Abstract:

Babesia is a kind of zoonotic pathogen transmitted by ticks, surviving in human or animal red blood cells (RBCs). After infection, it can cause clinical symptoms such as fever, anemia and hemoglobinuria in humans or animals, and even death, posing a severe threat to the health of humans, especially those with asplenia, malignant tumors, and immunosuppression. Previous epidemiological surveys demonstrated the significant differences in the infection rates of Babesia in different domestic animals, but the specific pathogenic mechanisms remain unclear. This study aims to provide a basis for further elucidating the pathogenic mechanisms of Babesia, through comparing the infectivity, changes in blood indices, and immune responses of Babesia microti in different common laboratory animals. In this study, mice, rabbits and sheep were inoculated with B. microti respectively. Then parasitemia, blood parameters and anti-parasite specific protein antibodies were monitored from 0 to 36 d post-infection. The results showed that the parasitemia in mice reached the peak (62.9%) on the 4th day after innoculation, with a copy number of 6.05×10⁷ copies·μL^-1, while the parasitemia in New Zealand rabbits and sheep was about zero. Hematology tests demonstrated that the number of RBCs, hemoglobin concentration (HGB), and hematocrit (HCT) in the mice blood were significantly decreased during the early and middle monitoring term (0-20 d post-inoculation). During 0-8 d post-infection, the number of platelets (PLT) and platelet packed volume (PCT) were prominently lower than those in the control group, but with a backup later to a normal level. ELISA test showed that anti-B. microti secreted antigen 1 (BmSA1) antibody in mice increased continuously to the highest level at 16 d post-infection and then kept at high level until the end of the tested term. Anti-BmSA1 antibody in New Zealand white rabbits speedly rose up to the highest level at 8 d post-infection and persisted to the end of the mornitoring time, while sheep showed low immune response. The results suggest that mice were susceptible to B. microti which led to abnormal changes in the blood, while rabbits and sheep were resistant to the B. microti infection. This specificity may be associated with the genetic diversity of the recognition site in hosts. Rabbits and sheep demonstrated different anti-BmSA1 antibody levels, which hinted the innate differences in the anti-Babesia mechanisms of the two species. The present study laid a base for further research on the Babesia-host interaction mechanism, as well as vaccine and drug exploration.

Key words: Babesia microti, susceptibility, infection rate, antibody, platelet