Chemotactic behaviour of Strongyloides stercoralis infective larvae on a sodium chloride gradient

Chemotactic responses of Strongyloides stercoralis infective larvae (L3) to sodium chloride (NaCl) were investigated by recording larval tracks on a saline gradient in agarose. On agarose, larvae migrated randomly, whereas when placed at 0·01 M NaCl larvae moved to approximately 1·1 M NaCl where they turned, headed down the gradient and eventually remained circling at a favoured salinity (0·03–0·07 M). Conversely, when placed at 2·85 M NaCl, the L3 larvae moved unidirectionally to lower, more favoured salt concentrations. Here they circled, changing directions frequently while making 'loop-like' tracks. Larvae were immobilized within 5 min at salt concentrations exceeding 3 M NaCl. When placed at 0·01 M NaCl, 51·1%±26·9 migrated to 1·1 M NaCl after 2 min, and 80%±18·7 did so after 8 min, at an average velocity of 4·1±1·4 mm/min. Larvae (53·6%±21·6) were repelled from 2·85 M NaCl to lower concentrations after 2 min. After 8 min, 95%±11·1 were repelled, moving at an average velocity of 6·2±1·1 mm/min. Using this bioassay, the influence of neuronal control over chemotactic behaviour of S. stercoralis and other parasitic nematodes can be elucidated.