Why do Body Lice Spread Disease, While Head Lice do not?
Even though they look and act very differently, head lice and body lice are of the same species. This was determined in 2012 after a genetic analysis revealed that despite morphological differences, they are indeed the same.
University of Illinois entomology professor Barry Pittendrigh and his colleagues compared the sequences of all protein-coding genes in head and body lice and determined that the two belonged to the same species, despite the fact that body lice are bigger than head lice, cling to clothing instead of hair, and can transmit disease.
Why then do body lice spread diseases such as trench fever or typhus, while head lice do not? A new study published in the journal Insect Molecular Biology seeks to answer this question.
In the new study, Pittendrigh worked with John M. Clark, a professor of environmental toxicology and chemistry at the University of Massachusetts. Clark’s group infected head and body lice with Bartonella quintana, the bacterium that causes trench fever. Pittendrigh’s laboratory then looked at gene expression in each to see how the insects responded to the infection.
“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.”
The researchers discovered that several immune genes were regulated differently in head and body lice after infection with the bacteria, and the infection progressed further in body lice over time.
“By eight days post-infection, head lice had killed or contained the invading B. quintana, whereas the bacteria were still proliferating and spreading in body lice,” the researchers reported.
The team cannot yet say why head and body louse immune responses differ from one another, but Pittendrigh hypothesizes that the body louse has a reason to be more tolerant of bacterial 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.”
The body louse’s dampened immune response would allow other invading bacteria, such as those that cause disease in humans, to also survive in its gut, he said.
“So body lice may grow bigger, but they also are more likely to get sick with the trench fever bacteria and pass the disease to humans,” Pittendrigh said.
The study team also includes researchers from Purdue University, Seoul National University, the University of California at San Francisco, the University of Massachusetts and the University of Illinois department of animal biology.
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