title: Theileria equi isolates vary in susceptibility to imidocarb dipropionate but demonstrate uniform in vitro susceptibility to a bumped kinase inhibitor.
abstract: BACKGROUND
  The apicomplexan hemoparasite Theileria equi is a causative agent of equine piroplasmosis, eradicated from the United States in 1988. However, recent outbreaks have sparked renewed interest in treatment options for infected horses. Imidocarb dipropionate is the current drug of choice, however variation in clinical response to therapy has been observed.
  
  
  METHODS
  We quantified the in vitro susceptibility of two T. equi isolates and a lab generated variant to both imidocarb dipropionate and a bumped kinase inhibitor compound 1294. We also evaluated the capacity of in vitro imidocarb dipropionate exposure to decrease susceptibility to that drug. The efficacy of imidocarb dipropionate for clearing infection in four T. equi infected ponies was also assessed.
  
  
  RESULTS
  We observed an almost four-fold difference in imidocarb dipropionate susceptibility between two distinct isolates of T. equi. Four ponies infected with the less susceptible USDA Florida strain failed to clear the parasite despite two rounds of treatment. Importantly, a further 15-fold decrease in susceptibility was produced in this strain by continuous in vitro imidocarb dipropionate exposure. Despite a demonstrated difference in imidocarb dipropionate susceptibility, there was no difference in the susceptibility of two T. equi isolates to bumped kinase inhibitor 1294.
  
  
  CONCLUSIONS
  The observed variation in imidocarb dipropionate susceptibility, further reduction in susceptibility caused by drug exposure in vitro, and failure to clear T. equi infection in vivo, raises concern for the emergence of drug resistance in clinical cases undergoing treatment. Bumped kinase inhibitors may be effective as alternative drugs for the treatment of resistant T. equi parasites.
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. siddra@vetmed.wsu.edu.
authors.forename: Siddra A
authors.initials: SA
authors.lastname: Hines
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. jdr105@vetmed.wsu.edu.
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Ramsay
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. lkapp@vetmed.wsu.edu.|Animal Disease Research Unit, Agricultural Research Service, USDA, Pullman, WA, 99164-6630, USA. lkapp@vetmed.wsu.edu.
authors.forename: Lowell S
authors.initials: LS
authors.lastname: Kappmeyer
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. laua@vetmed.wsu.edu.
authors.forename: Audrey Ot
authors.initials: AO
authors.lastname: Lau
authors.affiliation: Division of Allergy and Infectious Diseases and Center for Emerging and Re-emerging Infectious Diseases, School of Medicine, University of Washington, Seattle, WA, 98109-4766, USA. ojo67kk@u.washington.edu.
authors.forename: Kayode K
authors.initials: KK
authors.lastname: Ojo
authors.affiliation: Division of Allergy and Infectious Diseases and Center for Emerging and Re-emerging Infectious Diseases, School of Medicine, University of Washington, Seattle, WA, 98109-4766, USA. WVanVoorhis@medicine.washington.edu.
authors.forename: Wesley C
authors.initials: WC
authors.lastname: Van Voorhis
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. dknowles@vetmed.wsu.edu.|Animal Disease Research Unit, Agricultural Research Service, USDA, Pullman, WA, 99164-6630, USA. dknowles@vetmed.wsu.edu.
authors.forename: Donald P
authors.initials: DP
authors.lastname: Knowles
authors.affiliation: Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA, 99164-7040, USA. rhm@vetmed.wsu.edu.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Mealey
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000981:Antiprotozoal Agents / Q000627:therapeutic use*; D016000:Cluster Analysis; D004352:Drug Resistance, Microbial / Q000235:genetics*; D005434:Flow Cytometry; D051418:Focal Adhesion Kinase 2 / Q000037:antagonists & inhibitors; D006734:Horse Diseases / Q000188:drug therapy / Q000469:parasitology*; D006736:Horses; D007095:Imidocarb / Q000031:analogs & derivatives / Q000627:therapeutic use; D020128:Inhibitory Concentration 50; D008969:Molecular Sequence Data; D047428:Protein Kinase Inhibitors / Q000627:therapeutic use; D016415:Sequence Alignment; D013045:Species Specificity; D016794:Theileria / Q000235:genetics*; D013801:Theileriasis / Q000188:drug therapy / Q000453:epidemiology / Q000469:parasitology*; D014481:United States / Q000453:epidemiology
chemical_list: D000981:Antiprotozoal Agents; D047428:Protein Kinase Inhibitors; D007095:Imidocarb; D051418:Focal Adhesion Kinase 2; C031719:imidocarb dipropionate