Abstract
Case summary
A 1-year-old male castrated Savannah cat presented with chronic recurrent lethargy, stiffness, right pelvic limb lameness and spinal hyperaesthesia. Eight months before referral, the cat was treated with prednisolone, remdesivir and a 1-week course of amoxicillin/clavulanic acid and marbofloxacin for suspected feline infectious peritonitis. Multiple recurrences were reported after initial presentation, and were treated with 1-week courses of amoxicillin/clavulanic acid. Neurological examination did not reveal further findings. Haematology showed neutrophilia/monocytosis. Spinal, limb and thoracic radiographs revealed irregular endplates and narrowing of T12–T13 and L7–S1 intervertebral disc spaces, metaphyseal lesions of multiple long bones with heterogeneous medullary bone and reduced corticomedullary distinction, and two areas of increased opacity in the left lung lobes. Feline leukaemia virus, feline immunodeficiency virus, toxoplasma serology and urine culture were negative. Blood culture was positive for Salmonella species. Amoxicillin/clavulanic acid (20 mg/kg q12h) was started with clinical improvement, but no resolution of haematological/imaging changes. Relapse occurred 7 months into treatment. Blood culture showed Phocaeicola massiliensis, a suspected contaminant. Metronidazole (11 mg/kg q12h) was added based on sensitivity, with clinical improvement but relapse after discontinuation 4 months later. Neutrophilia and monocytosis were again identified, alongside hyperproteinaemia and globulinaemia. Recheck radiographs showed a worsening of the osteomyelitis but an improvement of the discospondylitis. Salmonella species were cultured again from blood and bone biopsies. Marbofloxacin (4.5 mg/kg q24h) was initiated. At the 6-month follow-up, complete resolution of clinical and laboratory findings was documented alongside radiographic improvement of the previous lesions. Treatment was discontinued with no relapses over the 10-month follow-up.
Relevance and novel information
To our knowledge, this is the first reported case of feline discospondylitis and osteomyelitis caused by Salmonella species. Marbofloxacin, but not amoxicillin/clavulanic acid, led to resolution of the infection.
Introduction
Discospondylitis is an inflammatory process of one or more intervertebral discs and the adjacent cartilaginous endplates. Similarly, osteomyelitis refers to inflammation affecting bone and bone marrow. In both cases, the underlying pathophysiology is related to haematogenous spread of bacteria or fungi from another infected site or due to trauma or iatrogenic causes. 1
Feline discospondylitis is less commonly described in the veterinary literature compared with dogs. In a recent study, Escherichia coli and Staphylococcus species were found to be the most common infectious agents. 2 Osteomyelitis has also been rarely described in cats, caused by Staphylococcus species,3,4 Mycobacterium species, 5 Streptococcus species, 6 Microsporum canis and Bartonella henselae. 7
Salmonella species are Gram-negative facultative anaerobic rods, and the main source is contaminated food. Despite multiple previous reports of Salmonella species infection in cats,8–13 discospondylitis and osteomyelitis are not described in this species. Instead, two case reports of salmonella discospondylitis and two of osteomyelitis were found in dogs.14–17
Case description
A 1-year-old male castrated Savannah cat was presented to the neurology service with a chronic history of recurrent lethargy, right pelvic limb lameness, low pelvic carriage and spinal hyperaesthesia. Eight months before presentation, the cat was presented to the primary care veteriarian as it had collapsed, and was hypotensive and hypoglycaemic. It received treatment with prednisolone, remdesivir and a 1-week course of amoxicillin/clavulanic acid and marbofloxacin for suspected feline infectious peritonitis (FIP). After initial presentation, multiple episodes of recurrent lethargy and spinal discomfort were reported, which responded to 1-week courses of meloxicam and amoxicillin/clavulanic acid. At the time of presentation, the cat was only under analgesic treatment with gabapentin (10 mg/kg q8h) and meloxicam (0.05 mg/kg q24h). The cat was fed a raw food diet with the breeder before being rehomed and was then switched to a commercial complete wet food diet.
The general physical examination was normal. The orthopaedic and neurological examination revealed a low pelvic carriage, bilateral grade 2 patellar luxation, short-strided gait in all four limbs with partially weightbearing lameness in the right pelvic limb, and multifocal pelvic limb and thoracolumbar spine hyperesthesia. No neurological deficits were detected, suggesting a multifocal localisation to the axial and appendicular musculoskeletal system
The haematology results are described in Table 1. The biochemistry was normal including pre-prandial bile acids, creatine kinase and aspartate transferase as markers of muscular damage, and vitamin B12 levels and folate levels as markers of malabsorption. The ELISA for feline leukaemia virus antigen and feline immunodeficiency virus antibody, and the indirect fluorescent antibody Toxoplasma gondii IgG/IgM serology were negative.
Haematological changes at presentation, 3-month follow-up, 7-month follow-up and 17-month follow-up (last recheck) showing the evolution of the laboratory findings
AGP = alpha-1-acid glycoprotein; HCT = haematocrit; N/A = not applicable; WBC = white blood cells
Full spinal, thoracic and pelvic limb radiographs were performed under general anaesthesia (Figures 1 and 2), showing irregular vertebral endplates and narrowing of the intervertebral disc spaces at T12–T13 and L7–S1, consistent with discospondylitis (Figure 1a). The radiographs of the limbs revealed a heterogeneous, coarse appearance of the medullary bone with reduced corticomedullary distinction in the proximal and distal diaphyseal and metaphyseal regions of multiple bones, including the femur, tibia, fibula, humerus, ulna and radius bilaterally, with the left side being worse. In addition, there was a mild patchy increased opacity in the mid diaphysis of the right femur, in the area of the nutrient foramen. These findings were consistent with multifocal osteomyelitis (Figure 2a). Moreover, there were two areas of increased opacity in the left lung lobes, compatible with pneumonia.
Lateral radiographs of the thoracolumbar spine showing (a) the evolution of the two discospondylitis sites (arrows) at the time of initial presentation, (b) relapse of the clinical signs after discontinuing amoxicillin/clavulanic acid and metronidazole and (c) after 7 months of marbofloxacin
Lateral radiographs of the left and right radius and ulna, showing the evolution of the osteomyelitis sites (arrows) at (a) the time of initial presentation, (b) relapse of the clinical signs after discontinuing amoxicillin/clavulanic acid and metronidazole and (c) after 6 months of marbofloxacin
Abdominal ultrasound showed enlarged medial iliac lymph nodes (thickness of 5–6 mm). Fine-needle aspiration showed mixed lymphoid population and mild neutrophilic inflammation, compatible with reactive lymphoid hyperplasia. The urine analysis was normal with negative culture.
A blood sample was collected from the left jugular vein using an aseptic technique and submitted for culture (aerobic and anaerobic). In the meantime, treatment with amoxicillin/clavulanic acid (20 mg/kg q12h) was initiated, alongside meloxicam (0.05 mg/kg q24h for 2 weeks) and gabapentin (10 mg/kg q8h for 4 weeks). Blood culture yielded Salmonella species sensitive to amoxicillin/clavulanic acid; therefore, continuation of the treatment was recommended (Table 2).
Culture and sensitivity results from blood and bone marrow cultures
Aerobic culture: moderate growth of Salmonella species. The anaerobic culture was negative
MIC = minimum inhibitory concentration
On this treatment, the cat markedly improved, becoming more comfortable and without any lameness in the right pelvic limb. After 3 months of antibiotic treatment, recheck radiographs were unchanged, apart from collapse of the T12–T13 intervertebral disc space with the development of ventral spondylosis. Alpha-1-acid glycoprotein (AGP) was elevated (1 g/l, reference interval [RI] 0.1–0.5). Based on all of the above, it was decided to prolong the treatment with the same antibiotic.
Seven months after starting amoxicillin/clavulanic acid, the cat was presented again with relapse of the clinical signs, including a not-full-weightbearing lameness in the left thoracic limb and discomfort when manipulating both thoracic limbs. Recheck blood results are summarised in Table 1. The AGP remained mildly elevated (0.6 g/l, RI 0.1–0.5). The recheck radiographs showed a worsening of the previous osteomyelitis, with the discospondylitis remaining unchanged. The left lung lobe lesions were resolved. A repeat blood culture identified Phocaeicola massiliensis sensitive to metronidazole. At that stage, it was unclear if this represented a real infection or a contaminant. However, given the worsening of clinical and radiographic findings, treatment with metronidazole (11 mg/kg q12h) was added, together with amoxicillin/clavulanic acid and meloxicam (0.05 mg/kg q24h for 2 weeks).
After 4 months, resolution of the left thoracic limb lameness and the discomfort was reported; however, the neutrophilia and globulinaemia persisted. Further diagnostic imaging was declined. At that stage, given the clinical improvement, discontinuation of both the antibiotics was attempted.
Two months after discontinuation of the antibiotics, the cat was re-presented because of reluctance to exercise, stiff gait and discomfort. Haematology and biochemistry findings were again similar. An echocardiography ruled out endocarditis and cardiac troponin I was normal. The AGP was within normal limits (0.4 g/l, RI 0.1–0.5). Repeat radiographs showed further worsening of the osteomyelitis, with marked heterogeneity of the medullary bone, cortical thinning and associated bony expansion (Figure 2b), but not of the discospondylitis, where the T12–T13 vertebral bodies appeared partially fused (Figure 1b). A repeat blood culture alongside histology and culture of bone biopsies of both distal ulnas were performed. The bone biopsies showed osteomyelitis, with lymphocytic, plasmocytic, histiocytic and neutrophilic inflammation, and mild to moderate fibrosis. The blood and bone cultures were again positive for Salmonella species with the same sensitivity panel (Table 2).
Based on the unsuccessful prolonged treatment with amoxicillin/clavulanic acid, marbofloxacin (4.5 mg/kg q24h) was started, together with a 2-week course of meloxicam (0.05 mg/kg q24h) and gabapentin (10 mg/kg q8h).
Six months later, the cat was reported to have completely recovered. The clinical examination did not reveal any abnormalities or any focus of discomfort. Recheck haematology and biochemistry were normal. The spinal radiographs showed improvement of the endplates at the L7–S1 intervertebral disc space (Figure 1c). Moreover, the radiographs of the limbs showed marked improvement (Figure 2c). Given the significant clinical and diagnostic improvement, the antibiotic was discontinued. At a telephone follow-up 10 months later, the cat was still reported to be healthy.
Discussion
To our knowledge, this report presents the first documented case of concurrent discospondylitis and osteomyelitis caused by Salmonella species in a cat. Although feline salmonellosis has been previously reported in isolated cases,8–13 salmonella discospondylitis or osteomyelitis has not been described in this species. In our case, the cat was fed a raw food diet when it was still with the breeder, which was considered a potential source for the infection.
Culturing the causative agent in feline discospondylitis can be challenging, suggesting that these infections often present in an isolated form. 2 In contrast, osteomyelitis typically allows for easier isolation of the infectious agent through blood or direct tissue cultures. 4 In cats with osteomyelitis, Staphylococcus species is most commonly identified. 4 In this case, blood cultures were positive three times at different stages of the disease. Two cultures yielded Salmonella species and one yielded Parabacteroides massiliensis, which was suspected to be a contaminant based on the persistent isolation of Salmonella species in subsequent cultures but not of P massiliensis.
Current recommendations for treating discospondylitis and osteomyelitis involve broad-spectrum antibiotics pending culture results. Amoxicillin/clavulanate is commonly used.2,4 However, definitive treatment choice and duration guidelines are lacking, with a minimum of 6 weeks generally recommended. In this case, despite initial clinical improvement and prolonged amoxicillin/clavulanate therapy based on sensitivity testing, a relapse occurred. Subsequent treatment with marbofloxacin resulted in complete resolution of clinical, laboratory and radiographic abnormalities, despite a shorter treatment duration.
Although both antibiotics demonstrated in vitro sensitivity and are known for good bone penetration, 1 the clinical outcomes differed significantly. We cannot exclude that this may be due to the development of in vivo resistance from prior intermittent amoxicillin/clavulanate use in short courses, though this remains speculative without confirmation on sensitivity testing. Fluoroquinolones, like marbofloxacin, are known for excellent bone:serum concentration ratios and high bioavailability, and are recommended for human osteomyelitis,18,19 which may explain the successful outcome in this patient.
As a learning point from this case, clinical improvement alone proved unreliable, as the cat initially responded positively to all three antibiotics. However, only marbofloxacin led to radiographic resolution of osteomyelitis and normalisation of haematological parameters. Interestingly, amoxicillin/clavulanate resulted in radiographic healing of one discospondylitis site but not the osteomyelitis lesions.
Repeat imaging is recommended for monitoring discospondylitis in dogs and cats, although radiographic improvement may lag behind clinical improvement. 20 MRI findings of discospondylitis in cats have been described and recheck MRI could potentially bypass this limitation. 21 C-reactive protein is a useful inflammatory marker in dogs,22,23 but its reliability in cats is limited. 23 Other acute phase proteins, such as serum amyloid A (SAA) and alpha-1-acid glycoprotein (AGP), have been explored. 24 AGP has been studied in cats with FIP,25,26 but no study investigated its reliability in discospondylitis or osteomyelitis cases in cats. In this case, AGP and serum globulins fluctuated between normal and elevated, suggesting they were not reliable single monitoring parameters. Leukocytosis and radiographic findings were more consistent with treatment response. SAA was not tested in this case at any stage.
This case underscores the limitations of relying solely on clinical signs for monitoring treatment response. Based on this report, our recommendation would be to always perform haematological and radiographic rechecks in similar cases to guide decisions on therapy. Although this might initially appear to be a more expensive approach to the owner, early identification of treatment failure/efficacy can potentially reduce treatment duration and subsequently cost. Optimal timing for rechecks remains to be determined because of the limited literature.
Conclusions
This report describes the first case of multifocal discospondylitis and osteomyelitis caused by Salmonella species in a cat. Despite in vitro sensitivity to both amoxicillin/clavulanate and marbofloxacin, only marbofloxacin resulted in clinical, haematological and radiographic resolution of the infection. This case highlights the need for further research to establish optimal antibiotic treatment, duration, imaging rechecks and biomarkers for monitoring these conditions in cats.
Footnotes
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Ethical approval
The work described in this manuscript involved the use of non-experimental (owned or unowned) animals. Established internationally recognised high standards (‘best practice’) of veterinary clinical care for the individual patient were always followed and/or this work involved the use of cadavers. Ethical approval from a committee was therefore not specifically required for publication in JFMS Open Reports. Although not required, where ethical approval was still obtained, it is stated in the manuscript.
Informed consent
Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or non-experimental animals, including cadavers, tissues and samples) for all procedure(s) undertaken (prospective or retrospective studies). No animals or people are identifiable within this publication, and therefore additional informed consent for publication was not required.
