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A 5-year-old, 520 kg, working Quarter Horse gelding was examined in mid-Spring for acute onset of pyrexia, tachycardia, and stiff gait. The referring veterinarian recorded a fever of 39.8 °C (103.8°F) and a heart rate of 60 bpm, and the horse appeared reluctant to move forward. A tentative diagnosis of equine granulocytic anaplasmosis (EGA) was made based on the clinical signs and the location of the horse within an area known to be endemic for the disease. The gelding was treated with oxytetracycline (IV) and flunixin meglumine (IV), but there was no improvement in the horse's condition in the 24 hours after treatment. The gelding was referred for further investigation. The horse had moved within northern California and Nevada several times within the preceding months to work cattle. Exercise intolerance had not been recorded, and the gelding had not displayed similar clinical signs before the acute onset of this episode. The horse had not undertaken strenuous exercise in the week before presentation, and no other horses in its environment were affected. The gelding was fed grass and alfalfa hay, with no pasture grazing and no concentrate supplementation.

On examination, the gelding was mildly obtunded and moved slowly and deliberately from the trailer. Physical examination revealed tachycardia (68 bpm) and tachypnea (24 bpm). Rectal temperature was within normal limits. No cardiac arrhythmias were ausculted. Moderate episcleral injection and icterus was evident in the oral, conjunctival, and nicitans mucus membranes, and capillary refill time was within normal limits. The gluteal and epaxial musculature were firm, painful, and nondepressible on palpation.

A complete blood count (CBC) revealed lymphopenia (934 μL−1; reference range 1,600–5,800 μL−1) and thrombocytopenia (44,000 μL−1; reference range 100,000–225,000 μL−1). Serum biochemical analysis identified azotemia (serum creatinine concentration 3.0 mg/dL; reference range 0.9–2.0 mg/dL) and a serum urea nitrogen (SUN) concentration (64 mg/dL; reference range 12–27 mg/dL), hyperglycemia (217 mg/dL; reference range 50–107 mg/dL), hyperfibrinogenemia (1,000 mg/dL; reference range 100–400 mg/dL), increased serum creatine kinase (CK) activity (101,077 IU/L; reference range 86–285 IU/L), aspartate aminotransferase (AST) (20,705 IU/L; reference range 168–494 IU/L), sorbitol dehydrogenase (SDH) (10 mg/dL; reference range 0–8 mg/dL) activities, and total bilirubin concentration (3.1 mg/dL; reference range 0.5–2.3 mg/dL). Analysis of the buffy coat identified numerous small smooth blue inclusions within many of the neutrophils. Most of the inclusions had a clear cytoplasmic vacuole around them. These findings were consistent with those observed after treatment of Anaplasma phagocytophilum infection with pyknotic, condensed organisms within the neutrophils. Blood was submitted for polymerase chain reaction (PCR) analysis for A. phagocytophilum, and blood samples were submitted for electron microscopy in an effort to better characterize the cytoplasmic inclusions. A coagulation profile was within normal limits, other than reduced antithrombin III (57%; reference range 80–120%) activity. Rectal examination and abdominal ultrasound identified no abnormalities.

The gelding was administered a 10-L bolus of polyionic fluids (Plasmalyte 148 solution, IV) followed by a 2 L/h constant rate IV infusion. Urinalysis obtained approximately 4 hours after the start of fluid therapy was dark red, with a specific gravity of 1.026 and a pH of 9.0. The urine was positive for myoglobin and protein. Both hyaline and granular casts were present in low numbers, and urinary white and red blood cell numbers were within normal limits. Treatment for the suspected A. phagocytophilum infection was delayed initially because of the potential nephrotoxic effects of oxytetracycline in a horse with myoglobinuria and impaired renal function.

Twelve hours after initiation of fluid therapy, serum creatinine and SUN concentrations were reduced to 2.2 and 59 mg/dL, respectively. The horse received oxytetracycline (6 mg/kg IV diluted in 1 L 0.9% saline, administered over 15 minutes, q24h) and butorphanol tartrate (0.02 mg/kg IM q4h) in an attempt to reduce the discomfort associated with rhabdomyolysis. Cold water hosing of the affected muscle groups was also undertaken. Biopsy of the gluteal, semimembranosus, and sacrocaudalis dorsalis lateralis (SCDL) muscles was performed to characterize the source of the increased serum CK activity, and muscle tissue obtained was frozen when still fresh. Serum for measurement of troponin I concentrations was submitted to identify myocardial damage that may have contributed to the increased serum CK activity, which was suspected because of sustained resting tachycardia (56–68 bpm during the first 24 hours of treatment).

The gelding's condition deteriorated after 24 hours. The horse appeared more painful and reluctant to move, and partial anorexia was noted. A CBC indicated a mild anemia (27%; reference range 30–46%), but was otherwise unremarkable. Serum biochemistry identified increased serum CK activity (387,970 mg/dL), AST (17,116 IU/L), SDH (30 IU/L), and SUN (40 mg/dL), hyperglycemia (120 mg/dL), and hyperfibrinogenemia (600 mg/dL). Total protein was reduced (5.1 g/dL; reference range 5.8–7.7 g/dL) because of hypoalbuminemia (2.1 g/dL; reference range 2.7–4.2 g/dL). Given the possibility of a toxic rhabdomyolysis and resultant oxidative muscle damage, the gelding received supplementation with vitamin E (10,000 IU PO q24h), vitamin C (1 g/L polyionic fluid, IV), and dimethyl sulfoxide (1 g/kg IV q24h).

PCR results confirmed the diagnosis of EGA. The serum was negative for equine infectious anemia and equine viral arteritis. The troponin I concentration was 0.17 ng/mL (reference range 0.00–0.06 ng/mL). PCR analysis of the muscle tissue samples was positive for A. phagocytophilum, and histopathologic analysis of the biopsies indicated no evidence of inflammatory infiltrates, vasculitis, or streptococcal protein A-associated immunoglobulin G. Occasional subsarcolemmal and central vacuoles were identified in the SCDL sample in which type I myofibers predominated. Mild myodegeneration (Fig 1) and a diffuse increase in lipid accumulation (Fig 2) were identified in all 3 muscle groups sampled. There was no evidence of abnormal polysaccharide accumulation or amylase-resistant inclusions. These findings were consistent with myodegeneration and suggestive of oxidative stress of undetermined cause.

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Figure 1.  Gluteus medius muscle on hematoxylin and eosin at × 10. A, normal muscle fibers; B, degenerated muscle fibers; H, hemorrhagic area between normal and degenerated muscle fibers.

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Figure 2.  (a) Gluteus medius muscle on oil red O at × 20. Note lipid droplets (red inclusions) accumulated in perymisial (black arrows), endomysial (hollow arrows), and intrasarcoplasmic (*) areas in both fiber types, more prominent in type I myofibers (identified as type I myofibers on ATPase reactions at various pre-incubation pH [9.8, 4.6, and 4.3] in previous sections). (b) Gluteus medius muscle on oil red O at × 20 from an asymptomatic horse.

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From the 3rd day of hospitalization, the gelding began to show clinical improvement in that it was more comfortable at the walk and the epaxial and gluteal musculature were less firm on palpation. The heart rate was lower. Administration of butorphanol was discontinued, but antioxidant, fluid, and oxytetracycline therapy was continued. Serum CK activity, AST, and SDH on the 4th day of hospitalization were reduced from previous levels (75,633 and 20,043 mg/dL and 15 IU/L, respectively). All other values were within normal limits. The horse was discharged from the hospital 5 days after admission after discontinuation of fluid therapy. Clinical signs had resolved. The horse received doxycycline (10 mg/kg PO q12h) for 5 days after discharge. The gelding returned to full work 14 days after discharge, and the owner confirmed that the horse had no further signs of rhabdomyolysis in the next 6 months.

Infection with A. phagocytophilum can result in a wide variety of clinical signs, including fever, petechial hemorrhage, edema, stiff gait, anorexia, icterus, orchitis, and recumbency.1–3 One recent report documented sudden death in a horse associated with experimental infection with the organism.4 Apparent myalgia has been reported in cases of human granulocytic anaplasmosis,5 and this is suspected to be the cause of the stiff gait that appears to be a common feature of the disease in horses. Increases in serum CK activity and AST have been reported previously in the horse associated with A. phagocytophilum infection,6 although the increases were mild (1,325 and 838 IU/L, respectively) and associated with abnormal muscle glycogen accumulation. Acute rhabdomyolysis has sporadically been associated with infection with both A. phagocytophilum and Ehrlichia chafeensis in humans.7–9

Moderate to marked increases in serum CK activity occur with both exertional and nonexertional rhabdomyolysis. One commonly identified cause of rhabdomyolysis in Quarter Horses is polysaccharide storage myopathy (PSSM).10 In the 3 muscles examined in this horse, there was no evidence of abnormal polysaccharide or amylase-resistant inclusions consistent with PSSM. Myonecrosis may occur as a result of infection with clostridial organisms, Streptococcus equi, viral diseases, and Sarcocystis species.11–14 A dietary deficiency of vitamin E or selenium may lead to peracute or subacute cardiac and skeletal muscle myodegeneration.15,16 Toxic causes of rhabdomyolysis include ionophore or gossypol ingestion; forage toxins include the Cassia species and tremetone-containing plants such as white snakeroot (Eupatorium rugosum), which may also induce rhabdomyolysis in horses.17 Atypical myopathy or atypical myoglobinuria is a severe seasonal pasture-associated myopathy that has been described in Europe; a similar condition has recently been documented in horses in the midwestern United States18 in which high serum CK activity (46,487–959,499 IU/L) and AST (>1,500 IU/L) were found in combination with generalized weakness, muscle fasiculations, recumbency, lethargy, and death. Postmortem examination of affected horses revealed acute severe myonecrosis and lipid accumulation within the diaphragm, neck, and intercostal muscles and muscles of the proximal thoracic and pelvic limbs. Atypical myopathy is suspected to be caused by a toxin on pastures that disrupts lipid metabolism. White snakeroot toxicity was unlikely because E. rugosum is not known to grow in Northern California or Nevada. Ionophore toxicity was considered unlikely given the lack of grain feeding and incompatible clinical history. Although rhabdomyolysis secondary to the use of oxytetracycline has not been reported in the horse, several cases of hypersensitivity to minocycline manifesting with myalgia and myonecrosis have been documented in the human literature.19,20 The exact mechanism by which A. phagocytophilum induces rhabdomyolysis is unknown. Interferon γ has been shown to be critical for the induction of histopathologic changes in mice, even in the absence of substantial bacterial load21 and despite its ability to avoid killing by innate immunity, A. phagocytophilum paradoxically induces some innate responses that have been documented to contribute to tissue injury.22

A considerable disparity was present between the ongoing myonecrosis suspected after physical examination and serum biochemical analysis and that identified after histopathologic examination of the biopsies of the muscle groups sampled in this horse. Although myocardial damage was present as documented by the increase in serum troponin I, the level of serum CK activity identified was considered too high to be the result of this moderate myocardial damage. It is therefore likely that the biopsies obtained were not representative of those muscle groups with the most severe myonecrosis. An increased number of samples or biopsy of deeper muscle groups may have been warranted in an effort to identify a focal or deep muscle necrosis had clinical resolution not occurred.

This horse demonstrates a temporal relationship between an acute infection with A. phagocytophilum (as demonstrated by a lymphopenia and thrombocytopenia, cytoplasmic neutrophilic inclusions, and positive blood PCR) and severe rhabdomyolysis. A positive PCR analysis of the muscle tissue would apparently suggest A. phagocytophilum as the potential source of the rhabdomyolysis; however, this result should be interpreted with caution given the inevitable blood contamination within these samples. Given the failure to identify another reasonable pathogenesis for the clinical condition, the authors presumed that the rhabdomyolysis occurred either directly or indirectly as a result of EGA and that infection with A. phagocytophilum should be considered as differential diagnosis for acute severe rhabdomyolysis and myalgia in the horse.

Acknowledgments

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The authors thank Drs Dori Borjesson and Keith De Jong for their help in this case.

References

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  2. Acknowledgments
  3. References