Carbon dioxide is an absolute requirement for exsheathment of some, but not all, abomasal nematode species.

The first step in the infection process of grazing ruminants by gastrointestinal nematodes is the exsheathment of the third-stage larva (L3). Exsheathment of various species can be achieved in vitro using carbon dioxide (CO) under the appropriate temperature and pH conditions. However, it remains unclear whether elevated CO levels are an absolute requirement for exsheathment. Exsheathment of four abomasal species was investigated in both the presence and absence of CO, in either rumen fluid (cow or sheep) or buffer (standard or enriched). Exsheathment of Ostertagia ostertagi, Teladorsagia circumcincta and Ostertagia leptospicularis was observed in CO-depleted rumen fluid and enriched buffer (respectively 46%, 22% and 15% in rumen fluid and 28% 18% and 26% in enriched buffer after 24 h). The level of this response was dependent on the species as well as the medium, and exsheathment was significantly higher in the presence of CO. For Haemonchus contortus, exsheathment could only be achieved under CO-saturated conditions. In conclusion, even though these parasite species exsheath in the same environment, there were significant differences in the minimal requirements to trigger their exsheathment. Some abomasal species were capable of exsheathment in the absence of CO, which is likely facilitated by cofactors present in the rumen fluid and/or enriched buffer.