Could lice be the secret to preventing asthma?

Research on mice shows that those carrying the most lice had calmer immune systems than uninfested rodents, and they said their finding may have implications for studying the causes of asthma and allergies in people.

The study, published in the BioMed Central journal BMC Biology, adds to evidence supporting the so-called hygiene hypothesis, which holds that the rise in asthma and allergies can be linked to hyper-clean living.

The idea is that if the immune system is not properly primed in childhood, immune cells can improperly react to harmless triggers such as pollen or bits of dander. Bacterial and viral infections do not seem to be the priming factor, but researchers have been focusing lately on parasites.

Joseph Jackson of Britain's University of Nottingham and colleagues wanted to test real, wild mice, not hygienic lab mice that had been raised for generations in ultra-clean conditions.

"Our understanding of mammalian immunology is largely based on rodents reared under highly unnatural pathogen- and stress-free conditions," Nottingham's Janette Bradley, who helped lead the study, said in a statement.

They trapped mice and studied their immune systems.

Mice uninfested with the louse Polyplax serrata had much more excitable immune systems than the mice carrying a heavy load of the parasites, they found.

It might be suppressing the immune system in some way, they speculated, perhaps by transmitting some other parasite or microbe or perhaps in its saliva as it feeds on the blood of its host.

The hygiene hypothesis holds that the immune system evolved when people were constantly infected by a host of worms and other parasites — from the mosquito-transmitted malaria parasite to various lice and ticks.

"Much like laboratory mice, people in developed countries are currently exposed to a very different profile of infections to that encountered by their ancestors," the researchers wrote.

"It is possible that the immune dysfunctions we see today are the result of immune systems calibrated for a set of challenges completely different to those they now routinely face."

Humans can also be infested with lice, although the species that affects humans does not affect other animals.