Benzimidazole-resistant Haemonchus contortus--the effect of cryopreservation on the resistance of successive generations.

Infective larvae (L3) of a strain of benzimidazole-resistant Haemonchus contortus were stored in the gas phase of liquid nitrogen, thawed and used to infest worm-free sheep from which, in turn, larvae were cultured, frozen and thawed. This cycle was repeated 5 times. Thereafter, the progeny of the cryopreserved larvae were compared with the progeny of the original untreated larvae for susceptibility to benzimidazole anthelmintics. Repeated freezing of consecutive generations of L3 in liquid nitrogen did not appear to affect their relative resistance to benzimidazoles and, although other strains must also be tested, it would seem that cryopreservation may be used for storing resistant strains.