Introduction

Trapped neutrophil syndrome (TNS) is a rare autosomal recessive genetic disease of border collies that has been documented in Australia, New Zealand, Japan, the United Kingdom, and Israel.^1–6 Affected dogs develop lameness, joint swelling, and neutropenia before 4 mo of age. Although allele prevalence in border collies is estimated to be 6–12% worldwide, there have been no North American cases described to date.^7,8 In addition, although the radiographic findings associated with TNS are distinctive and their recognition is paramount to making a diagnosis, radiographic images available in the veterinary literature are currently limited to a single image of an atypical finding (metaphyseal fracture).¹ This report is the initial description of TNS in a North American border collie and documents multiple radiographic abnormalities, including their progression over more than 6 mo.

Case Report

A 10 wk old female border collie was presented for evaluation of hemorrhagic diarrhea, lethargy, and left pelvic limb lameness that had progressed to nonambulation. Physical examination revealed pyrexia (103.3°F, 39.6°C) and moderate swelling and pain in both carpi, tarsi, and stifles, most severe in the left pelvic limb. On complete blood count (CBC), the patient’s leukogram was unremarkable with an absolute segmented neutrophil count within the reference interval (4.1 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL).

Radiographs of the right carpus (Figures 1A, B) showed capsular soft tissue swelling with a region of ill-defined lucency at the right distal radial and ulnar metaphyses just proximal to the distal physes. Radiographs of the right femorotibial joint (Figure 2) showed capsular swelling as well as a large heterogeneous area of lucency with surrounding sclerosis in the cranial and proximal aspect of the distal femoral metaphysis. The distal femoral physis was irregular and indistinct. The right femoral condyles were irregularly marginated and the femoral epiphysis was diffusely mottled. There was a thin, irregularly marginated, ill-defined lucency in the right distal tibial metaphysis just proximal to and parallel with the distal physis. Radiographs of the left pelvic limb showed capsular swelling of the left femorotibial joint and a thin, irregularly marginated, and ill-defined lucency in the distal femoral metaphysis parallel to and just proximal to the physis. The epiphysis was heterogeneous with an irregular articular border. In addition, there was capsular soft tissue swelling of the left tibiotarsal joint associated with a thin, irregularly marginated, ill-defined lucency with adjacent sclerosis just proximal to the distal tibial physis.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Cytologic examination of joint fluid from the left stifle and carpus showed nondegenerate neutrophilic inflammation; aerobic culture of joint fluid was negative. A single oral dose of carprofen (2 mg/kg) was administered and treatment with clindamycin (15 mg/kg, IV, q 12 hr) and ampicillin sulbactam (30 mg/kg, IV, q 8 hr) was initiated. Three days after the initial presentation, there was no clinical improvement and a recheck CBC showed mild neutropenia (2.9 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL). The neutropenia in combination with the signalment, radiographic findings, and lack of response to antibiotic therapy raised suspicion for trapped neutrophil syndrome (TNS), prompting treatment with prednisone (1.4 mg/kg, per os [PO], q 12 hr), omeprazole (1 mg/kg, PO, q 12 hr, for 14 days), and tramadol (5 mg/kg, PO, q 8–12 hr, for 10 days). Within 24 hr, the dog showed significant improvement. When discharged the following day, 4 days after presentation, the dog was afebrile, was eating and drinking normally, and was nonpainful with normal ambulation. Blood submitted for gene sequencing to the Veterinary Genetics Laboratory^a showed that the dog was homozygous for the TNS mutation of the VPS13B gene. A recheck CBC 1 wk after discharge (11 days after presentation) showed neutropenia (2.4 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL).

One month after initial presentation, at 3.5 mo of age, the dog was clinically normal and received a rabies vaccine. Prednisone was continued at 1 mg/kg, PO, q 12 hr. One week later, the dog was presented for acute lameness and right carpal swelling with no known history of trauma. Examination showed moderate, diffuse soft tissue swelling of the right thoracic limb distal to the elbow. A small crust was present on the lateral aspect of the right thoracic limb just distal to the elbow. A CBC revealed moderate neutropenia (1.8 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL). Radiographs showed extracapsular soft tissue swelling of the right carpus (Figures 1C, D). The previously noted regions of lucency in the right distal radial and ulnar metaphyses were no longer present. The dog was discharged on amoxicillin clavulanate (24 mg/kg, PO, q 12 hr, for 14 days), tramadol (5 mg/kg, PO, q 8–12 hr, for 14 days), and continued prednisone therapy. At re-evaluation 3 days later, the dog was ambulatory and nonpainful and had decreased swelling of the right forelimb.

Approximately 5 mo after diagnosis, at 7.5 mo of age, the dog was presented for recurrent lameness and swelling of both stifles and tarsi. Repeat CBC showed neutropenia (2.6 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL) and moderate monocytosis (5.0 cells × 10³/μL; reference interval 0.1–1.4 cells × 10³/μL). Radiographs of both carpi showed sclerosis surrounding the distal ulnar and radial physes with resolution of the previously identified soft tissue swelling. Radiographs of both femorotibial joints showed severe intracapsular swelling of the right stifle joint (Figure 3). The previous irregularity of the epiphysis and articular margin had progressed to severe lysis of the medial aspect of the right distal femoral condyle affecting the articular margin with a surrounding rim of sclerosis. Similar but less severe changes were present at the medial aspect of the left femoral condyle and medial tibial condyle. Additional diagnostics were declined by the owner, and the dog received amoxicillin-clavulanate for 14 days, tramadol for 5 days, and continued prednisone therapy (0.6 mg/kg, PO, q 12 hr).

View Figure

• Download as Powerpoint
• Download Figure

Six weeks later, 6.5 mo after the initial presentation and at approximately 9 mo of age, the dog was re-presented for lameness. She was small (8.7 kg) and short in stature for her age and breed, had severe effusion of both stifles, and was nonambulatory in the pelvic limbs. A CBC showed a segmented neutrophil count that was within the reference interval (11 cells × 10³/μL; reference interval 3–11.5 cells × 10³/μL) with increased band neutrophils (1.4 cells × 10³/μL; reference interval 0–0.3 cells × 10³/μL). Non–contrast enhanced computed tomography at 0.625 cm slice thickness in a bone algorithm of both femorotibial joints was performed to obtain a more complete assessment of the osseous lesions for diagnostic and prognostic reasons. Severe lysis of the medial femoral condyles with surrounding sclerosis was present bilaterally and was more extensive on the right (Supplementary Figure I). Severe erosive polyarthritis was diagnosed. Cytology of synovial fluid from the stifles showed mildly degenerate neutrophilic inflammation. Aerobic culture of synovial fluid yielded a small number of colonies of Staphylococcus pseudintermedius from the right stifle but not the left. Sepsis secondary to septic arthritis was suspected and antibiotic therapy was reinstituted, but the dog died the following day. Necropsy was declined by the owner.

Discussion

Similar to the dog described here, dogs with TNS typically develop failure to thrive, fever, gastrointestinal signs, lameness, joint swelling, and neutropenia between 6 and 12 wk of age.^1–6 They are often small compared with litter mates and may have abnormal craniofacial development characterized by a narrow, elongated skull or “ferret-like” features.^1–5 Cytologically, the polyarthritis associated with TNS is most often characterized by nondegenerate neutrophilic inflammation, consistent with immune-mediated polyarthritis.³ However, septic arthritis can also occur, presumably secondary to persistent neutropenia.

The mutation that causes TNS is a 4 base pair deletion in exon 19 of the VPS13B gene on chromosome 13, which codes for a trans-membrane protein used for vesicle-mediated transport and sorting within the cell and is specifically involved in Golgi glycosylation and endo-lysosomal maintenance.^7,9 In people, mutations in VPS13B lead to the rare autosomal recessive disorder Cohen syndrome (CS), which is characterized by delayed development, microcephaly, neutropenia, and facial dysmorphism typified by a beak-shaped nose and grimace.¹⁰

Based on the nine previously described cases, the neutropenia associated with TNS is typically mild to moderate and occurs despite myeloid hyperplasia in the bone marrow, hence the name of the syndrome.¹ Although the specific mechanism for neutropenia is unknown, the importance of vacuolar protein sorting genes in granulocytic development is reinforced by the finding of neutropenia in VPS45-deficient people.¹¹ Studies of bone marrow and neutrophil function in people with CS revealed normal marrow cellularity without granulocytic dysplasia or maturation arrest and with appropriate superoxide anion release.^12,13 However, neutrophils from people with CS were found to have increased adhesion capacity and decreased expression of L-selectin (CD62L) and B2 integrin (CD11b), suggestive of increased neutrophilic margination.¹³ Although these studies have yet to be performed in dogs with TNS, it can be speculated that the mechanism of neutropenia is similar because bone marrow evaluation from dogs with TNS has largely been unremarkable.^2,3

The most common radiographic and computed tomographic abnormalities previously described in border collies with TNS are similar to those identified in the dog described here, and they include intracapsular joint swelling and metaphyseal lucencies surrounded by sclerosis, most often in the distal femur, tibia, radius, and ulna. However, lesion severity has ranged from the absence of radiographic abnormalities^4,5 to fractures associated with metaphyseal lysis,¹ and locations have also included proximal humerus,^1,3 distal fibula,² and phalanges.⁵ Histologically, areas of radiographic metaphyseal lucency have corresponded to areas of hypercellularity associated with marked neutrophilic infiltration, hemorrhage, and osteonecrosis with variable loss of the trabecular bone.¹ Segmental necrosis of the physis has also been described and may account for the physeal sclerosis and irregularity observed radiographically in our patient.¹

The case described here was unique in that there was an opportunity to evaluate the progression of radiographic abnormalities over more than 6 mo. Lesions were initially characterized by capsular joint swelling and heterogeneous areas of metaphyseal lucency and sclerosis in the distal radius, ulna, femur, and tibia but progressed to include bilateral epiphyseal lysis of the distal femur and tibia as well as articular lysis of both stifles. Although previous reports have described lysis at the joint margin and articular swelling associated with nonseptic inflammation, articular lysis has not been previously reported in dogs with TNS and may reflect changes secondary to septic arthritis or may be a newly described feature due to progression of disease. The distal radial and ulnar metaphyseal lucencies resolved in this dog after initial therapy, and it is unclear whether some component of hypertrophic osteodystrophy (HOD) caused the radiographic features and were self-limiting, as was suspected in one previously described dog with TNS,² or whether immunosuppressive therapy contributed to their resolution.

The clinical signs and radiographic lesions associated with TNS have a number of similarities to those caused by HOD, and as a result, TNS may be misdiagnosed as severe HOD that fails to resolve.² Similar to TNS, the signs associated with HOD typically develop at 2–8 mo of age and include lameness, lethargy, fever, and anorexia.^14,15 Radiographically, both diseases cause metaphyseal lesions in long bones, including irregular, faint, radiolucent bands aligned parallel to the physis in the distal radius, ulna, and tibia. However, there are a number of differences between the radiographic features associated with these diseases. First, swelling of the distal extremities is often the result of intracapsular joint effusion in dogs with TNS, whereas in HOD, swelling is typically extracapsular and is most prominent at the physis. Second, HOD is infrequently described in the distal femur, with the most common sites being the distal radius, ulna, and tibia,¹⁵ whereas distal femoral lysis is common in dogs with TNS. Third, in addition to the faint, lucent metaphyseal bands typically associated with HOD, metaphyseal lysis associated with TNS often has an indistinct, moth-eaten appearance with surrounding sclerosis consistent with osteomyelitis^1,3 and may extend to the metaphyseal/diaphyseal junction¹ or into the epiphysis, as was seen in our patient. Finally, the progressive articular lysis of the stifles observed in the dog described here has not previously been associated with HOD.

A clinical diagnosis of TNS can be made on the basis of signalment combined with typical clinical features, including fever, neutropenia despite myeloid hyperplasia, effusive polyarthropathy, and radiographic evidence of osteolytic and sclerotic irregular metaphyseal lesions in long bones. Because of their clinical similarities, TNS should be considered in any young border collie with signs consistent with HOD, especially when gastrointestinal signs, neutropenia, or joint effusion are present. The diagnosis of TNS can be confirmed by gene sequencing that indicates the animal is homozygous for the VPS13B gene mutation.⁷ In the United States, there are at least three laboratories that offer testing for TNS from blood samples or cheek swabs^a,b,c. Results can differentiate normal dogs from those that are carriers (heterozygous for the mutation) or affected (homozygous for the mutation).

Unfortunately, the long-term prognosis for dogs with TNS is guarded to poor, with therapy aimed at controlling clinical signs, improving neutropenia, and treating secondary bacterial infections. The mainstay of treatment is immunosuppressive therapy, which in the few cases that have been described has often resolved the signs associated with polyarthropathy and improved neutropenia in the short term.^1,3–5 Glucocorticoids have been used successfully to decrease joint effusion and increase neutrophil counts, but signs typically recur as immunosuppressive therapy is tapered. It is suspected that glucocorticoid administration promotes neutrophil demargination and migration from the bone marrow into the peripheral circulation by decreasing gene transcription of L-selectin.¹⁶ The dosage of prednisone used in the dog described here was based on dosages that were deemed effective for control of clinical signs in previously described cases.^3,5 Although this dog's clinical signs improved initially, the neutropenia was persistent and signs of polyarthropathy recurred twice despite therapy. In addition, it was not possible to reevaluate the dog frequently. Therefore, rather than attempting to more aggressively taper the dosage of prednisone, we elected to maintain the same tablet size of prednisone over the course of treatment. Because of the dog's growth, this resulted in a decrease in the prednisone dosage over 6.5 mo from 1.4 mg/kg q 12 hr to 0.6 mg/kg q 12 hr.

Of the nine dogs with TNS previously described in the literature, four died or were euthanized before 1 yr of age because of persistent neutropenia, recurrent bacterial infection, sepsis, or recurrent polyarthritis.^1,2,4 However, one report described a dog that was treated for at least 3 yr,³ and a second report described two cases that were managed for 3 and 6 yr, respectively,⁵ so longer outcomes are possible. In the two dogs described in the second report, azathioprine was used in addition to prednisone for immunosuppression, but the authors concluded that its use did not improve clinical signs or allow lowering of the prednisone dosage.⁵ The addition of newer immunosuppressive agents such as mycophenolate may be considered for these purposes in future cases. In the dog described here, it is possible that earlier identification of potential septic arthritis and reinstitution of antibiotic therapy may have delayed the outcome; however, the necessity for ongoing immunosuppressive therapy combined with recurrent or persistent neutropenia makes long-term management of patients with TNS challenging.

Conclusion

Trapped neutrophil syndrome is a rare, autosomal recessive, heritable disease of border collies that has not previously been described in a dog from North America, despite the fact that estimates of worldwide allele prevalence in the breed are 6–12%.^7,8 It is our hope that the description of clinicopathologic and radiographic findings in this case will raise awareness in North America and prompt veterinarians to include TNS as a differential diagnosis when a young border collie with signs consistent with HOD, immune-mediated polyarthritis, or septic arthritis is encountered, especially when combined with gastrointestinal signs or neutropenia. Veterinarians who counsel US border collie breeders should consider recommending that genetic testing for the TNS mutation be included when screening breeding animals for heritable diseases.

Supplemental Figure I

Right (R) and left (L) femorotibial joints 6.5 months after initial presentation at approximately 9 months of age; computed tomography performed in a bone algorithm scanned at 0.625cm slice thickness. Capsular fluid-attenuating distention is present bilaterally and is severe on the right. Lysis of the medial femoral condyles with surrounding sclerosis (arrowheads) is present bilaterally, but is more severe on the right. There are defects in the distal articular margin of medial condyles (arrows). Multifocal, small, variably shaped mineral attenuating foci are present within the epiphysis bilaterally.