Infective larvae (L3) of two different batches of anthelmintic-susceptible Haemonchus contortus, cryopreserved for 15,6 and 15,7 years in liquid nitrogen appeared dead when examined 10-20 min after thawing. However, having become motile later in the day, their viability was tested the next day in two separate groups of sheep. The mean development of 41 ,8 % and 66,3 % of the H. contortus compared favourably with previous trials with this strain, in which the L3 were injected into the abomasa of sheep after various periods of cryopreservation. It is speculated that the relatively slower recovery rates after thawing of the L3 used in this trial, when compared with those in previous trials in which one of the two batches of susceptible H. contortus was used after shorter periods of cryopreservation, may be an indication that these L3 were gradually approaching the limits of survival after almost 16 years in liquid nitrogen.
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