Infectivity of Ancylostoma caninum larvae after freezing over liquid nitrogen.

Third-stage larvae of Ancyclostoma caninum were exsheathed either by carbon dioxide or sodium hypochlorite treatment. Exsheathment per se reduced the infectivity of third-stage larvae by 45% to 59% without affecting larval motility. Both ensheathed and exsheathed larvae were frozen over liquid nitrogen for 90 days, and thawed. Frozen ensheathed larvae were all killed; but frozen exsheathed larvae were 40-54% as infective as non-frozen exsheathed larvae, and the addition of 10% dimethyl sulphoxide raised the infectivity to 89%, relative to non-frozen exsheathed larvae. Taken together, the deleterious effects of exsheathment and freezing resulted in an infectivity of 16-37%, relative to normal larvae.