Equine piroplasmosis spreading into more temperate regions – review

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Protozoa that cause piroplasmosis. Photo: Centers for Disease Control/ Steven Glenn (Public domain)

Equine piroplasmosis is spreading into more temperate regions previously considered free of the disease, according to researchers.

The infection, caused by the blood-dwelling parasites Theileria equi, Theileria haneyi, and Babesia caballi, is already prevalent in most parts of the world.

Cases affect animal performance and welfare, and the disease also has important economic impacts related to limitations in horse transport between endemic and non-endemic regions.

Sharon Tirosh-Levy and her fellow researchers, in a review published recently in the journal Pathogens, analyzed the epidemiological, serological, and molecular diagnostic data published in the last 20 years, including all DNA sequences submitted to the GenBank database, to describe the current global prevalence of the parasites behind the disease.

“We concluded that EP is endemic in most parts of the world, and is spreading further into more temperate climate zones previously considered parasite-free,” the review team concluded.

“The use of genotyping to monitor the spread of infection is important for better surveillance and control,” they said.

The life cycle of Theileria equi (TE) and Babesia caballi (BC) in the tick vector and in the equine host. RBC refers to equine red blood cells, WBC refers to equine white blood cells, and SG stands for tick salivary glands. Image: Tirosh-Levy et al. https://doi.org/10.3390/pathogens9110926

“In the future, whole genome sequencing of the different genotypes should be established to better understand differences in virulence and the clinical impact of different parasite strains.”

It is estimated that 90% of the global horse population resides in areas where the tick-borne disease is endemic.

Since no effective, commercially available vaccines against equine piroplasmosis are yet available, control is based on a combination of drug therapy, vector control, and restricted transport of infected horses.

The aims of treatment and control strategies differ between endemic and non-endemic regions.

In non-endemic areas, the aim is to keep the area disease-free. Thus, treatment of infected horses is aimed at a complete clearance of infection, while control is mainly based on monitoring and restricting the entrance of infected horses.

Several non-endemic countries, including the United States, Australia, and Japan deny entrance of seropositive horses, and either export, quarantine, or euthanize any positive animal within the country.

In endemic areas, treatment is usually aimed only to reduce clinical signs of acute infection, while strategic use of tick-killing agents is recommended to reduce, but not eliminate, exposure to ticks.

“Since different approaches for treatment and control should be implemented in endemic versus non-endemic areas, the fact that most parts of the world harbor these parasites should highlight the importance of distinguishing between susceptible and resistant equine subpopulations in order to reduce the clinical and economic impacts of this disease,” the review team said.

The review team comprised Tirosh-Levy, Yuval Gottlieb and Amir Steinman, all with The Hebrew University of Jerusalem; and Lindsay Fry and Donald Knowles, with Washington State University.

Tirosh-Levy, S.; Gottlieb, Y.; Fry, L.M.; Knowles, D.P.; Steinman, A. Twenty Years of Equine Piroplasmosis Research: Global Distribution, Molecular Diagnosis, and Phylogeny. Pathogens 2020, 9, 926.

The review, published under a Creative Commons License, can be read here.

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