Figure 1. Life cycle of protostrongylid parasite: Umingmakstrongylus pallikuukensis in muskoxen definitive and gastropod intermediate hosts (12). Adult nematodes (for U. pallikuukensis, located in the lungs) lay eggs, which hatch to first-stage larvae (L1). L1 move up the airways, are swallowed, and pass in the feces, where they must invade the foot of gastropod intermediate hosts for further development to the infective third-stage larvae (L3). Development to L3 requires a minimum amount of heating and does not occur below a critical threshold; these development parameters vary among different protostrongylid species (6,7,13). Definitive hosts become infected by ingesting a gastropod containing L3 or, for some protostrongylids such as U. pallikuukensis, by ingesting L3 that have emerged from the gastropods and are free in the environment.