Cryopreservation of Dirofilaria immitis microfilariae and third-stage larvae.

Microfilariae of Dirofilaria immitis retained their infectivity for susceptible mosquitoes after cooling to -196 degrees C in the presence of 5% dimethylsulphoxide (Me2SO) using a two-step cooling sequence. Motility and in vitro development of cryopreserved microfilariae also compared favourably with unfrozen controls. Third-stage larvae frozen by the same cooling sequence in the presence of either 5% Me2SO or 16% hydroxyethyl starch were motile upon thawing. Thawed larvae completed the third- to fourth-stage moult in vitro at a frequency approximately 5 to 10% of that seen in unfrozen controls.