(ISNS) -- In a new study, researchers have found that head and body lice -- one a mere annoyance, the other a serious health threat -- may actually be the same species.
There are some big differences in the way the blood-suckers look and act. In an evolutionary quirk, almost every species of mammal or bird hosts a distinct species of lice -- so dogs and humans don't feed the same critters. Head lice are highly dependent upon human body warmth and will die if separated from their human host for 24 hours.
Body lice are larger and hardier since they live on clothing and can survive if separated from human contact for up to a week without feeding.
"Head lice are a nuisance, whereas body lice represent a true direct health risk," said Barry Pittendrigh , a professor of entomology at the University of Illinois and an author of the new study, which appears in the journal Insect Molecular Biology.
A working definition of whether two organisms are part of the same species is if they breed, or can potentially breed, with one another. Previous studies have found that even when head and body lice are both present on the same host, they don't move into each other's territories. The two provinces of lice don't breed with one another in the wild, but they can generate offspring under specific laboratory conditions. The presence of head lice has little to do with human hygiene conditions, but body lice seem to appear out of nowhere when hygiene suffers.
Head lice don't carry diseases, but body lice also carry diseases like louse-born typhus and trench fever.
The researchers collected head and body lice at every stage of development and put them through a battery of challenges -- exposing them to cold, heat, pesticides, bacteria -- to coax the lice to express as many genes as possible. Lice have a tiny genome compared to other pests, with only about 10,000 protein-coding genes.
When the researchers compared the number and sequences of all of the protein-coding genes expressed at every stage louse life cycle, they found very few differences between the head and body lice -- 14 genes, to be exact.
Pittendrigh said that the reasons why there is so much physical difference in two creatures who share so much genetic material remain to be determined.
"Head lice live on people regardless of hygiene conditions. But when hygiene conditions drop, a new ecological niche opens up, and head lice could move down to start being body lice. That's one hypothesis," said Pittendrigh.
The body lice have to survive longer without a blood meal and evolved to increase their body size to compensate.
"There are wonderful things that could be tested in the future," said Pittendrigh.
Richard Pollack, an instructor with the Harvard School of Public Health, said that he's not sure that the genetics make the case for different species.
"We've got more than 100 years or so of observations that head and body lice behave differently, look different, and they don't interbreed except in the laboratory," said Pollack, adding that he doesn't question the research group's work, but he does diverge from them on the conclusions. "How much difference would be necessary to call the things different species?"
As a matter of practicality, a louse by any other name bites the same -- but Pollack said that there could be implications for public health interventions if the two types are lumped together.
"If we muddy the waters and say they're the same things, then there will be folks out there who will use this as ammunition and say -- well, if there's no difference then all bets are off. Head lice must be as dangerous as body lice," said Pollack. "People who don't know enough about the biology and public health significance of these two creatures will reverse the gains that have been made in dealing more rationally with these organisms."