Equine piroplasmosis, much like the well-known tick-borne illness Lyme disease, is transmitted to horses via tick saliva. However, the protozoan parasites Theileria equi and Babesia caballi cause piroplasmosis, whereas bacteria called Borrelia burgdorferi are responsible for Lyme disease.

Equine piroplasmosis is endemic in South and Central America, the Caribbean, Africa, the Middle East, and Eastern and Southern Europe. This disease attacks erythrocytes (red blood cells) and can cause fever, anemia, and jaundice. In severe cases, it can be life-threatening. Researchers on a new retrospective study have revealed the prevalence of equine piroplasmosis in 26 European countries.

Equine Piroplasmosis: A Growing Concern Worldwide

Hard ticks (Ixodidae), such as the Dermacentor spp (dog tick), transmit T. equi and B. caballi. “In addition to infecting equids, T. equi has also been found in other ungulates (hoofed mammals),” said Carla Axt, DVM, clinical laboratory veterinarian at Lakoklin Laboratory, in Bad Kissingen, Germany. “It is, therefore, conceivable that there are other reservoirs besides horses.”
While piroplasmosis typically occurs in tropical and subtropical regions, reports of the disease in nonendemic areas rise as the global movement of horses increases, including to and from the U.S. To better understand the prevalence and risk factors of piroplasmosis in horses, Axt led a team of German researchers conducting a retrospective study involving 4,060 horses across Europe.

Prevalence and Risk Factors for Equine Piroplasmosis

Axt’s team collected data from 2008 to 2021 using polymerase chain reaction (PCR) tests to find protozoan DNA in whole blood treated with ethylenediaminetetraacetic acid (EDTA, an anticoagulant agent). They also used competitive enzyme-linked immunosorbent assay (cELISA) tests on blood (serology) to detect the pathogens (disease-causing organisms).

The researchers collected PCR results for 1,589 of the horses, finding a 9.7% positivity rate for piroplasmosis. On serology they found 15.2% positivity for T. equi and 6.8% for B. caballi. They also noted that breed appeared to play a role, with heavy horses, such as draft breeds more likely to test positive for B. caballi and Warmbloods for T. equi, respectively.

The odds of positive serology results increased annually for both pathogens, indicating an increased incidence of the infection across the horses in the study. Each horse’s geographic region significantly influenced both PCR and serology results, with Southern Europe showing the highest odds of positive tests. Unsurprisingly, the study found that piroplasmosis prevalence was correlated with the distribution of tick populations.

Limitations of the Study

Axt noted two main pitfalls in the results drawn from her team’s research. Both centered around the lack of contextual information. “The main limitations of this study are linked to the retrospective study design with firstly missing data regarding the reason for testing—such as routine screening or acute onset of disease—as well as lack of information regarding the travel history of the horse and the clinical examination findings,” she said. Meaning the study’s conclusions are limited because it was based on past records, which didn’t always include important details like why the horses were being tested for piroplasmosis, or if any clinical abnormalities were noted during their physical exams.

Take-Home Message

The increasing prevalence of equine piroplasmosis underscores the need for continued disease surveillance and tick prevention, especially as it spreads to nonendemic areas globally. Horse owners should work with their veterinarians to determine the best ways to prevent and monitor tick-borne illnesses in their horses.