Immunological, Biochemical, and Histological Evaluation of Sonicated Theileria annulata Antigens and Methionine in Rabbit Models

Background: Theileria annulata is a hemoprotozoan parasite transmitted by ticks, responsible for tropical theileriosis, an important disease in cattle that leads to high mortality and morbidity rates. The disease causes economic losses to the animals due to reduced productivity, treatment costs, and death, especially in endemic areas.
Objectives: This study examined rabbit immunization with sonicated T. annulata antigen, determined the effects of administering immunomodulatory L-methionine, and performed the immunological, biochemical, and histopathological analysis to discover strategies to control the parasite’s spread.
Methods: Forty-eight rabbits were divided into four groups, each comprising 12 rabbits. The first group (control) administered an injection of distilled water and functioned as the control group. The second group (antigen) was inoculated intraperitoneally with 500 μg of purified sonicated T. annulata piroplasm antigen. The third group (antigen + L-methionine) was inoculated similarly to the second group and received supplementation with L-methionine. The fourth group administered L-methionine. The immunological analysis of the rabbit groups was performed on day 21 of the trial. The investigation included evaluations of immune response parameters (IFN-γ, IL-10, TGF-β1, and IgG), biochemical serum analysis (malondialdehyde [MDA] and total antioxidant capacity [TAC]), and histopathological analysis of the mesenteric lymph nodes.
Results: Immunization significantly elevated IFN-γ, IL-10, TGF-β1, and IgG levels compared to controls. Immunization combined with L-methionine (G3) resulted in the highest levels, indicating synergy. However, L-methionine alone (G4) raised these values to a lesser extent. Group G4 (L-methionine) improved antioxidant capability, but group G2 (immunized) exhibited higher MDA levels, indicating oxidative stress. Histologically, group G2 showed follicular hyperplasia and hemorrhages; group G3 exhibited mixed immune cell infiltration and minor vascular congestion; the group G4 displayed mild apoptotic alterations and enhanced cortical cellularity. L-methionine enhanced immunological and antioxidant responses, especially when combined with vaccination. 
Conclusion: Immunization significantly enhances immune responses, while L-methionine appears to further amplify these effects when combined with immunization. L-methionine alone also contributes to immune regulation, but to a lesser extent.