Investigation of the parasite elimination and transmission via vector ticks in treated and untreated sheep following experimental Babesia ovis infection

In this study, the elimination of the parasite, the persistence of infection, and the transmission potential (via blood transfusion and vector ticks) were experimentally investigated in sheep that were experimentally infected with the Babesia ovis-Alacakaya strain and either treated with 1.2 mg/kg imidocarb dipropionate (IMDP) following infection or allowed to recover without treatment. A total of 20 sheep were used in the study. The experimental groups consisted of untreated (#021, #668, #671) and treated (#842, #953, #1006) sheep. In addition, in sheep monitored for six months after experimental infection, both blood transfusion and infestation experiments with sterile Rhipicephalus bursa larvae were planned. This period was not only intended to assess the persistence of infection but also biologically designed to model the potential encounter between clinical babesiosis cases emerging in spring and the activation of R. bursa larvae in autumn under natural conditions. In both groups, clinical signs typical of babesiosis such as high fever, anemia, and jaundice were observed following infection, with clinical symptoms subsiding more rapidly in the treated group. However, nPCR revealed fluctuating parasitemia between days 70–144 in the treated group and days 59–180 in the untreated group. Serologically, B. ovis-specific antibodies were detected by ELISA between days 153–180 in the treated group and persisted until day 180 in the untreated group. In both groups, antibody responses developed early and remained detectable for an extended period. xx On day 180, blood was collected from the infected sheep and transfused into naïve recipient sheep (#361, #400, #625, #1763, #1751, #1759). Infection developed in recipients transfused with blood from both the treated and untreated groups, with some recipients exhibiting fever and nPCR positivity. These findings indicate that latent infections can be transmitted via blood transfusion and that nPCR/ELISA negativity does not preclude infectivity. To evaluate whether infected sheep could serve as a source of infection for R. bursa larvae, sterile R. bursa larvae were applied to both treated and untreated sheep six months after infection. The larvae and nymphs completed their development on these sheep, molted into adults, and were used to infest naïve recipient sheep (#H7, #A9, #M26, #4577, #4513, #4520). After tick infestation, recipient sheep were monitored for B. ovis infection via microscopic, serological, and molecular methods. No evidence of infection was detected in any of the recipient animals. Furthermore, B. ovis DNA was not detected in the engorged adult ticks or in pools of eggs laid by these ticks. These results suggest that sheep recovering from clinical infection, regardless of treatment status, did not serve as a source of infection for R. bursa larvae. In conclusion, while IMDP treatment was effective in alleviating clinical signs, it did not result in complete elimination of the parasite. Treated sheep retained the potential to transmit infection via blood transfusion. However, both treated and untreated sheep were found to have parasite loads below the acquisition threshold for vector ticks six months post-infection, and thus were not infectious to R. bursa larvae. These findings highlight the importance of using highly sensitive molecular diagnostic methods in detecting latent infections and suggest that when parasite burden remains below the transmission threshold, R. bursa larvae are unable to acquire and transmit the infection. Keywords: Babesia ovis, Experimental infection, Blood transfusion, Imidocarb dipropionate, Rhipicephalus Bursa, Sheep.