the infection progressed further in body lice over time.
"Our experiments suggest that the head louse immune system is fairly effective in fighting off the bacteria that cause trench fever," Pittendrigh said. "However, the body lice don't seem to have as good an immune response."
While the reason the immune responses differ hasn't been determined, Pittendrigh suggested the body louse has a reason to be more tolerant of infection.
"Head and body lice have beneficial bacteria living inside them," he said. "These bacteria make vitamins that the lice need to grow and survive. Body lice tend to grow larger than head lice. It may be that a suppressed immune system allows body lice to grow more of the bacteria that make the vitamins they need, and they grow larger."