Age of Haemonchus contortus third stage infective larvae is a factor influencing the in vitro assessment of anthelmintic properties of tannin containing plant extracts.

•The effect of Haemonchus contortus L3 age on the in vitro assessment of anthelmintic properties of plant extracts was evaluated.•Older L3 require higher plant extract concentrations to inhibit their exsheathment.•More stable exsheathment inhibition efficacy values were found within the range of 2- to 5-week old larvae.•Strong negative correlations were found between extract EC50 values measured with LEIA and L3 motility.•These results provide scientific evidence and indicators to standardize the LEIA between laboratories.