Tapeworms get a bad rap. They’re voracious parasites that burrow into gut walls and devour nutrients like a nightmarish version of The Very Hungry Caterpillar. But not all tapeworms are bad news. In fact, one type appears to protect against memory loss in rats. The discovery, researchers say, could lead to new treatments for some neurological diseases in humans, like Alzheimer’s.
“A lot of people talk about [brain-gut connections]. Very few people have data supporting it,” says Judy Van de Water, an internal medicine specialist at the University of California, Davis, who did not take part in the research. “This study’s getting data that can begin addressing [the connection] more directly.”
In the study, researchers divided 30 rats into two main groups: those infected with the rat tapeworm Hymenolepsis diminuta, and those with clean guts. To test the potential protection offered by H. diminuta, the team infected both groups of rodents with a one-two punch: a mild case of Escherichia coli shortly after birth to stimulate an immune response, and an injection of lipopolysaccharide (LPS) after they had been “wormed” as adults. LPS is a noninfectious component of bacteria that trips the immune system’s alarm, leading to a potentially overactive response in the brain that can include swelling and confusion.
The idea behind the one-two punch is this: Rats without worm protection will have an overreactive immune response to the initial E. coli infection, which increases levels of a specific type of signaling molecule in the brain called interleukin-1 β (IL-1β). (Although IL-1β is important for proper brain function, excessive levels can damage cognition and are associated with some neurological diseases.) The high levels of IL-1β leave the rats’ brains susceptible to damage, but the symptoms will only surface if a second infection occurs, in this case, LPS.
To test the effects on the brain, specifically memory, Williamson and colleagues exposed the rats to a new environment that the rodents soon came to recognize. After getting to know the box, the rats got an LPS injection. The following day, researchers placed them back in the box and administered an electric shock. Twenty-four hours later, the team tested and compared memory of the shock between the wormed and wormless mice.
Just as the researchers predicted, when the rats with worms were reintroduced to the scary shock box, they froze in fear, indicating they had a full memory of the previous day’s shock. But the wormless rats paused only half as much—suggesting their memories of the box were incomplete, the team reports in Brain, Behavior, and Immunity. The “wormy” rats froze as much as the double control group that was both worm-free and infection-free, suggesting that the worms had somehow protected the rats against memory loss after the infections.
In another indication of worm-facilitated protection, the team saw that rat pups whose mothers had been wormed still benefitted even if they themselves had not received worms. To test this, researchers took one pup from a wormy mother and one pup from a worm-free mother 5 days after birth and infected them with E. coli. They monitored levels of IL-1β. Pups born to wormy moms showed no increase in IL-1β after the E. coli infection, whereas the levels in pups born to worm-free moms shot up, suggesting immune benefits could be conferred through the mother’s milk or transferred in the womb.
The researchers tally with a commonly held theory called “biome depletion,” says co-author Staci Bilbo, a neuroscientist at Duke University in Durham, North Carolina. The theory suggests immune systems that evolve without enough exposure to infections (like the wormless rats) overreact when exposed to even milder forms of infection (like E.coli or the LPS injections). Following that theory, more exposure to microbes may help mammals avoid the oversensitive responses, which in the rats’ case could help prevent memory loss.
Bilbo says her next plans involve tweaking the rat experiment in an effort to zero in on how the worms convey cognitive benefits and how they could be applied in potential human treatments. “I’m not advocating that people [ingest worms],” she says. “But the information available is compelling and I think it has a lot of therapeutic potential.”