From Benign Parasite to Emerging Pathogen: A Review of Theileria orientalis in Ruminants

Abstract

Simple Summary Theileria orientalis is a tick-transmitted parasite that infects cattle worldwide and was long considered harmless. However, in recent decades, severe outbreaks causing anaemia, reproductive losses, and deaths have been reported in several countries, drawing increasing attention from the veterinary and scientific communities. Disease severity depends largely on the parasite genetic type, with the Ikeda and Chitose genotypes being the most dangerous. Diagnosis can be challenging, particularly in animals that carry the parasite without showing clinical signs. Treatment options remain limited, and no vaccine is currently available. Based on a comprehensive search of the available scientific literature, this narrative review brings together current knowledge on how this parasite is classified, how it spreads, how it causes disease and how infections can be managed. It also highlights the areas where research is still lacking, including a better understanding of which tick species can transmit the parasite and the development of effective vaccines. As tick distributions continue to expand due to climate change, understanding and controlling this infection are becoming increasingly important.