CRYOPRESERVATION OF THE FIRST-STAGE LARVAE OF TRICHOSTRONGYLID NEMATODE PARASITES

First stage (L1) larvae of Haemonchus contortus, Trichostrongylus colu briformis and Ostertagia circumcincta can be cryopreserved in the pres ence of DMSO using a two-step freezing protocol involving an initial p eriod at -80 degrees C prior to transfer to liquid nitrogen. Thawed L1 larvae continue development in vitro producing third stage (L3) larva e that are infective to sheep when dosed per os. Establishment rates f or L3 larvae grown from thawed L1 larvae were 40 and 80% for A. contor tus and T. colubriformis, respectively. There was no difference in sur vival or infectivity between benzimidazole (BZ)-susceptible and BZ-res istant A contortus isolates and cryopreservation caused no shift in th eir BZ-resistance status as indicated in an in vitro larval developmen t assay. Cryopreservation also had no effect on the sensitivity of the se isolates to the avermectins or levamisole in vitro. High survival r ates (60-70%), good levels of establishment and the stability of anthe lmintic resistance status of isolates indicate that little if any sele ction occurs during the cryopreservation process. L1 larvae of all 3 s pecies have been successfully recovered after 16 months storage in liq uid nitrogen, cultured to the L3 stage and established in sheep.