Cytokine Profiles in Naturally Infected Cattle Reveal Immune Responses to Theileria orientalis Infection

Theileria orientalis, long considered a benign haemoprotozoan, is now recognised as an emerging pathogen causing clinical theileriosis in cattle across the globe. This study investigated the clinical presentation and immunopathological responses associated with natural T. orientalis infections in northern Kerala, India. Blood samples were collected from clinically suspected (n = 52) and healthy cattle (n = 148). Infection was confirmed via Giemsa-stained blood smear examination, species-specific PCR targeting the major piroplasm surface protein (MPSP) gene, and subsequent sequencing. Among clinically suspected cases, 63.5% were smear-positive and PCR-confirmed, while subclinical infection was identified in 46.6% of healthy cattle. Morphological analysis revealed marked polymorphism of intraerythrocytic piroplasms. Clinical signs included anaemia, pyrexia, lymphadenopathy, and systemic debilitation. To elucidate host immune responses, expression levels of key pro-inflammatory cytokines (IL-6, IL-1β, TNF-α) were quantified in peripheral blood mononuclear cells using SYBR Green-based quantitative real-time PCR. Clinically affected animals exhibited significantly elevated expression of IL-6 and IL-1β (p < 0.05), indicative of a heightened inflammatory response, whereas TNF-α expression showed a non-significant downward trend. Correlation analysis revealed that parasite burden was positively associated with IL-6 and IL-1β expression but not with TNF-α, linking parasite load to the inflammatory response. The suppressed TNF-α response suggests a possible immune evasion strategy unique to T. orientalis, contrasting with the elevated TNF-α typically seen in T. annulata and T. parva infections. These findings indicate that IL-6 and IL-1β may serve as potential biomarkers of severe oriental theileriosis. These findings provide new insights into the immunopathogenesis of oriental theileriosis and suggest that cytokine dysregulation contributes to clinical disease severity. Furthermore, the high prevalence of subclinical infections underscores the role of carrier animals in sustaining transmission cycles, complicating disease control efforts in endemic regions. This study highlights the need for improved diagnostic strategies and targeted immunomodulatory interventions in managing T. orientalis infections.