Abdominal Aortic Aneurysm Associated with Systemic Fungal Infection in a German Shepherd Dog

Introduction

Abdominal aortic aneurysms are characterized by an evagination of the vessel wall, often secondary to a loss of vessel wall integrity. Presenting clinical signs may vary from being asymptomatic to an acute onset of paraparesis and collapse. Most often cases are idiopathic; however, aortic aneurysms associated with systemic infections have been reported in humans, and less commonly, in veterinary patients. This is the first report of an abdominal aortic aneurysm in the dog associated with a mycotic organism.

Case Report

A previously healthy 2 yr old, 28.7 kg, spayed female German shepherd presented to the emergency service of a veterinary teaching hospital with a chief complaint of progressive paraparesis, pyrexia, and weight loss (7.3 kg) over the previous 30 days. The dog had been to the primary care veterinarian 6 days prior to presentation with a complaint of hind limb weakness. The dog was prescribed doxycycline^a 5mg/kg per os [PO] q 12 hr due to a positive result on an enzyme-linked immunosorbent assay (ELISA)^b SNAP 4Dx for Borrelia spp. antibody. Over the next several days, the paraparesis worsened and the dog was presented to an emergency practice. The dog was in lateral recumbency, pyrexic (39.3°C), and nonambulatory. Postural test reactions were markedly diminished in the pelvic limbs but the thoracic limbs and cranial nerves were assessed as normal. The dog was admitted to the hospital and administered meloxicam^c 0.2mg/kg subcutaneously (SC) once, doxycycline^d 10 mg/kg IV q 12 hr, enrofloxacin^e 4.8 mg/kg IV once, and lactated Ringer's solution^f at 4 mL/kg/hr.

No clinical improvement was seen over the next 36 hr and the dog was subsequently referred to a veterinary teaching hospital for further diagnostics and supportive care. On initial physical examination the dog was laterally recumbent, had an elevated temperature of 39.8°C, generalized muscle wasting, and normal femoral pulse quality. Neurologic examination revealed a loss of conscious proprioception in the pelvic limbs with normal muscle tone and pain sensation. Pelvic limb spinal nerves were assessed to be mildly decreased to normal, including the patellar reflexes, hind limb flexor reflexes, and anal sphincter tone. The forelimbs appeared neurologically normal. The lesion was localized to the thoracolumbar spinal cord with the possibility that diffuse or multifocal lesions could exist due to the decreased pelvic limb spinal reflexes. Initial treatment consisted of isotonic crystalloids (lactated Ringer's solution) at a rate of 3.5 mL/kg/hr, doxycycline 5.2 mg/kg IV q 12 hr, and enrofloxacin 10.1 mg/kg IV q 24 hr. The antibiotic spectrum was broadened 7 hr later with the addition of ampicillin^g 22 mg/kg IV q 8 hr and amikacin^h 15 mg/kg IV q 24 hr.

Thoracic radiographs were normal but the abdominal ultrasound revealed a mildly enlarged and hypoechoic liver, mild splenomegaly with inhomogeneous echotexture, and bilaterally enlarged kidneys with mild left pylectasia. In addition, a focal dilation of the terminal aorta was noted (2.5 cm in diameter and 4 cm in length) with swirling echogenic blood flow within the dilation. (Figure 1) Color flow Doppler revealed an absence of signal within the dilation and a normal signal in the adjacent aorta and external iliac arteries. A thrombus was not identified. Treatment with a low molecular weight heparinⁱ 150 IU/kg SC q 12 hr was instituted due to concern for an increased risk of thromboemoblic disease secondary to vascular stasis.

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A CBC revealed a leukocytosis (WBC was 37,800 × 10³/μL; reference range, 4,900–6,900 × 10³/μL) characterized by a mature neutrophilia (segmented neutrophil count was 30.240 × 10³/μL; reference range, 2,800–11,500 × 10³/μL) with a left shift (band neutrophils was 0.378 × 10³/μL; reference range, 0–0.300 × 10³/μL). Serum chemistry abnormalities included hyperbilirubinemia (2.5 mg/dL; reference range, 0.10–0.30 mg/dL), an elevated serum alkaline phosphatase (725 U/L; reference range, 12–121 U/L), an elevated serum alanine aminotransferase (345 U/L; reference range, 18–86 U/L), and an elevated serum aspartate aminotransferase (130 U/L; reference range, 16–54 U/L). Urinalysis collected via cystocentesis revealed a urine specific gravity of 1.011. The urine culture and a single blood culture were both negative for aerobic bacterial growth. Lyme C6 quantitative antibody ELISA^j was within normal limits at <10 U/mL, which differed from the SNAP 4Dx results. This was most likely due to the presence of a low antibody level detected by the qualitative test (i.e., the SNAP 4Dx), but not by the quantitative test (i.e., the Lyme C6 ELISA).

Due to the complexity of disease and financial concerns, the owners elected to have the dog euthanized 1 day after presentation. The dog was euthanized with an IV injection of pentobarbital^k and a full necropsy was authorized and performed.

Gross Postmortem Examination

Post mortem examination of the abdominal cavity revealed the presence of a sacculated aneurysm within the caudal abdominal aorta located 2.5 cm cranial to the iliac bifurcation and measuring 7 cm in length and 3 cm in width (Figure 2). The mesenteric lymph nodes were enlarged with the largest lymph node (pancreatic) measuring 6 cm × 2.5 cm × 1.5 cm. Multiple spherical white/tan, raised, soft nodules ranging in diameter from 0.2 cm to 0.8 cm were observed on the cut surface (especially within the red pulp) of the entire spleen without capsular involvement. Multiple triangular areas of white/tan discoloration were found on both the cortical and medullary regions of the kidneys bilaterally. These areas ranged in size from 0.5 cm × 0.5 cm to 3 cm × 2 cm and were more prominent within the medullary regions. Examination of the spinal column revealed the presence of a white/tan, soft, round nodule measuring 0.9 cm × 2.0 cm × 2.0 cm within the skeletal muscle (longus colli) immediately adjacent to the ventral aspect of the second and third thoracic vertebral bodies. The mass extended into the adjacent cortical bone and marrow cavity of both vertebral bodies and protruded into the spinal canal with mild focal compression of the ventral aspect of the adjacent spinal cord. The segment of the spinal cord adjacent to and compressed by the mass appeared grossly normal. The examination of the thoracic cavity did not reveal significant gross abnormalities.

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Histopathology

Tissues collected at necropsy were fixed in 10% neutral buffered formalin, routinely processed, and embedded in paraffin according to accepted histologic technique. Sections measuring 5 μm thick were stained with hematoxylin and eosin (H&E) for microscopic examination.

Examination of the H&E stained sections revealed severe transmural granulomatous and necrotizing arteritis of the aorta with intralesional fungal organisms. There was severe chronic granulomatous lymphadenitis of the mesenteric and axillary lymph nodes, as well as granulomatous splenitis, nephritis (bilateral), hepatitis, pneumonia, and myocarditis. The granulomas were all characterized by the presence of fungal organisms that appeared similar to those observed in the aortic wall (Figure 3). The nodule observed macroscopically within the second and third thoracic vertebral bodies and adjacent skeletal muscle was characterized histologically by granulomatous inflammation with similar intralesional fungal organisms as observed in the other organs. The segments of the spinal cord adjacent to the previously described mass (second and third thoracic vertebral bodies) showed large numbers of dilated axonal bodies and dilated axonal spaces occasionally containing macrophages (digestion chambers) within the ventral and lateral white matter funiculi. The previously described symmetrical lesions were more prominent within the ventral funiculi and decreased progressively within the lateral funiculi to almost complete absence in the dorsal white matter. A Gomori-Grocott methenamine silver (GMS) stain and a periodic acid-Schiff stain were performed on 5μm sections of the affected aortic wall. The fungal organisms observed on the H&E sections stained positive for both the GMS stain and periodic acid-Schiff (Figure 4). Both free and intracellular (within macrophages) fungal organisms were observed within most of the areas of granulomatous inflammation. The morphology of both the yeast and pseudohyphae as well as the special stains were highly suggestive of a Candida sp.; however, definitive organism speciation cannot be determined histologically.

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Immunohistochemistry^l (IHC) was performed on paraffin imbedded, formalin fixed tissue from the wall of the aortic aneurysm. The result was negative for both Candida and Aspergillus. PCR testing^m was attempted on formalin fixed and paraffin embedded tissue from the wall of the aortic aneurysm. This test was also unable to identify the fungal organism.

Discussion

Systemic fungal infections are often difficult to diagnose in veterinary medicine. Fungal sepsis involves many different organ systems and shares the same clinical signs as other systemic infections. Effective treatment typically requires accurate diagnosis and prompt intervention. Cytologic or histopathologic examination of infected tissues allows for recognition of mycotic organisms and speciation is confirmed with fungal culture. The morphology of the fungal organisms in this case is most consistent with a Candida sp. Antemortem blood and urine cultures did not grow fungal organisms and post mortem samples for culture were not collected. Identification of the fungus was attempted with IHC and PCR 18 mo after tissue collection and preservation in formalin. The IHC was negative for Aspergillus and Candida indicating that the fungal organism may not have been a member of either genus. The IHC results can be effected by a lack of cross reactivity between the antibody and the specific fungal species or formalin fixation and length of time elapsed prior to testing causing cross-linking of antigen and altered antibody binding during the test.¹ PCR can be used to identify fungal organisms from paraffin embedded formalin-fixed tissues; however, the procedure was unsuccessful in this case.² Ultimately, the species of fungus could not be confirmed leaving the possibility that while morphologically similar to Candida, this cannot be stated definitively.

The presenting complaint in this case was paraparesis. The cause of the paraparesis was likely invasion of the fungal organism into the thoracic vertebrae and spinal canal. Post mortem examination confirmed the suspected thoracolumbar lesion. At the time of the post mortem examination, the lumbosacral area appeared grossly normal but, unfortunately, histology of this region was not performed. Spinal cord invasion has been previously reported in a case of systemic candidiasis in a dog.³ Arteritis has been recognized in human as well as canine systemic fungal infection but arteritis has not been reported to be either associated with or a cause of aneruysmal dilation in canines.^3,4

Abdominal aortic aneurysms are an uncommon finding in humans and have only been sporadically reported in veterinary patients.^5–7 Abdominal aortic aneurysms are a consequence of a weakening of the aortic wall and subsequent localized dilation.⁸ The pathogenesis is most commonly associated with degenerative change with no underlying infectious etiology. In humans, the incidence of aortic aneurysms is correlated with family history, genetics, and smoking. There is only a weak association between systemic hypertension, hypercholesterolemia, and aneurysm formation.^8,9 Mycotic (i.e., Cryptoccus spp.,¹⁰ Aspergillus spp.,¹⁰ Capnocytophaga spp.,¹¹ and Candida spp.) aortic aneurysms have also been reported in people. Such reports do not exist in dogs.⁴

Aortic aneurysms in dogs have been found to be associated with both infectious and noninfectious etiologies. Other than a report of familial thoracic aortic aneurysms in Leonberg dogs, there appears to be no clear risk factors for the development of aortic aneurysms in dogs.¹² Most abdominal aortic aneurysms in dogs have not been associated with an infectious etiology, but concurrent infection with Oomyctes sp. and Spirocerca lupi has been reported.^6,7

A variety of risk factors have been identified with the development of systemic fungal infections. Reported risk factors in humans include broad-spectrum antibiotic use, mechanical ventilation, parenteral nutrition, hemodialysis, renal failure, prior surgery, neutropenia, chemotherapy, severe illness, age, and indwelling catheters (especially central venous catheters).¹³ Although less commonly reported, veterinary patients appear to have similar risk factors.^14,15 Interestingly, the dog described in this case report did not have any of the previously described risk factors in the human or veterinary literature.

German shepherds appear to be predisposed for developing systemic fungal disease, primarily Aspergillus spp. The underlying etiology has yet to be determined; however, a congenital immunodeficiency is suspected. Prior research has been directed toward investigating a suspected link between immunoglobulin A (IgA) deficiency and a predisposition for the development of fungal disease. IgA is part of the innate immune system and acts by neutralizing pathogens and preventing entry through the mucosa as part of the mucosal barrier.¹⁶ Unfortunately, serum or fecal IgA levels were not measured in this dog. Nonetheless, it is unlikely that a deficiency in IgA would be the sole predisposing factor for the development of disseminated disease in this dog, as cell-mediated and innate immunity are both important defense mechanisms against the development of systemic fungal disease.¹⁷

Conclusion

Systemic fungal infection is a recognized condition in veterinary medicine and it can be associated with a poor prognosis.¹⁸ The dog in this report was severely affected by a systemic mycotic infection with the organism invading numerous vital organs. To the authors’ knowledge, this is the first report of an aortic aneurysm in a dog associated with a fungal organism. This case would suggest that if an abdominal aortic aneurysm is documented in a dog then systemic fungal disease should be considered as a potential cause for the aneurysm and associated systemic illness.