Humanity’s bug-infested past might be why we have more allergies today (Image: James Gathany/ Center for Disease Control and Prevention, USA)

It is well established that intestinal parasites dampen mammalian immune reactions. But in a surprise result, scientists have found that another kind of parasite – the body louse – does too. That means the epidemic of allergic disorders in modern, urban people might be due to our having rid ourselves of lice and worms.

The “hygiene hypothesis” holds that our immune systems evolved to compensate for continual infections with parasitic gut worms, which secrete chemicals that reduce our immune responses. People who are now worm-free have overreactive immune systems, which can lead to asthma and autoimmune disorders.

Yet mammals’ immune systems have mainly been studied in clean, well-fed and parasite-free lab mice. Jan Bradley, a parasitologist at Nottingham Universtiy, UK, and colleagues reasoned that normal, wild mice might be different.


Need for nasties

The researchers trapped 100 wood mice in the fields around Nottingham and measured their loads of several different parasites, including various worms, mites, ticks, fleas and lice.

As expected, immune cells from mice with roundworms produced less of an inflammatory chemical called TNF-alpha in response to germs than cells from uninfected mice. But surprisingly, says Bradley, mice with heavy louse infestations showed even stronger immune suppression, producing half as much TNF-alpha as uninfested mice.

None of the other parasites, nor the animal’s age, sex or general condition, was related to the production of the immune chemical, suggesting that it wasn’t just a matter of stressed or sickly animals having more lice and less immunity.

“The hygiene hypothesis was always associated just with worm infections, but this shows other kinds of parasites may have the same effect,” Bradley says. That means ridding ourselves of lice, as well as worms and perhaps other parasites, might have caused the modern epidemic of immune hypersensitivity.

Bradley wants to delouse wild mice to see whether this reverses the immune suppression in them or their descendants.

Journal reference: BMC Biology, DOI:10.1186/1741-7007-7-16