Tulathromycin and Diclazuril Lack Efficacy against Theileria haneyi, but Tulathromycin Is Not Associated with Adverse Clinical Effects in Six Treated Adult Horses

Abstract

Equine theileriosis, caused by Theileria haneyi and Theileria equi, leads to anemia, exercise intolerance, and occasionally, death. Theileriosis-free countries prohibit the importation of infected horses, resulting in significant costs for the equine industry. Imidocarb dipropionate is the only treatment for T. equi in the United States, but lacks efficacy against T. haneyi. The goal of this study was to assess the in vivo efficacy of tulathromycin and diclazuril against T. haneyi. Fourteen T. haneyi-infected horses were utilized. Six were treated with eight weekly 2.5 mg/kg doses of tulathromycin. Three were treated daily for eight weeks with 2.5 mg/kg diclazuril. Three were pre-treated with 0.5 mg/kg diclazuril daily for one month to determine whether low-dose diclazuril prevents infection. Following infection, the dose was increased to 2.5 mg/kg for eight weeks. Two infected horses remained untreated as controls. The horses were assessed via nested PCR, physical exams, complete blood counts, serum chemistry panels, and cytology. Tulathromycin and diclazuril failed to clear T. haneyi and the treated and control groups exhibited similar parasitemia and packed cell volume declines. To obtain additional safety data on tulathromycin use in adult horses, necropsy and histopathology were performed on tulathromycin-treated horses. No significant lesions were detected.

Transient efficacy of buparvaquone against Theileria haneyi in chronically infected horses

“…This nding is similar to what was observed in the treatment of T. equi infected horses with BPQ [11], suggesting that BPQ can be used in the management of acute T. haneyi infection. This suppression below the limits of detection further suggests that BPQ is more effective in the treatment of T. haneyi than imidocarb [6], tulathromycin and diclazuril [7]. Unlike imidocarb, tulathromycin and diclazuril that did not clear the parasite during infection, BPQ led to rapid abatement of the parasite that was maintained for a minimum of 7 weeks before recrudescence was observed.…”

“…Seven Welsh ponies (HO-270, HO-364, HO-776, HO-777, HO-784, HO-411 and HO-413) that had been used in previous studies were reused in this study [6,7].…”

“…HO-364, HO-776, HO-777 and HO-784 were infected intravenously using 2-4 ml of T. haneyi blood stabilate at 8.8 percent parasitized erythrocytes (PPE) [6]. HO-411 and HO-413 were infected with the T. haneyi blood stabilate as previously described [7]. At the time of this study, all the animals remained in a chronic phase of T. haneyi infection, consistently testing PCR positive for the parasite for at least 2 years, and yet exhibited no symptoms.…”

“…The IDEXX VetConnect PLUS software (IDEXX Laboratories) was used for visualization of data from the ProCyte DX™ and the Catalyst One Veterinary Blood Chemistry Analyzer. Evaluation of parasitemia was performed on blood smears stained with the PROTOCOL™ Hema 3™ stain set (Fisher Healthcare, Houston, TX, USA), and the percent of parasitized erythrocytes (PPE) was determined using the formula; (Total parasites in 5 elds)/(Erythrocyte count in ¼ of a eld × 20)) × 100, as previously described [7]. Pyrexia was assessed weekly by measurement of rectal temperature of the horses in degrees Fahrenheit (°F).…”

“…For instance, a study conducted to investigate the e cacy of imidocarb dipropionate on T. haneyi infection in horses showed that the drug failed to clear the infection, and the parasite interfered with the e cacy of the drug in horses co-infected with T. equi [6]. Additionally, a study of the effectiveness of tulathromycin and diclazuril on T. haneyi infected horses showed that both drugs were unable to clear the infection [7]. The lack of effective treatment poses a major challenge in the control of T. haneyi, and this is exacerbated by the lack of knowledge of tick species that transmit the parasite making it di cult to predict the parasite's epidemiology globally.…”