Factors in the resistance of rats to re-infection and super-infection by Clonorchis sinensis.

Rats develop resistance to re-infection by Clonorchis sinensis while humans do not. We investigated factors involved in the development of resistance to re-infection and super-infection in rats. Sprague-Dawley rats were infected by C. sinensis metacercariae and treated with praziquantel, then re-infected after immune modulation. The rats were also subjected to super-imposed infection after primary infection. Resistance to re-infection was observed by lowered rates of worm recovery after various durations from treatment; 1% at 5days and 24.4% at 11months vs 56.2% in the control. Similar significant resistance was observed in the super-infected rats after 3weeks of primary infection. The re-infected or super-infected worms grew very slowly and remained immature. Resistance disappeared in immune-suppressed and nude rats but not in splenectomized rats. Immunization with antigens of C. sinensis produced no resistance. Local tissue reaction and immune response in the infected bile duct may play an important role in the resistance, although the crowding effect may also be implicated in resistance in super-infected rats.