Review of equine piroplasmosis.
Authors: Wise L N, Kappmeyer L S, Mealey R H, Knowles D P
Journal: Journal of veterinary internal medicine
Summary
Equine piroplasmosis, caused by either *Babesia caballi* or *Theileria equi*, represents a significant tick-borne threat to horses worldwide, with both parasites establishing persistent infections that primarily manifest as haemolytic anaemia through destruction of red blood cells. Although these two erythrocytic parasites share similar transmission routes and pathogenic outcomes, they differ substantially in their lifecycle dynamics—notably that only *B. caballi* transmits transovarially through tick generations, whilst *T. equi* follows a distinct tissue tropism by initially infecting peripheral blood mononuclear cells rather than circulating erythrocytes. This comprehensive review synthesises current knowledge on piroplasmosis aetiology, pathogenesis, and epidemiology, with particular emphasis on the recent re-emergence of *T. equi* in North America and the underlying factors driving this resurgence. Understanding these distinctions between causative agents is critical for equine practitioners, as diagnostic approaches, transmission control strategies, and treatment responses may differ significantly between *B. caballi* and *T. equi* infections. The review underscores the need for heightened awareness among farriers, veterinarians, and facility managers regarding tick control protocols and diagnostic protocols, particularly in regions where piroplasmosis prevalence is increasing or where international horse movement may introduce these pathogens.
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Practical Takeaways
- •Clinicians should recognize tick-borne piroplasmosis as a cause of anemia in horses, particularly with T. equi re-emergence in the US, requiring appropriate diagnostic testing
- •Tick control is critical for disease prevention since both parasites are tick-transmitted; however, B. caballi's transovarial transmission makes eradication more challenging
- •Long-term carrier status in recovered horses necessitates screening protocols before movement or breeding to prevent disease transmission
Key Findings
- •Equine piroplasmosis is caused by two erythrocytic parasites: Babesia caballi and Theileria equi, both transmitted by ticks and causing anemia through erythrolysis
- •B. caballi transmits across tick generations and immediately infects erythrocytes, while T. equi infects peripheral blood mononuclear cells first
- •Both parasites can persist indefinitely in equid hosts despite immune responses
- •T. equi has re-emerged in the United States, increasing clinical awareness and diagnostic/control challenges