tEquine piroplasmosis is a hemoprotozoan tick-borne disease with worldwide distribution that is causedby Theileria equi and Babesia caballi. However, the geographical distribution of equine piroplasmosis inIran is unknown. The aim of the current study was to determine the causative agents and vector ticksof equine piroplasmosis in horses in the North Khorasan Province. In the year 2011, 100 horses wererandomly selected from 14 villages. Blood samples and ixodid ticks were collected and examined usingmicroscopical, molecular, and serological methods. Theileria equi infection was microscopically detectedin 5 (5%) of the blood smears with low parasitemia, while serum samples were tested by the indirectimmunofluorescent antibody test (IFAT). Antibodies against T. equi, B. caballi, and a mixed infection weredetected in 48 (48%), 2 (2%), and 3 (3%) of the serum samples, respectively. A multiplex PCR was used todetect T. equi and B. caballi DNA in blood samples. No B. caballi infections could be found, but Theileriaequi DNA was detected in 45 (45%) of the blood samples, and a BLAST analysis of the sequenced sam-ples indicated a 99% similarity with T. equi 18S rRNA gene sequences in GenBank. Both molecular andserological results did not identify any significant association between T. equi infection and risk factors. Acomparision of the results of 3 diagnostic methods demonstrated a poor agreement between microscopi-cal examination with IFAT and PCR and a moderate agreement between IFAT and PCR. Thirty-seven adultticks (20 females and 17 males) were collected from 15 horses. The most common tick was Hyalommamarginatum marginatum (n = 19), followed by Hyalomma anatolicum excavatum (n = 10), Rhipicephalusbursa (n = 4), Hyalomma marginatum turanicum (n = 3), and Hyalomma anatolicum anatolicum (n = 1). Thesalivary glands and ovaries were also examined using PCR. The genomic DNA samples of the salivaryglands of 3 ticks, H. a. excavatum (n = 2) and R. bursa (n = 1), had a positive reaction for T. equi, but no tickcontained B. caballi DNA. Thus, our results indicate that T. equi occurs more frequently than B. caballi inthe investigated geographical region

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