Fungal osteomyelitis of the axial border of the proximal sesamoid bones in a horse

Kevin M. Sherman Myhre Equine Clinic, 100 Ten Rod Rd, PO Box 1673, Rochester, NH 03866.

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Grant D. Myhre Myhre Equine Clinic, 100 Ten Rod Rd, PO Box 1673, Rochester, NH 03866.

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Elizabeth I. Heymann Myhre Equine Clinic, 100 Ten Rod Rd, PO Box 1673, Rochester, NH 03866.

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Abstract

Case Description—A 12-year-old Standardbred gelding was referred for swelling of the right metacarpophalangeal joint.

Clinical Findings—Ultrasonography of the right metacarpal area revealed hypoechoic areas in the right digital sheath and metacarpophalangeal joint consistent with synovial effusion. Radiography of the right metacarpophalangeal joint revealed lysis of the axial border of the proximal sesamoid bones. Aspergillus fumigatus was detected on fungal culture of synovial fluid.

Treatment and Outcome—Regional limb perfusion (150 mg of amikacin in 60 mL of saline [0.9% NaCl] solution perfused for 30 minutes) was performed 2 and 4 days after admission. Itraconazole (5 mg/kg [2.27 mg/lb], PO, q 24 h) was administered for approximately 9 weeks. Joint lavage with amikacin (500 mg) in 1 L of saline solution was performed 4 times. Three months after discharge, the owner reported that the horse was mildly lame during trotting but was moving freely and comfortably during all gaits and had gained a considerable amount of weight. Because the osteoarthritis was not expected to improve and because it was recommended that the horse not return to purposeful exercise, the owner decided to retire the horse from racing.

Clinical Relevance—Various diagnostic imaging methods and fungal cultures are useful for diagnosing fungal osteomyelitis of the axial borders of the proximal sesamoid bones in horses. Fungal osteomyelitis of the sesamoid bones and erosive arthritis should be considered as a differential diagnosis for horses in which corticosteroids have been administered intra-articularly.

Abstract

Case Description—A 12-year-old Standardbred gelding was referred for swelling of the right metacarpophalangeal joint.

Clinical Findings—Ultrasonography of the right metacarpal area revealed hypoechoic areas in the right digital sheath and metacarpophalangeal joint consistent with synovial effusion. Radiography of the right metacarpophalangeal joint revealed lysis of the axial border of the proximal sesamoid bones. Aspergillus fumigatus was detected on fungal culture of synovial fluid.

Treatment and Outcome—Regional limb perfusion (150 mg of amikacin in 60 mL of saline [0.9% NaCl] solution perfused for 30 minutes) was performed 2 and 4 days after admission. Itraconazole (5 mg/kg [2.27 mg/lb], PO, q 24 h) was administered for approximately 9 weeks. Joint lavage with amikacin (500 mg) in 1 L of saline solution was performed 4 times. Three months after discharge, the owner reported that the horse was mildly lame during trotting but was moving freely and comfortably during all gaits and had gained a considerable amount of weight. Because the osteoarthritis was not expected to improve and because it was recommended that the horse not return to purposeful exercise, the owner decided to retire the horse from racing.

Clinical Relevance—Various diagnostic imaging methods and fungal cultures are useful for diagnosing fungal osteomyelitis of the axial borders of the proximal sesamoid bones in horses. Fungal osteomyelitis of the sesamoid bones and erosive arthritis should be considered as a differential diagnosis for horses in which corticosteroids have been administered intra-articularly.

A 12-year-old Standardbred gelding that was used for harness racing was referred for swelling of the right metacarpophalangeal joint. Two months earlier, a chronic lameness problem had been localized to that same joint. Signs of lameness resolved after intra-articular administration of cortisone (dose unknown). Six weeks before admission, the metacarpophalangeal joint became swollen and no growth was detected on bacterial culture of synovial fluid. Penicillin (dosage unknown) and gentamicin (dosage unknown) were administered, but the lameness did not improve. Bacterial culture of synovial fluid was repeated; results were negative. Administration of ceftiofur sodium (dosage unknown) and intra-articular administration of amikacin (dose unknown) did not result in improvement of the lameness.

On physical examination, severe lameness (scale, 4/5)1 of the right forelimb was detected. The lameness became worse (scale, 5/5)1 after flexion of the right metacarpophalangeal joint. A moderate amount of swelling was detected around the joint and digital sheath. Ultrasonography of the right metacarpal area revealed hypoechoic areas in the right digital sheath and metacarpophalangeal joint consistent with synovial effusion. Radiography of the right metacarpophalangeal joint revealed lysis of the axial border of the proximal sesamoid bones (Figure 1).

Figure 1—
Figure 1—

Dorsopalmar radiographic view of the right metacarpophalangeal joint in a 12-year-old Standardbred evaluated for swelling of the right metacarpophalangeal joint of 6 weeks' duration. Notice moderate soft tissue swelling medial and lateral to the joint space. The axial borders of the proximal sesamoid bones are irregular (arrowheads).

Citation: Journal of the American Veterinary Medical Association 229, 10; 10.2460/javma.229.10.1607

Arthrocentesis of the right metacarpophalangeal joint and tenocentesis of the digital sheath were performed after the skin was aseptically prepared. Synovial fluid was submitted for cytologic examination, bacterial culture, and antimicrobial susceptibility testing.

Synovial fluid from the right metacarpophalangeal joint was red, and the viscosity of the fluid was watery; the fluid contained WBCs (19.3 × 103 cells/μL), and the total solids concentration was 5.4 g/dL. Synovial fluid from the digital sheath was also red, and the viscosity of the fluid was watery; the fluid contained WBCs (18.1 × 103 cells/μL), and the total solids concentration was 5.4 g/dL. Cytologic examination of both samples revealed a large number of band and segmented neutrophils, occasional synovial cells, RBCs, and small lymphocytes suspended in a basophilic mucoproteineous background. Fibrin cellular clumps were detected in direct smear preparations. The cytologic diagnosis was chronic neutrophilic inflammatory synovial effusion. Rare numbers of bacteria were detected after Gram staining. No growth was detected on bacterial culture of synovial fluid; therefore, samples of synovial fluid were submitted for fungal culture.

The horse was treated with phenylbutazone (4.4 mg/kg [2.0 mg/lb], IV, q 12 h), and enrofloxacin (5.0 mg/kg [2.3 mg/mg], IV, q 24 h) for approximately 4 weeks, and omeprazole (2 mg/kg [0.91 mg/lb], PO, q 24 h) until it was discharged from the hospital (approx 12 weeks). A frog support pad was used on the left fore foot and maintained until discharge. Ice was applied topically to the distal portion of the right third metacarpal bone and the metacarpophangeal joint daily for 20 minutes; support wraps were applied to both forelimbs and reset each day until discharge. Diclofenac sodium cream (1%) was applied topically over the right metacarpophalangeal joint and digital sheath during bandage changes until discharge.

Regional limb perfusion (150 mg of amikacin in 60 mL of saline [0.9% NaCl] solution perfused for 30 minutes) via the right lateral digital palmar vein was performed 2 and 4 days after admission. Eleven days after admission, Aspergillus fumigatus was detected on fungal culture of synovial fluid obtained from the right metacarpophalangeal joint and the digital sheath and ketaconazole (30 mg/kg [13.6 mg/lb], PO, q 12 h) was added to the treatment regimen pending results of antifungal susceptibility testing. Flunixin meglumine (1 mg/kg [0.45 mg/lb], IV, q 24 h) was administered for 10 days. Morphine (0.3 mg/kg [0.14 mg/lb], SC, q 24 h) was administered for 21 days for pain management. Acepromazine (0.03 mg/kg [0.01 mg/lb], IM, q 24 h) was administered to prevent development of laminitis.

Nineteen days after the initial evaluation, the horse was non–weight bearing on the right forelimb. Results of antifungal susceptibility testing were received 6 days later; administration of ketoconazole was discontinued, and itraconazole (5 mg/kg, PO, q 24 h) was administered for approximately 9 weeks. Intravenous administration of phenylbutazone, flunixin meglumine, and enrofloxacin was discontinued; however, oral administration of those medications was continued using the same dosages.

Approximately 1 month after the initial evaluation, ultrasonography of the right metacarpophalangeal joint and digital sheath was performed. Findings were not notably changed, compared with the initial evaluation. A 10% povidone iodine solution (10 mL) in 1 L of saline solution was used to lavage the right metacarpophalangeal joint. Synovial fluid was obtained from the joint; the WBC count was 24.9 × 103 cells/μL. The following day, joint lavage was performed with 1 L of saline solution containing amikacin (500 mg). Synovial fluid was obtained from the joint; the WBC count was 15.3 × 103 cells/μL. Joint lavage with amikacin and 1 L of saline solution was performed on 3 additional occasions. After the last joint lavage, the WBC count in synovial fluid was 18.5 × 103 cells/μL. Approximately 6 weeks after initial evaluation, radiography was performed; notable changes in the radiograph lesions were not detected.

Approximately 9 weeks after the initial evaluation, ultrasonography of the right metacarpophalangeal joint revealed that the volume of synovial fluid in the tendon sheath of the deep digital flexor tendon and the joint space was mildly increased (Figure 2). In addition, dystrophic mineralization within the lateral branch of the suspensory ligament was also detected (Figure 3). Findings were compatible with synovial effusion and tenosynovitis. The attachment of the intersesamoid ligament on the axial border of the sesamoid bones was irregular, suggesting erosion of subchondral bone.

Figure 2—
Figure 2—

Transverse ultrasonographic image of the right metacarpophalangeal joint of the horse in Figure 1 at the level of the proximal sesamoid bones 2 months after the radiograph in Figure 1 had been obtained. An anechoic line (arrow) representative of fluid accumulation in the tendon sheath of the deep digital flexor tendon (DDF) is evident. Subjectively, the intersesamoidean ligament (asterisk) is misshapen. The large acoustic shadow (Med) represents the area of the medial proximal sesamoid bone. The axial border of the lateral proximal sesamoid bone is irregular (arrowhead). SDF = Superficial digital flexor tendon.

Citation: Journal of the American Veterinary Medical Association 229, 10; 10.2460/javma.229.10.1607

Figure 3—
Figure 3—

Longitudinal ultrasonographic image of the right metacarpophalangeal joint of the horse in Figure 1 at the level of the suspensory ligament attachment. The large acoustic shadow is generated by the sesamoid bone (asterisk). Several hyperechoic foci (arrows) within the branch of the suspensory ligament (indicated by electronic caliper marks) are representative of foci of dystrophic mineralization.

Citation: Journal of the American Veterinary Medical Association 229, 10; 10.2460/javma.229.10.1607

Lameness was notably improved, and metacarpophalangeal joint swelling had decreased so that the size of the joint was near normal. Radiography of the metacarpophalangeal joint performed 1 month later revealed progression of the aggressive process and exuberant secondary periarticular new bone formation (Figures 4 and 5). Lysis associated with the proximal sesamoid bones had become more severe, compared with radiographic findings obtained at the initial evaluation. In addition, a well-defined lytic process affecting the articular areas of the distal extremity of the third metacarpal bone caused narrowing of the joint space. Radiographic findings were compatible with an aggressive reaction and erosive arthritis.

Figure 4—
Figure 4—

Lateral radiographic view of the right metacarpophalangeal joint of the horse in Figure 1. Notice moderate soft tissue swelling surrounding the joint space. Exuberant irregular periarticular new bone formation associated with the proximal sesamoid bones (white arrow) as well as the dorsoproximal aspect of the proximal phalanx and dorsodistal aspect of the third metacarpal bone can be seen (curved arrow). Well-defined subchondral bone lysis associated with the palmar and proximal aspect of the distal extremity of the third metacarpal bone is also evident (arrowheads). The heterogeneous appearance of the proximal sesamoid bones (asterisk) also indicates loss of bone mass most likely resulting from lysis.

Citation: Journal of the American Veterinary Medical Association 229, 10; 10.2460/javma.229.10.1607

Figure 5—
Figure 5—

Dorsopalmar radiographic view of the right metacarpophalangeal joint of the horse in Figure 1. Lysis (arrowheads) associated with the axial surfaces of the proximal sesamoid bones is evident. Narrowing of the joint space indicating a severe degree of subchondral erosion is also evident. Periarticular new bone formation (arrows) associated with the proximal aspect of the first phalanx can be seen laterally and medially.

Citation: Journal of the American Veterinary Medical Association 229, 10; 10.2460/javma.229.10.1607

Approximately 12 weeks after the initial evaluation, the horse was discharged from the hospital. The owners were instructed to administer phenylbutazone (4.4 mg/kg, PO, q 12 h), enrofloxacin (3.3 mg/kg [1.5 mg/lb], PO, q 12 h), isoxuprine (1.0 mg/kg, PO, q 12 h), and omeprazole (2 mg/kg, PO, q 24 h) for an additional 30 days; flunixin meglumine (0.55 mg/kg [0.25 mg/lb], PO, q 24 h) for an additional 2 weeks; itraconazole (5 mg/kg, PO, q 24 h) for an additional 6 weeks; and highly viscous defined fractions of hyaluronic acid sodium salt and chondroitin sulfates A and C in a 10% solution of N-acetyl-D-glucosamine (0.55 mg/kg, IM, q 5 d) for the life of the horse.

The horse was reexamined 10 weeks after discharge. Lameness (scale, 3/5)1 of the right forelimb persisted during trotting. Swelling of the right metacarpophalangeal joint and the digital sheath had diminished. Three months after discharge, the owner reported that the horse was mildly lame during trotting but was moving freely and comfortably during all gaits and had gained a considerable amount of weight. Because the osteoarthritis was not expected to improve and because it was recommended that the horse not return to purposeful exercise, the owner decided to retire the horse from racing.

Discussion

Fungal infections of the musculoskeletal system are not common in horses. Pythiosis, a fungal disease of horses that is usually confined to soft tissues, has been reported in bone lesions in 4 horses.2 Several cases of septic arthritis in horses have reportedly been caused by fungal infections.3–5

In horses, the cause of osteomyelitis of the axial borders of the proximal sesamoid bones is not known, although trauma has been suspected.6 Dorsiflexion of the metacarpophalangeal and metatarsophalangeal joints puts severe stress on the planter and palmar supporting structures, particularly the proximal sesamoid bones and intersesamoidean ligaments; such stress could result in tearing of the intersesamoidean ligaments.6 Proposed biomechanics of the cause of condylar fractures suggests that the palmar aspect of the condyles rotates medial to lateral, then lateral to medial during maximal dorsiflexion of metacarpophalangeal and metatarsophalangeal joints. Such rotation would result in severe strain on the palmar and axial supporting structures of the joint, particularly the proximal sesamoid bones and intersesamoidean ligament.7

The medial and lateral palmar and planter digital arteries supply blood to the intersesamoidean ligament.8 In clinically normal horses, the blood supply to the intersesamoidean ligaments when perfused with dilute barium sulfate is sparse and the axial borders of the proximal sesamoid bones are perfused last.8 Destructive lesions of the proximal sesamoid bones and septic arthritis of the metacarpophalangeal or metatarsophalangeal joints attributable to catheterization of the dorsal metatarsal artery in a horse have been reported.9 The authors of that report suggest that septic thrombosis of the microvasculature of the proximal sesamoid bones can result in osteomyelitis and septic arthritis. Avulsion of the intersesamoidean ligament may disrupt the blood supply to the proximal sesamoid bones, resulting in bone resorption along their axial borders.6

Osteomyelitis of the axial borders of the proximal sesamoid bones may be considered as an extension of a primary digital sheath infection,10 or it may be a primary condition leading to extension of infection into the digital sheath. Of the 15 horses reported with osteomyelitis of the axial borders of the proximal sesamoid bones, 6 had infected synovial structures and 5 had septic arthritis of a metacarpophalangeal or metatarsophalangeal joint and also had sepsis of the digital sheath6,8; possible causes for the infections were not given. Thrombosis of vessels in the intersesamoidean ligament described by Wisner et al8 may promote an environment conducive to harboring microbial growth.

The percentage of horses with septic arthritis resulting from intra-articular administration of corticosteroids is not known; however, the percentage of humans with septic arthritis after intra-articular administration of corticosteroids is low.11 In immunocompetent humans, repeated intra-articular administration of corticosteroids has been implicated in the development of septic arthritis caused by A fumigatus.12 In horses, fungal infections have not been reported as a complication after intra-articular injections.13

In the horse reported here, the effectiveness of itraconazole for treatment of septic arthritis caused by A fumigatus is difficult to determine because numerous medications and treatments were administered simultaneously during this horse's recovery. Itraconazole has been used successfully for treatment of fungal infections in horses.14–17 Horses with mycotic infections should be treated for 4 to 6 weeks.14 In immunocompromised humans, use of itraconazole for treatment of arthritis caused by A fumigatus resulted in initial improvements in clinical and radiographic findings but did not stop deterioration of joint function.18 In humans, voriconazole has been used successfully for treatment of septic arthritis attributable to A fumigatus.12 To the authors' knowledge, voriconazole has not been used to treat fungal infections in horses.

Various diagnostic imaging methods and fungal cultures are useful for diagnosing fungal osteomyelitis of the axial borders of the proximal sesamoid bones in horses. Fungal osteomyelitis of the sesamoid bones and erosive arthritis should be considered as differential diagnoses for horses in which corticosteroids have been administered intra-articularly.

References

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