A comparison of the infectivity of cryopreserved versus unfrozen infective larvae of Haemonchus contortus, Trichostrongylus colubriformis and Trichostrongylus axei. Results of the Onderstepoort Veterinary Institute and collaborators from 1977 to the present.

The infectivity for sheep of cryopreserved infective larvae (L3) of various strains of Haemonchus contortus, Trichostrongylus colubriformis and Trichostrongylus axei is compared using previously published results of trials conducted at the Onderstepoort Veterinary Institute laboratories, and of collaborators. The means and ranges of development were similar for both frozen and unfrozen larvae of two of the three worm species reviewed. A mean of 33.4% (range, 12.7-63.0%) of cryopreserved H. contortus L3 developed, compared to a mean of 43.7% (range 2.4-78.7%) of unfrozen L3 of this worm species. The corresponding values for T. colubriformis were 33.0% (range 10.3-62.7%), and 33.5% (range 8.3-52.2%), respectively. In the case of T. axei, the development of the cryopreserved L3 (tested in only three trials) was markedly lower than that of unfrozen L3 in the single trial in which the latter was evaluated. It is concluded that development of cryopreserved L3 is probably similar to that of unfrozen L3 and that, for several reasons, maintaining nematode larvae in the frozen state in liquid nitrogen is a much superior method to that of one which entails cycling worm strains continually in their final hosts.