A shockingly large number of us – nearly a quarter – are infected with worms. But to infect us, these tiny parasites must navigate an incredible journey. For example, threadworms (Strongyloides stercoralis) wriggle through soil to the surface, where they anchor onto the legs of passing victims. Despite being shorter than a millimeter, they are strong enough to break through our skin and hike toward our lungs and intestines to multiply. Such a long distance for a worm. For Navonil Banerjee and a team of researchers led by Elissa Hallem at the University of California, Los Angeles, USA, this was an intriguing mystery. What prompts worms to leave home, and how do they know where their travels end? After all, the body is a large place, so how can worms navigate through the human body so readily on their first trip there?
Banerjee and colleagues noted that parasites are often picky about carbon dioxide, the gas molecule we exhale. They reasoned that threadworms live in high carbon dioxide environments for much of their life cycle: our bacteria-filled intestines are loaded with this gas, as is the feces in which larvae exit the body. So, the researchers performed an experiment: they placed the worms in a Petri dish, with an air inlet on one side and a carbon dioxide source on the other side. Then they measured whether the worms migrated towards or away from carbon dioxide. Most of the threadworms weren't picky at all. Adults neither moved towards the carbon dioxide, nor did they run away.
Then, the scientists repeated the experiment with younger worms; the youngest larvae were just as indifferent as adults. But adolescent threadworms reacted strongly. When threadworms became mature enough to infect a new host, they scattered away from carbon dioxide. These ‘infective’ worms can continue to mature until they become ‘activated’ – preparing for life inside their new host. These slightly older, ‘activated’ threadworms didn't run like their younger siblings; they moved eagerly towards carbon dioxide. As Banerjee notes, this makes sense: to infect a new host, larvae must leave cozy, carbon dioxide-heavy feces and trek into fresh surface soils where humans might be passing through. Later, inside their new host, they must navigate into the carbon dioxide-rich intestines to settle down. This gas makes them run away from home, as well as settle down again. But how are they even aware of the odorless gas in the first place?
Banerjee's team pulled up all the available information on worm genetics they could find and scanned it for carbon dioxide detectors. As it turns out, the worms had such a detector – on just two lonely nerve cells in their entire brain. Were two mere cells responsible for such a dramatic change – first a sudden distaste, and then a renewed love for carbon dioxide? They removed this gene in the threadworms and placed the genetically modified worms back in a Petri dish with carbon dioxide and air on either end. This time, the threadworms ignored carbon dioxide at all ages. So, threadworms navigate with a compass, of sorts: a carbon dioxide detector deep in their brain, telling them when it's time to venture out, and when it's time to settle down again.
It is fascinating to know why such tiny animals go on such circuitous adventures. But for the roughly 1.5 billion humans infected with worms, it may become more than an intellectual fancy. The more we understand what motivates parasites, the more we can limit infections. If we understand how they get in, we may learn how to keep them out.
