Walsh, B.; Banerjee, N.; Bartolo, G.; Hallem, E. A.

Parasitic nematodes infect over a billion people worldwide and cause some of the most prevalent neglected tropical diseases (1-5). Many of these parasites are skin penetrating and have both extra-host life stages that inhabit host feces and surrounding soil, and intra-host life stages that inhabit host niches such as skin, vasculature, and intestine (2,6-8). Across life stages, these parasites encounter oxygen (O2) levels that range from ~21% at the soil-air interface to near-anaerobic levels in the host intestine (9-12). However, whether parasitic nematodes detect and respond to changes in O2 levels was unknown. Here, we examine O2 sensation in skin-penetrating parasitic nematodes and find that they show robust responses to changes in O2 levels. Moreover, their O2-evoked behaviors differ from those of the free-living nematode Caenorhabditis elegans. We then investigate the molecular and neural mechanisms of O2 sensing in Strongyloides stercoralis, a genetically tractable human-infective nematode, and find that parasite-specific behavioral responses to O2 arise in part from evolutionary changes in their soluble guanylate cyclase repertoire. Finally, we find that neuronal O2 sensing regulates intra-host development in S. stercoralis. Our results demonstrate that skin-penetrating nematodes exhibit neuronally mediated O2 responses that are critical for multiple steps of their parasitic life cycle.