Radiolabeling of infective larvae of Haemonchus contortus (Nematoda: Trichostrongyloidea) with 75Se-methionine and their performance as tracers in sheep.

Haemonchus contortus infective larvae incorporated between 5 and 12 pCi/larva for each muCi of 75Se-methionine added per gram of fecal sediment. Thorough admixture of 75Se-methionine and fecal sediment was necessary to obtain approximately normal distribution and low variance of individual larval radioactivities. Ecdysis induced by treatment with 0.025% HClO in vitro resulted in loss of approximately 40% of the 75Se label of infective larvae. Loss of 75Se by parasitic larvae and adult H. contortus in vivo conformed to a two-component negative exponential function with half lives of 3.1 and 56 days acting on compartments representing 90% and 10%, respectively, of the 75Se label remaining after ecdysis. Labeled and unlabeled worms were readily distinguished by autoradiography 37 days after infection. No effect of gamma radiation arising from decay of 75Se in the range 130 to 1,300 pCi/larva could be measured in terms of survival or sex ratio of worms recovered at 17 days PI.