Diversity and Distribution of Theileria Species and Their Vectors in Ruminants from India, Pakistan and Bangladesh

Tropical theileriosis, caused by the apicomplexan hemoparasite of the genus Theileria, is a major constraint to livestock production in various parts of world, including South Asia. Several studies have been carried out over the last five decades; however, comprehensive information in this region regarding the diversity and distribution of Theileria is lacking. Therefore, keeping in mind the economic importance of theileriosis, we have systematically reviewed the current knowledge about Theileria spp. diversity and distribution affecting cattle, water buffaloes, goats and sheep in three countries included India, Pakistan and Bangladesh of the Indian sub-continent. The data collected indicated that the microscopic method is the widely used method for evaluating Theileria species in the three countries from 1970 to 2021. This is the first study in this region to compile a comprehensive knowledge about the diversity and distribution of Theileria. Our study revealed the existence of 11 different species of Theileria, including Theileria spp. Theleria annulata, T. orientalis, T. mutans, T. velifera circulating in cattle and buffalo while T. annulata, T. lestoquardi, T. luwenshuni T. ovis, Theileria spp. and T. lestoquardi-like spp., were infecting goats and sheep from various regions of India, Pakistan and Bangladesh. We find that T. annulata can be found in both small and large ruminants and is widely distributed in the different regions of India, Pakistan and Bangladesh. In addition, our analysis revealed that the existence of possible tick vectors of the genera Hyalomma, Haemophysalis, Rhipicephalus and Amblyomma may be responsible for the diverse and wide distribution of different Theileria species. However, the competence of these tick vectors for different Theileria species still need to be explored. Therefore, further studies are needed to bridge this gap and to improve the health and production of livestock and reduce economic losses due to theileriosis in India, Pakistan and Bangladesh. Furthermore, we selected representative 18S rRNA sequences for T. annulata from the different regions to infer phylogenetic relationship. Phylogenetic analysis of the selected isolates clustered in different clades which might be due to the variation in a hypervariable region of 18S rRNA. The outcome of this analysis is expected to provide a coherent and integrated framework about the different Theileria species prevailing in these countries and contribute to improving the surveillance and control plans of various Theileria species in the region.

Automated summary

This study systematically reviewed the diversity and distribution of Theileria species affecting cattle, water buffaloes, goats, and sheep in India, Pakistan, and Bangladesh. The findings revealed the presence of multiple Theileria species and identified potential tick vectors responsible for their wide distribution. Further research is needed to bridge knowledge gaps and improve livestock health and production.