The effect of heat shock exposure duration on in vitro exsheathment and subsequent viability of Haemonchus contortus third-stage larvae

Abstract Helminth infections in livestock are a health concern across age groups and cause substantial global economic losses. Three main anthelmintic classes, benzimidazoles, macrocyclic lactones, and cholinergic agonists, are used in small ruminants; however, resistance has emerged against all, with Haemonchus contortus resistant to all major classes since 1977. Understanding nematode physiology may help identify stages suitable for intervention, both inside and outside the host. A critical but often overlooked stage is the transition of free-living third-stage larvae to the parasitic form, triggered upon ingestion by the host. Environmental triggers, including heat shock, carbon dioxide, and pH, induce exsheathment, but both the minimal and optimal durations of heat shock for exsheathment and larval viability are unknown. This study compared in vitro exsheathment rates of H. contortus L 3 exposed to heat shock for 45-, 60-, and 240-minutes, respectively. Further, experimental evaluation of viability of larvae stored in phosphate-buffered saline for 4- and 24-hours, respectively, was also conducted. All three groups that were exposed to heat shock resulted in high exsheathment rates (92, 86, and 96%, respectively) while the control group had no exsheathment (0%). Viability remained consistent across treatments at both time points. Viability after 4 h for 0-, 45-, 60- and 240-minutes were 94, 84, 81 and 3.25%, respectively, and after 24 h, the percentage viable larvae was 95, 78, 84, and 2.48%. These findings confirm the viability of L 3 larvae as a potentially effective target for in vitro exsheathment.