Some Aspects of Resistance in the Mice Experimentally Infected with Toxacara canis Larvae

A series of experiments were carried out to obtain some informations on host resistance in mice infected with Toxocara canis. Approximately 210 male BLAB/c mice were used and given various doses of infective eggs. For the purpose. the mean percentages of eosinophils were counted. and recovery rates of larvae from the liver, lung, brain, carcass and other tissues were examined separately at various intervals according to the experimental schedules.

Experiment I: Mice in groups I-1 and I-2 were given a single dose of 30 eggs and 100 eggs, respectively. Eosinophils and larvae were examined daily for 6 days and weekly for 21 weeks. Experiment II : Mice in groups II-1 and II-2 were given 100 eggs weekly for 2 weeks and 50 eggs for 4 weeks from 1 week after an initial administration of 30 eggs. Eosinophils were counted weekly and larvae were examined at 3 weeks after the final administration, respectively. Experiment III : Mice in 6 groups from III-1 to III-6 were given a challenge dose of 100 eggs at 1. 2. 3. 5. 7, and 9 weeks after an initial administration of 30 eggs. Eosinophils were counted weekly for 3 weeks and larvae were examined at the 3rd week after the challenge infection, respectively.

The means of eosinophils in groups I-1 nd I-2 respectively were 6.4% and 8.2% at 6 hr. and reached a maximum peak of 16.8% and 21.6% at 2 weeks, and the means decreased rapidly for the next 6 weeks. then fell gradually. The superinfection and the challenge infection in Experments II and III caused quicker and more intense rises of the eosinophils.

The recovery rates of larvae in groups I-1 and I-2 respectively showed 3 peaks at 48 hr. 1 week and 3 weeks, then decreased gradually. At 48 hr. 88.2% and 93.5% of the larvae recovered and 38.0% and 40.8% of the total larvae given were found in the liver, then decreased gradually until none was found at the end of the experiment. On the contrary. in the brain and carcass, the larvae accuulated repidly through the 1st week and the rates persisted up to the 8th week without remarkable variation. The recovery rates of larvae in total, and from the brain and carcass in groups of Experiments II and III decreased remarkably, whereas those from the liver increased significantly compared with groups I-1 and I-2 at the corresponding weeks of examination.

Transition patterns of the means of eosinophils in groups of Experiments I and II and those of total recovery rates of larvae in groups of Experiment I were essentially similar each other in rise and fall throughout the experiments.

From the overall results, it is strongly suggested that eosinophils do affect the development of resistance to invading Toxocara larvae, and the higher recovery rates of larvae in the liver indicates that the liver is the organ which may play a role of protection against infection with T. canis larvae by means of immunomechanism in mice.