The effects of freezing on the viability of Toxocara canis and T. catiembryonated eggs

Suspensions of embryonated (L2) Toxocara canis and T. cati eggs were maintained under freezing conditions in order to study the impact on their long term viability. The eggs had been removed from the uteri of adult Toxocara spp. worms and the suspended in a 0.4% formalin solution before being frozen in 20 ml plastic tubes in the freezer compartment of a domestic refrigerator. Assessment of embryonated egg viability over a period of 34 days was conducted by microscopic examination under a high light intensity. Embryonated T. cati eggs were found to exhibit a greater resistance to freezing than those of T. canis. A difference in the size, design of the egg shell or the metabolism of the larval form may have accounted for the observed differences in the percentage viabilities over time between the two species of Toxocara.