Olfactory circuits and behaviors of nematodes

•C. elegans encodes complex olfactory behaviors with only a small number of neurons.•Microcircuit motifs that are fundamental across computational systems encode these behaviors.•Odor preferences of parasitic worms reflect their host ranges and infection modes.•Olfactory neuron function is at least partly conserved across nematode species.

Over one billion people worldwide are infected with parasitic nematodes. Many parasitic nematodes actively search for hosts to infect using volatile chemical cues, so understanding the olfactory signals that drive host seeking may elucidate new pathways for preventing infections. The free-living nematode Caenorhabditis elegans is a powerful model for parasitic nematodes: because sensory neuroanatomy is conserved across nematode species, an understanding of the microcircuits that mediate olfaction in C. elegans may inform studies of olfaction in parasitic nematodes. Here we review circuit mechanisms that allow C. elegans to respond to odorants, gases, and pheromones. We also highlight work on the olfactory behaviors of parasitic nematodes that lays the groundwork for future studies of their olfactory microcircuits.