Carbon dioxide shapes parasite-host interactions in a human-infective nematode

Skin-penetrating nematodes infect nearly one billion people worldwide. The developmentally arrested infective larvae (iL3s) seek out hosts, invade hosts via skin penetration, and resume development inside the host in a process called activation. Activated infective larvae (iL3as) traverse the host body, ending up as parasitic adults in the small intestine. Skin-penetrating nematodes respond to many chemosensory cues, but how chemosensation contributes to host seeking and intra-host navigation-two crucial steps of the parasite-host interaction-remains poorly understood. Here, we investigate the role of carbon dioxide (CO₂) in promoting host seeking and intra-host navigation in the human-infective threadworm Strongyloides stercoralis. We show that S. stercoralis exhibits life-stage-specific behavioral preferences for CO₂: iL3s are repelled, non-infective larvae and adults are neutral, and iL3as are attracted. CO₂ repulsion in iL3s may prime them for host seeking by stimulating dispersal from host feces, while CO₂ attraction in iL3as may direct worms toward high-CO₂ areas of the body, such as the lungs and intestine. We also identify sensory neurons that detect CO₂; these neurons display CO₂-evoked calcium activity, promote behavioral responses to CO₂, and express the receptor guanylate cyclase Ss-GCY-9. Finally, we develop an approach for generating stable knockout lines in S. stercoralis and use this approach to show that Ss-gcy-9 is required for CO₂-evoked behavioral responses in both iL3s and iL3as. Our results highlight chemosensory mechanisms that shape the interaction between parasitic nematodes and their human hosts and may aid in the design of novel anthelmintics that target the CO₂-sensing pathway.