Selection and identification of a precocious line of Eimeria intestinalis with enlarged oocysts and deletion of one generation of schizogony

Rabbit coccidiosis is a common parasitic disease and responsible for enormous economic losses in the rabbit industry. Eimeria intestinalis, one of the highly pathogenic and common Eimeria species infecting rabbits, is considered as an indispensable species for the development of live oocyst vaccines against rabbit coccidiosis. In this study, we report the successful selection of a precocious line (EIP8) from a wild-type strain of E. intestinalis (WT) by successively collecting and propagating the early excreted progeny oocysts. The EIP8 line had a prepatent period of only 132 h compared to 204 h for the WT. Oocysts of EIP8 were notably different from those produced by the WT strain by their significantly larger size (mean length: 29.3 vs 27.6 μm and mean width 20.5 vs 19.8 μm). Examination of tissue sections prepared from EIP8-infected rabbits revealed that this precocious line undergoes only two generations of schizogony before differentiating into gametocytes by 120 h post-infection. In contrast, WT parasites undergo three generations of schizogony and gametocytes are present by 168 h post-infection. EIP8 multiplication capacity reduced by more than 35-fold and a concomitant decrease in pathogenicity was detected. Interestingly, immunization with 103 or 104 EIP8 oocysts provided sufficient protection against homologous challenge with wild-type parasites, as body weight gain of immunized and challenged rabbits was similar to that of untreated animals, as well as more than 90% reduction of oocyst output was detected in immunized and challenged animals when compared to unimmunized and challenged animals. Together, these results show that the EIP8 precocious line of E. intestinalis is an attenuated immunogenic strain and a suitable candidate for the development of a live vaccine against rabbit coccidiosis.