Our immune systems protect us from disease, but could they also interfere with our brains? Experiments by an Irish scientist with honeybees give food for thought, writes Andrew Read.
In starved bees, artificially-induced immune responses speed death. Dr Eamonn Mallon wondered if they could also reduce memory formation.
Like Pavlov's dogs, honeybees can learn to associate a stimulus and a reward - in the case of bees, a floral scent with food. Once they've learned, the bees push out their mouth-parts in response to the scent alone.
Working with Dr Axel Brockmann and Prof Paul Schmid-Hempel, in the University of Würzburg in Germany and the Swiss Federal Institute of Technology in Zürich, Mallon tested the ability of the bees to remember the scent/food connection after they had been injected with an immune stimulant. The substance they used comes from bacteria and causes the insect's immune system to be switched on.
Stimulation of the immune system had a dramatic effect on memory. "Between 60 and 70 per cent of the bees in the control group remembered, but only 30 per cent of the immune-stimulated group," he says.
"The bees weren't sick," says the Dublin-born researcher. The immune stimulant was not toxic and all the bees responded to the scent. But the test bees were less likely to associate the stimulus with food, he said.
Why does immunity make bees forgetful? It may be that the two systems compete for the same resources, he suggests. "But more likely is that this is one of many costs of immunity."
For more than a century, it has been known that children with worms do less well at school. Could anti-worm immunity be the reason? "You extrapolate at your peril," says Mallon. "But it's one of the reasons sick birds don't do so well at finding their stored nuts." And in rats, a cytokine - an immune system hormone - interferes with a part of the brain and reduces memory consolidation, he said.
The effects of infections such as worms and malaria on children's performance at school has important implications for education in the tropics. Many of these infections, particularly worms, are contracted during the school years. Double-blind placebo trials over the last decade have shown conclusively that in areas where children have large numbers of worms, school performance can be substantially improved if worms are removed with drugs.
These drugs, which are used by farmers in the First World to treat worm infections in their animals, are very cheap. Improvements in child education in some parts of the developing world can often be achieved more cheaply with these drugs than by employing extra teachers to reduce class sizes.
There are several reasons why worm infestation may reduce school performance. Except at very high numbers, worms are not painful, but it may be that gastro-intestinal upset reduces attention in lessons or attendance at school. But the bee and rat experiments suggest it may also be due to chronic immune responses against the worms.