How Did Babesia equi Become Theileria equi? The Story of the Scientific Reclassification of the Equine Piroplasma Parasite

 

 

Dr. Nahlan Juwair Hassan Al-Samarrai

10-6-2026

Equine piroplasmosis is one of the most important tick-borne parasitic diseases in horses, caused by two main parasites: Babesia caballi and Theileria equi. However, many older references refer to the second parasite as Babesia equi, leading some to ask: Did Babesia become Theileria?

 

The answer is no. The parasite did not biologically change from one species to another. Rather, its scientific classification changed as a result of advancements in knowledge and the tools used to study parasites.

 

When the parasite was first discovered, its classification was primarily based on its microscopic appearance and the life cycle known at the time. Due to its similarity to other Babesia species within red blood cells, it was named Babesia equi. However, this classification was based on limited information compared to what is available today. With the advent of molecular biology techniques and gene sequencing, scientists began to study the evolutionary relationships between parasites with greater precision. Studies of the 18S rRNA gene showed that Babesia equi is more closely related to the genus Theileria than to the genus Babesia. These studies also revealed that the parasite exhibits a schizogony stage of asexual reproduction in mononuclear cells, a characteristic unique to Theileria and absent in true Babesia.

 

Based on this molecular and biological evidence, the parasite was officially reclassified as Theileria equi. Babesia caballi, however, remained within the genus Babesia because its genetic and biological characteristics were consistent with this classification.

 

This case clearly illustrates the evolution of taxonomy; with advancements in technology, reliance on genetic relationships has become more important than reliance on morphological similarity alone. This is why some older books and articles still use the name Babesia equi, while more recent references use the correct name, Theileria equi. In practice, both Babesia caballi and Theileria equi still cause equine piroplasmosis, but understanding the differences between them is crucial for epidemiological and diagnostic studies and for interpreting the results of recent research.

 

The story of Babesia equi’s transition to Theileria equi is not simply a story of one parasite changing from one species to another, but rather a story of evolution in scientific knowledge. Modern molecular evidence has led to a revision of the classification to reflect the true evolutionary relationship of this parasite.

Note: The name Babesia equi is still common in older veterinary literature, so when reviewing studies published before the 1990s or early 2000s, you may find both names used to refer to the same parasite.

 

References

 

1. Mehlhorn H, Schein E. The Piroplasms: Life Cycle and Sexual Stages. Advances in Parasitology. 1984;23:37–103.
2. Allsopp MTEP, Cavalier-Smith T, De Waal DT, Allsopp BA. Phylogeny and evolution of the piroplasms. Parasitology. 1994;108:147–152.
3. Ueti MW, Palmer GH, Scoles GA, Kappmeyer LS, Knowles DP. Persistence of Theileria equi in the horse. Experimental Parasitology. 2008;119(1):148–153.
4. Knowles DP Jr. Equine piroplasmosis: a review of epidemiology, pathogenesis, and diagnosis. Journal of Equine Veterinary Science. 1996;16(1):6–11.
5. World Organization for Animal Health. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals – Chapter on Equine Piroplasmosis.
6. Merck Veterinary Manual. Equine Piroplasmosis (Babesiosis). Modern veterinary reference recognizing Theileria equi as the current accepted name.
7. Kappmeyer LS, Knowles DP, Ueti MW, et al. Genetic diversity of Theileria equi and its relationship to other piroplasms. Veterinary Parasitology. 2012;189:64–73.