Theileria Equi Infection: What the Research Says
Evidence from 24 peer-reviewed studies
What Professionals Should Know
- •Current antiparasitic drugs have significant limitations—imidocarb causes toxicity and failures, buparvaquone is unreliable for T. equi—so expect treatment challenges and plan accordingly
- •Pre-export diagnostic testing is unreliable with current methods; use combined PCR and serology rather than single tests to reduce risk of unknowingly trading infected horses
- •Tick control alone is unsustainable due to environmental concerns and resistance development; integrated control strategies and novel therapeutics are needed and under development
- •Standardized nomenclature (genotypes A-D) is now established for T. equi, improving consistency in diagnostic reporting and research communication across laboratories and regions.
- •Understanding geographical distribution of genotypes can inform risk assessment and quarantine protocols for imported horses, particularly for genotypes A and C which span multiple continents.
- •Genetic diversity variations among genotypes may have implications for vaccine development and diagnostic test sensitivity, warranting investigation of whether different genotypes respond differently to treatment or prevention strategies.
- •T. equi is present in Kyrgyzstan grazing populations at ~7% prevalence; practitioners should consider serological/molecular screening protocols for horses in endemic regions, particularly older males
- •Knowledge of specific T. equi genotypes (A and E) circulating locally may inform vaccine selection and control strategies as they become available
- •Absence of B. caballi and Anaplasma species in this survey does not rule out transmission risk; continued surveillance and tick control remain important
- •Clinically healthy racehorses may harbour T. equi or B. caballi infections that cause subclinical inflammatory changes and impact performance; consider PCR testing in underperforming horses
- •Blood work abnormalities and reduced muscle mass in apparent clinically sound horses warrants investigation for piroplasmosis as a potential underlying cause
- •T. equi and B. caballi infections should be considered in the differential diagnosis for standardbred racehorses with unexplained poor racing performance or training difficulties
- •Implement rigorous tick control and monitoring programs on farms, as tick presence on horses or shared cattle increases infection risk 3-4 fold
- •Establish quarantine and testing protocols for horses entering tick-free zones to prevent introduction of T. equi, given the dramatic difference in seroprevalence between areas (6% vs 70%)
- •Consider geographic location and local tick burden when assessing disease risk and implementing herd health strategies for equine piroplasmosis prevention
- •Tick-borne pathogen infections in donkeys cause measurable changes in complete blood count parameters; routine hematology can aid in diagnosis alongside serological testing
- •Anemia (reduced RBC, hemoglobin, platelets) is a consistent finding in infected donkeys and may require supportive care during treatment
- •Elevated globulin fractions suggest active immune response; this marker could help monitor treatment efficacy and recovery in affected animals
- •If you work with horses in Mexican tropical/subtropical regions, expect high exposure rates to both parasites; serological testing alone underestimates active infections compared to molecular methods
- •Theileria equi appears more prevalent and persistent than B. caballi in this region—PCR testing is more reliable than serology for detecting current infections
- •Tick control and vector management are critical in endemic areas; consider screening horses before movement to non-endemic regions
- •EP remains rare in Dutch horses but is significantly more prevalent in imported horses—screening imported animals is essential before introducing them to your facilities
- •Although tick vectors are established in the Netherlands, the disease has not become endemic; continued surveillance is warranted but panic is not justified
- •If sourcing horses internationally, particularly from Southern Europe (Spain 6.0% prevalence), request EP serology results and consider quarantine protocols
- •Military and working horses in Portugal carry a significant burden of T. equi infection (>1 in 3 horses); implement tick control and biosecurity protocols appropriate to your region
- •Theileria equi can have two different genotypes, which may affect treatment responses and international movement restrictions—know the genotype status of horses being exported or moved between countries
- •Apparently healthy horses can be infected; use serological or molecular screening before moving horses between regions or internationally to prevent disease spread
- •Theileria equi infection triggers a cascade of oxidative stress markers that compromise cellular function; veterinarians should monitor for this parasite in horses showing signs of haemolysis or general malaise
- •The marked increase in free haemoglobin and oxidative damage suggests T. equi-infected horses may benefit from supportive antioxidant therapy and close monitoring of liver function
- •DNA damage detected in infected cells indicates potential for long-term health consequences; prevention through tick control and early detection via PCR screening is preferable to managing chronic infection
- •Donkeys in Southern Italy represent a significant reservoir for equine piroplasmosis; practitioners should consider serological screening before introducing donkeys to horse populations
- •Co-grazing with horses increases transmission risk for B. caballi; separate management of donkeys and horses may reduce infection spread
- •Although most infected donkeys are asymptomatic, clinical disease can occur; monitor donkeys with poor body condition or high antibody titres for acute piroplasmosis signs
- •Implement rigorous tick control programs in Indigenous equine communities, particularly targeting Dermacentor nitens, which carries both major equine piroplasmosis agents
- •Screen horses showing clinical signs of piroplasmosis and test tick vectors to assess local disease burden; consider tick surveillance as part of routine herd health monitoring
- •Work with community stakeholders to establish integrated vector management and disease surveillance systems as part of One-Health initiatives in endemic regions
- •Nested PCR offers superior sensitivity for detecting both T. equi and B. caballi but requires gel electrophoresis for differentiation; duplex qPCR is faster and directly distinguishes parasites but may miss low-level infections
- •Choice of diagnostic method should depend on laboratory capability and clinical priority: prioritize sensitivity (nPCR) for screening or specificity/speed (duplex qPCR) when infection type must be identified quickly
- •Mixed infections with both parasites occur in field populations (5.56% prevalence here), so diagnostic methods capable of detecting both organisms are essential for comprehensive screening programs
- •Donkeys in West Africa carry significantly higher piroplasm infection rates than horses and may serve as disease reservoirs—implement targeted screening and tick control for donkey populations
- •The presence of multiple T. equi genotypes suggests diverse parasite strains circulating in the region; consider this genetic diversity when selecting diagnostic tests and treatment protocols
- •Nigeria's role as an animal transport hub increases risk of EP dissemination across West Africa—veterinarians should maintain high clinical suspicion and implement biosecurity measures for imported equids
- •Consider T. equi infection in differential diagnosis for horses presenting with bilateral hyphema, even though this is an atypical manifestation
- •Anemia and thrombocytopenia in young foals with fever and lethargy warrant screening for tick-borne hemoparasites via blood smear examination
- •Prompt diagnosis and appropriate therapy can achieve complete clinical and laboratory resolution even in cases with ocular involvement
- •Donkeys respond differently to piroplasm infections than horses—do not assume equine clinical signs or disease progression will be identical in donkey patients
- •Treatment and management protocols developed for equine piroplasmosis may not be directly applicable to donkeys; species-specific approaches are needed
- •Clinicians should be alert to atypical presentations in donkeys with tick-borne parasitic infections and seek donkey-specific diagnostic and therapeutic guidance
- •Theileria equi establishes infection through innate immune mechanisms independent of adaptive immunity, meaning vaccination strategies must focus on non-lymphocyte-dependent pathways or early parasite stages
- •The intraleukocyte schizont stage can persist in multiple white blood cell types, complicating diagnostic and therapeutic targeting strategies
- •Understanding T. equi tropism to macrophages may inform treatment approaches targeting the intracellular parasitic stage
- •This novel lateral flow assay could enable rapid point-of-care diagnosis of equine piroplasmosis without requiring specialized laboratory equipment, improving early detection on farms and in field settings
- •The simplified testing protocol may reduce diagnostic delays and support faster treatment initiation, potentially improving outcomes in Theileria equi-infected horses
- •Field-deployable diagnostics of this type could improve disease surveillance and control in equine populations, particularly in resource-limited settings
- •Over half of horses in northern Italy show serological evidence of tick-borne pathogen exposure; implement robust tick control programs year-round
- •Multiple pathogen co-infections occur in approximately 12% of cases, complicating diagnosis and treatment decisions—serological testing for all three agents is warranted
- •Geographic location (elevation) influences infection risk; tailor monitoring and prevention strategies based on local geography and tick ecology
- •Equine practitioners in Spain should be aware that piroplasmosis remains a significant tick-borne disease threat affecting horse health and industry economics
- •Regional variation in EP prevalence suggests risk assessment and prevention strategies should be tailored to local epidemiological patterns
- •Serological and molecular screening may be warranted for horses with clinical signs consistent with piroplasmosis or in endemic regions
- •Theileria equi infection may present with clinical signs (anaemia, depression, poor performance) but acute phase proteins often remain within reference limits, so negative APP results should not rule out theileriosis
- •Serum iron concentration measurement may be a useful supplementary diagnostic indicator when T. equi infection is suspected
- •Thrombocytopoenia occurred in a significant proportion of stabled stallions (5/7) and should prompt consideration of T. equi testing in at-risk populations
- •This research identifies novel vaccine targets that could eventually reduce transmission of economically important parasitic diseases in cattle and horses
- •The species-specific and stage-specific expression patterns of CCp proteins suggest that any future vaccine approach would need to be tailored to the target parasite and life stage
- •Understanding parasite biology at the molecular level may lead to new control strategies beyond current tick management and treatment approaches
- •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
- •Imidocarb dipropionate offers a practical chemotherapeutic option for eliminating T. equi from persistently infected horses and removing transmission risk, facilitating safe international movement and trade.
- •Nested PCR is the appropriate diagnostic method to confirm treatment success rather than relying on antibody serology, which remains positive even after pathogen elimination.
- •A small percentage of horses may require retreatment; if PCR remains positive after initial treatment, a second course should be considered rather than assuming treatment failure.
Key Research Findings
Equine piroplasmosis is caused by three intraerythrocytic apicomplexan parasites (T. equi, B. caballi, T. haneyi) transmitted by multiple tick species and through iatrogenic/vertical routes
Imidocarb dipropionate (ID) has significant side effects and recurrent treatment failures, while buparvaquone (BPQ) is less effective against T. equi and unavailable in some countries
Current diagnostic limitations include lack of standardized PCR tests and serological assay limitations, creating risk of exporting infected animals
Combination of standardized PCR-based techniques and improved serological tests are proposed to enhance diagnostic accuracy and trade safety
Phylogenetic analysis of 736 T. equi sequences identified four distinct genotypes (A, B, C, D) with high bootstrap support, consolidating previous inconsistent nomenclature systems.
Genotype B was the most dominant (52.85%), followed by genotype A (27.58%), with genotypes C and D each representing 9.78% of sequences.
Genotype C showed the highest genetic diversity (91.0-100% identity), while genotypes B and D showed the lowest (95.7-100% identity).
Genotypes A and C had wide geographical distribution across 31 and 13 countries respectively, while genotypes B and D showed limited distribution restricted to Asian, African, and European continents.
T. equi detected in 7.4% (23/311) of grazing horses in Kyrgyzstan; B. caballi, hemotropic mycoplasmas, and Anaplasma species not detected
T. equi infection rate higher in males (8.11%) versus females (6.35%) and in horses >5 years (9.02%) versus 1-4 years (6.35%), though differences not statistically significant
Phylogenetic analysis identified A and E genotypes of T. equi circulating in Kyrgyzstan horse populations
First molecular investigation of A. capra in horses in Kyrgyzstan; pathogen not detected despite wide host spectrum in region
Theileria equi and/or Babesia caballi PCR positivity was detected in Italian Standardbred racehorses presented as clinically healthy
Haematological and blood chemistry parameters differed between PCR-positive and PCR-negative horses
Chronic equine piroplasmosis infection was associated with altered muscle mass scores in positive horses
Evidence Base
New insights in the diagnosis and treatment of equine piroplasmosis: pitfalls, idiosyncrasies, and myths.
Mendoza Francisco J, Pérez-Écija Alejandro, Kappmeyer Lowell S et al. (2024) — Frontiers in veterinary science
Nehra Anil Kumar, Kumari Ansu, Moudgil Aman Dev et al. (2024) — Frontiers in veterinary science
Altay Kursat, Erol Ufuk, Sahin Omer Faruk et al. (2024) — Frontiers in veterinary science
A cross-sectional study on performance evaluation in Italian standardbred horses' real-time PCR-positive for Theileria equi.
Coluccia Pierpaolo, Gizzarelli Manuela, Scicluna Maria Teresa et al. (2024) — BMC veterinary research
Comparison of Seroprevalence and Identification of Risk Factors for Theileria equi in Horses From Vector-Free and Infested Areas in Southern Brazil.
Pereira Marco Rocha, Trein Cristina Rodrigues, Webster Anelise et al. (2023) — Journal of equine veterinary science
Immune and Inflammatory Response of Donkeys (Equus asinus) Tested Positive to Tick-borne Pathogens.
Cocco Raffaella, Rizzo Maria, Carta Carlo et al. (2023) — Journal of equine veterinary science
Serological and molecular detection of Babesia caballi and Theileria equi in Mexico: A prospective study.
Salinas-Estrella Elizabeth, Ueti Massaro W, Lobanov Vladislav A et al. (2022) — PloS one
Low seroprevalence of equine piroplasmosis in horses exported from the Netherlands between 2015 and 2021.
Graham Heather, van Kalsbeek Paul, van der Goot Jeanet et al. (2022) — Frontiers in veterinary science
Survey of Zoonotic and Non-zoonotic Vector-Borne Pathogens in Military Horses in Lisbon, Portugal.
Fuehrer Hans-Peter, Alho Ana Margarida, Kayikci Feodora Natalie et al. (2020) — Frontiers in veterinary science
Oxidative stress and DNA damage in horses naturally infected with Theileria equi.
Radakovic M, Davitkov D, Borozan S et al. (2016) — Veterinary journal (London, England : 1997)
Seroprevalence and risk factors associated with Babesia caballi and Theileria equi infections in donkeys from Southern Italy.
Piantedosi D, D'Alessio N, Di Loria A et al. (2014) — Veterinary journal (London, England : 1997)
Babesia caballi and Theileria equi in ticks from horses in four Indigenous communities of Costa Rica.
Posada-Guzmán M F, Jiménez-Rocha A E, Sánchez-Bermúdez J F et al. (2026) — Journal of equine veterinary science
Lv Kunying, Zhang Yiwei, Yang Yixin et al. (2022) — Frontiers in veterinary science
Genetic Characterization of Piroplasms in Donkeys and Horses from Nigeria.
Sunday Idoko Idoko, Tirosh-Levy Sharon, Leszkowicz Mazuz Monica et al. (2020) — Animals : an open access journal from MDPI
First Report of Acute Bilateral Hyphema in a Theileria equi-Infected Kathiawari Horse.
Prasad Amit, Kumar Vineet, Kumar Binod (2019) — Journal of equine veterinary science
Clinical investigation on Theileria equi and Babesia caballi infections in Italian donkeys.
Laus Fulvio, Spaterna Andrea, Faillace Vanessa et al. (2015) — BMC veterinary research
Lymphocytes and macrophages are infected by Theileria equi, but T cells and B cells are not required to establish infection in vivo.
Ramsay Joshua D, Ueti Massaro W, Johnson Wendell C et al. (2013) — PloS one
Rapid and sensitive detection of Theileria equi using a novel integrated RPACRISPR/Cas13a lateral flow assay.
Alsultan Amjed, Karim Salah Mahdi, Al-Saadi Mohammed et al. (2025) — Journal of equine veterinary science
Seroprevalence of Tick-Borne Infections in Horses from Northern Italy.
Villa Luca, Gazzonis Alessia Libera, Allievi Carolina et al. (2022) — Animals : an open access journal from MDPI
Sero-molecular survey and risk factors of equine piroplasmosis in horses in Spain.
Camino Eliazar, Buendia Aranzazu, Dorrego Abel et al. (2021) — Equine veterinary journal
Show 4 more references
Acute phase proteins in Andalusian horses infected with Theileria equi.
Rodríguez Rocío, Cerón José J, Riber Cristina et al. (2014) — Veterinary journal (London, England : 1997)
Differential expression of three members of the multidomain adhesion CCp family in Babesia bigemina, Babesia bovis and Theileria equi.
Bastos Reginaldo G, Suarez Carlos E, Laughery Jacob M et al. (2013) — PloS one
Review of equine piroplasmosis.
Wise L N, Kappmeyer L S, Mealey R H et al. (2013) — Journal of veterinary internal medicine
Re-emergence of the apicomplexan Theileria equi in the United States: elimination of persistent infection and transmission risk.
Ueti Massaro W, Mealey Robert H, Kappmeyer Lowell S et al. (2012) — PloS one