Response to carbon dioxide by the infective larvae of three species of parasitic nematodes

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The response of infective third-stage larvae (L3) of three species of parasitic nematodes, Ancylostoma caninum, Strongyloides stercoralis, and Haemonchus contortus to carbon dioxide (CO 2) at physiological concentrations was investigated. L3 of the skin-penetrating species, A. caninum and S. stercoralis, were stimulated by CO 2 at the concentration found in human breath (3.3–4%); these larvae responded by crawling actively, but not directionally. Crawling was not stimulated by breath passed through a CO 2-removing ‘scrubber’ or by ‘bench air’. Both A. caninum and S. stercoralis L3 stopped crawling when exposed to 5% CO 2 for 1 min. L3 of A. caninum became active 9–14 min after exposure to 5% CO 2 ended, but activity resumed more rapidly (10–15 s) if larvae were subsequently exposed to breath or breath through the scrubber. L3 of S. stercoralis resumed crawling 30–35 s after exposure to 5% CO 2, but resumed crawling within a very few seconds when exposed to breath or breath through the scrubber. Thus, while 5% CO 2 was inhibitory, lower concentrations of this gas stimulated L3 of both species. Apparently, exposing immobilized larvae to breath or breath through the scrubber causes the environmental CO 2 concentration to drop to a level that is stimulatory. The L3 of H. contortus ceased crawling and coiled when exposed to human breath or to 1% CO 2, but continued to move within the coil in both cases. The crawling response of the L3 of the two skin-penetrating species, A. caninum and S. stercoralis, to stimulation by CO 2 probably relates to their active host-finding behavior, while the cessation response elicited by CO 2 in H. contortus larvae may relate to the fact that they rely on passive ingestion by a ruminant host.