Autoimmune and immune hypersensitivity disorders are increasingly common conditions in which dysfunctions of the immune system cause deleterious effects in an individual. These disorders include allergic responses, chronic inflammation and possibly even some psychiatric disorders, but little is known about their causes or why they are now appearing with greater frequency.

One possible explanation is the 'biome depletion theory', which considers the complete ecosystem of the human body and posits that our immune system evolved to function properly alongside a robust microbiome. According to this theory, modern hygiene's intense policing of the human microbiome can destabilize this ecosystem and dysregulate immune responses.

Recently, neuroscientist Staci Bilbo and colleagues at Duke University (Durham, NC) tested how this theory might apply to neonatal development in lab rats (Brain Behav. Immun. doi:10.1016/j.bbi.2015.07.006; published online 7 July 2015). Previous studies have found that lab rats, when infected with bacteria as neonates, show cognitive deficits later in life caused by the induced immune reaction.

“We have faced bacterial infections throughout our entire evolutionary history, presumably also during the neonatal period,” said Bilbo in a press release. “It always seemed kind of strange that the immune system would have evolved to overreact like that.”

In their most recent study, Bilbo's team again infected rats with bacteria, only this time some of the rats had been previously colonized with helmiths, or tapeworms. Helminths are common in wild rats, so Bilbo's team used them to model a more complete microbiome than is typical of lab rats. Once the rats matured, Bilbo's team administered tests for hippocampal-specific memory deficits, finding that rats with helminths did not show the cognitive impairments seen in worm-free lab rats.

When they examined cell samples from each rat, Bilbo's team confirmed that cells from rats with helminths expressed lower levels of inflammatory response factors during immune stimulation, compared to cells from the worm-free rats. This suggests that the small parasite load tempered the immune reaction that otherwise causes deleterious and enduring effects during development.

Coauthor William Parker noted that, in some ways, lab rats are a very appropriate model for testing the biome depletion theory. “In a real sense we've done the same things to our lab animals that we've inadvertently done to ourselves,” he said, referring to modern hygiene practices. Studies like this contribute to mounting evidence of the importance of microbiotic ecology, indicating new, if not classically hygienic, possible treatments for complex immune disorders.