Effect of Refrigeration on the Antinematodal Efficacy of Ivermectin

Several observations have suggested that the anthelmintic ivermectin can affect nematodes by non-oral entry into the nematode body. To investigate this possibility further, we refrigerated Caenorhabditis elegans at 5 C to prevent its locomotion and to block the pharyngeal pumping that is so prominent a feature of its feeding. Worms were exposed to ivermectin (1–25 µg/ml) at that temperature for 1 hr, after which the medium was replaced by unmedicated medium at room temperature. After 1 hr at room temperature the worms were examined and counted to determine the degree to which irreversible immobilization had occurred. The drug was significantly less effective at 5 C than at room temperature. This reduction in potency could be attributed to a general cold-induced decline in the rate of the biochemical processes involved in drug action. Alternatively, the reduction could be attributed to the cold-induced blockade of pharyngeal pumping, which would suggest that the efficacy of ivermectin is partially the resu...