Louse infestation calibrates immune system regulation

Some parasites can exert a moderating effect on the immune system, perhaps reducing the risk of developing immune dysfunctions like asthma, allergies and some forms of arthritis.

Researchers writing in the open access journal BMC Biology

studied the effects of several parasites on the immune activity of wild wood mice, finding that louse infestation was associated with a reduced readiness to mount an immune response.

Janette Bradley led a team of researchers from the University of Nottingham who carried out the tests on a population of wood mice captured in a Nottinghamshire forest. Unlike lab animals bred in captivity, these mice had 'natural' levels of parasite infestations and immune function. Bradley said, "Our understanding of mammalian immunology is largely based on rodents reared under highly unnatural pathogen- and stress-free conditions. Analyzing immune responses in wild populations can give crucial insights into how the immune system functions in its natural context".

The authors conducted post-mortem studies on the captured mice, assessing their weight, parasite load, and the responsiveness of their spleen cells to substances such as heat-killed listeria and bacteria, which bind receptors of the innate immune system and provoke a measurable reaction. They found that those mice uninfected with the louse Polyplax serrata showed markedly increased responses to these triggers of innate immune responses, compared to highly-infected animals. This suggests that the parasite is able to exert some kind of immunosuppressive effect, possibly directly by secreting some substance into the mice from its saliva, or indirectly by transmitting bacteria or other pathogens.

The authors speculate that this profound dampening of innate immune responsiveness supports the view that modern parasite-free human populations have a level of heightened immune responsiveness that would not have been typical during their recent evolutionary history, "Much like laboratory mice, people in developed countries are currently exposed to a very different profile of infections to that encountered by their ancestors. 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".