On June 1, 2009, a 7-year-old Quarter Horse gelding that was being used for unsanctioned horse racing was examined at Kansas State University Veterinary Medical Teaching Hospital because of fever, anorexia, and lethargy of 48 hours' duration. The horse had raced 8 days previously and had received an unknown amount of dexamethasone and furosemide the day prior to the initial examination. Communication with this owner was difficult, and a complete history was likely not obtained.
On day 1 (the day of hospital admission), physical examination revealed fever (39.7°C [103.5°F]), tachycardia (76 beats/min), and tachypnea (72 breaths/min). Mucous membranes were mildly hyperemic, and gastrointestinal tract sounds were increased in frequency. Results of rectal examination were unremarkable, and nonmalodorous, soft feces were present in the rectum. A CBC revealed mild anemia (PCV, 26%; reference interval, 31% to 47%), monocytosis (1,800 cells/μL; reference interval, 100 to 1, 000 cells/μL), thrombocytopenia (64,000 platelets/μL; reference interval, 100,000 to 350,000 platelets/μL), and hyperfibrinogenemia (700 mg/dL; reference interval, 100 to 400 mg/dL). Serum biochemical analyses revealed azotemia (creatinine concentration, 1.9 mg/dL [reference interval, 0.8 to 1.8 mg/dL]; BUN concentration, 28 mg/dL [reference interval, 9 to 22 mg/dL]); hyperbilirubinemia (8.0 mg/dL; reference interval, 0.5 to 2.7 mg/dL) with an increased indirect bilirubin concentration (7.7 mg/dL; reference interval, 0.3 to 2.6 mg/dL), and sodium, chloride, and potassium concentrations that were slightly lower than the lower limits of the reference intervals. Urinalysis revealed highly concentrated urine (urine specific gravity, 1.037) and 1+ blood and 2+ protein reactions on the assay reagent strip. Results of thoracic and abdominal ultrasonographic examinations were unremarkable. The primary differential diagnosis at the time of the initial examination was colitis; this differential diagnosis was determined on the basis of fever, soft feces, anorexia, dehydration, serum electrolyte abnormalities, and hyperemic mucous membranes. The horse was moved to an isolation stall. Treatments included lactated Ringer's solution (120 mL/kg/d [55 mL/lb/d], IV), potassium penicillin (20,000 U/kg [9,091 U/lb], IV, q 6 h), gentamicin (6.6 mg/kg [3.0 mg/lb], IV, q 24 h), flunixin meglumine (1.1 mg/kg [0.5 mg/lb], IV q 12 h), and omeprazole (1 mg/kg [0.45 mg/lb], PO, q 24 h).
A CBC and serum biochemical analyses were repeated on day 2, results of which revealed increased severity of anemia (PCV, 15%), a plasma protein concentration within the reference interval, and mild thrombocytopenia (91,000 platelets/μL); azotemia and serum electrolyte abnormalities had resolved. Results of an ELISA for detection of equine infectious anemia were negative. Results of a direct immunofluorescence assay were positive for erythrocyte membrane-bound IgM (percentage of erythrocytes with positive results, 16%) and IgG (6%); results were negative for IgA (< 1%; reference intervals for IgG, IgM, and IgA, < 3%). Because the horse was anemic and had a history of participation in unsanctioned Quarter Horse racing, Wright-Giemsa staining and microscopic examination of a blood smear were requested; results indicated rare ringed piroplasms (diameter, 0.5 to 1 μm) in erythrocytes, which were considered consistent with Babesia spp (Figure 1). Erythrocyte autoagglutination was detected during preparation of the blood smear and confirmed with a 1:10 saline (0.9% NaCl) solution dispersion test. A tentative diagnosis of piroplasmosis and IMHA was made. The office of the Kansas State Veterinarian was contacted, and the federal Area Veterinarian-in-Charge sent a foreign animal disease diagnostician to collect a blood sample for submission to the National Veterinary Services Laboratories, Ames, Iowa, for further testing. No ticks were found on the horse during a thorough examination. The horse was photographed for identification purposes.
Antimicrobial treatments were changed before results of further tests were available. Penicillin and gentamicin were discontinued, and oxytetracycline was administered (6.6 mg/kg, IV, q 12 h). The rate of IV administration of fluids was decreased (60 mL/kg/d [27 mL/lb/d]) for 12 hours, then administration was discontinued. Administration of flunixin meglumine and omeprazole was continued at the same doses that were used initially. The horse remained lethargic, tachycardic, and febrile during the next 48 hours, despite treatments. Therefore, on day 3, imidocarb dipropionate (2.2 mg/kg [1 mg/lb], IM, q 24 h for 2 days) was administered on the basis of the presumptive diagnosis of piroplasmosis and the need to provide supportive care during the regulatory investigation. Modest improvements in clinical signs and laboratory test results were detected during the subsequent 48 hours. Fever and anorexia resolved, and the heart rate of the horse decreased to 56 beats/min. Results of a CBC indicated a PCV of 22% and a platelet count (161,000 platelets/μL) within the reference interval. Intraerythrocytic piroplasms were not detected during examination of a blood smear. Local signs of pain and edema developed at the imidocarb injection site within 24 hours after administration of the first dose. No other signs of imidocarb toxicosis were detected.
The results of tests performed at the National Veterinary Services Laboratories were available on day 5. Results of a CFT were positive for Babesia equi (at a 1:5 dilution on a microtiter plate) and negative for Babesia caballi. However, results of a CFT performed via titration in tubes at an endpoint dilution of 1:5 were negative. Results of a competitive ELISA and an IFA were negative for both B equi and B caballi. Results of realtime and conventional nested PCR assays were positive for B equi and negative for B caballi. Examination of a blood smear revealed intraerythrocytic structures consistent with Babesia spp. Results of assays were interpreted by personnel at the National Veterinary Services Laboratories and indicated that the test results were consistent with acute infection of the horse with B equi.
Because of the lack of concordance in results of the 2 CFTs, serologic testing was repeated on day 8. A blood sample was collected from the horse by the foreign animal disease diagnostician, and results of repeated analysis were available on day 10. Results of a CFT and an IFA were positive for B equi at dilutions of 1:10 and 1:80, respectively. Results of PCR (realtime and conventional nested) assays were positive for B equi. Results of a competitive ELISA and examination of a blood smear were negative for B equi and B caballi. A blood sample was resubmitted on day 11 for performance of a direct immunofluorescence assay to detect erythrocyte surface antibodies; results were negative. On day 12, findings of a physical examination were unremarkable (other than imidocarb injection–site reactions) and the PCV had increased to 41%. Previously administered medications were discontinued, and doxycycline (10 mg/kg [4.5 mg/lb], PO, q 12 h) and phenylbutazone (2 mg/kg [0.9 mg/lb], PO, q 24 h) were administered to the horse for 7 days (to reduce inflammation at imidocarb injection sites).
After confirmation of a diagnosis of B equi infection in the index horse of the present report, Missouri state and federal animal health personnel instituted a quarantine for the home premises of the index horse (index premises) located in Jackson County, Mo, on June 6, 2009. The index premises, where the index horse had been housed since January 2009, comprised a boarding stable that housed 63 other equids (59 horses, 3 ponies, and 1 mule). All equids at the facility were tested for piroplasmosis by personnel at the National Veterinary Services Laboratories. Results reported for blood samples collected on June 9 and 12, 2009, indicated an additional 6 horses on the premises had positive results for B equi. On June 9, 5 of these horses had positive results for a competitive ELISA, 3 had positive results for a CFT, 5 had positive results for an IFA, and 4 had positive results for a PCR assay. On June 12, 5 of these horses had positive results for a competitive ELISA, 2 had positive results for a CFT, 6 had positive results for an IFA, and 2 had positive results for a PCR assay. All 6 of these horses with positive results for B equi were Quarter Horses participating in unsanctioned or bush track racing. The other 57 equids at the facility were tested multiple times for detection of B equi and B caballi infection during a 30-day surveillance period; results for these horses were negative.
Fourteen other horses had recently been moved from the index premises in Missouri prior to detection of piroplasmosis in the index horse; these horses were located and tested to detect B equi and B caballi infection. Thirteen of these horses had negative results for piroplasmosis, and one of these horses (a Quarter Horse participating in unsanctioned racing in Kansas) had positive results for B equi infection via competitive ELISA, IFA, and PCR assay but had negative results for a CFT. Thus, 8 Quarter Horses with a history of participation in unsanctioned racing under the management of the same trainer were identified that had positive results for B equi infection.
A complete examination of all equids on the index premises was performed for detection of ticks as a part of the regulatory response. No ticks were found on any of the horses. Additionally, a complete tick survey of the index premises was conducted from June 23 to 29, 2009, by personnel of the Southeast Cooperative Wildlife Disease Study, which included tick dragging and wildlife capture techniques. During this 7-day period, only 4 ticks were collected from the index premises, none of which were identified as competent tick vectors for the organisms that cause piroplasmosis in equids.
The index horse was discharged from the Kansas State University Veterinary Medical Teaching Hospital on day 12 under state and federal regulatory supervision. The federal Area Veterinarian-in-Charge and the Kansas state livestock commissioner were present when the owner came to the hospital for discharge of the horse. The horse was loaded on a trailer, the trailer was officially sealed, and the Kansas livestock commissioner followed the trailer back to the farm of origin in Missouri, which was already under quarantine. The 7 horses on the index premises in Missouri that had positive results for B equi infection (including the index horse) were housed together in a group of stalls away from the other horses on the premises. These horses received radiofrequency identification implants, and their stalls were padlocked. The affected horses, their stalls, and the interior and perimeter of the barn where they were housed were sprayed with an acaricide.
After in-depth discussions with the owners and trainer of the horses with piroplasmosis, the decision was made on June 17 that the infected horses would be euthanized under the supervision of Missouri state and federal animal health personnel the next day. When Missouri state and federal officials arrived on June 18 to euthanize the horses, they found that 2 of the horses with piroplasmosis had been illegally removed from the premises. Removal of horses had entailed cutting of paddocks on stalls and the gate of the premises during the previous night. The 2 missing horses belonged to 1 owner who was not present at the facility with the other owners on the scheduled euthanasia day. The 5 horses with piroplasmosis that remained on the premises were euthanized on June 18 (necropsies were not performed).
Kansas state and federal officials were unable to locate the horse with B equi infection that had been participating in unsanctioned racing in that state and were unable to locate the owner of that horse. Law enforcement officials were enlisted to assist in locating the 3 missing horses with positive results for B equi infection, but after months of investigation, the missing horses were not found. During the investigation, it was suggested that all 3 of the missing horses were likely moved illegally to Mexico within a few days after they were reported missing, but no official record of the horses entering Mexico could be found. Quarantine of the index premises in Missouri was discontinued on July 9, 2009, after all requirements were met (including repeated testing of horses with negative results for B equi infection).
Complement fixation test
Indirect immunofluorescent antibody assay
Immune-mediated hemolytic anemia
Short M, Equine Programs Manager, Division of Animal Industry, Florida Department of Agriculture and Consumer Services, Tallahassee, Fla: Personal communication, 2011.
Knowles D, College of Veterinary Medicine, Washington State University, Pullman, Wash: Personal communication, 2011.
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