Abstract
Two yearling bull elk (Cervus elaphus) from the same farm developed anorexia, weight loss, and lameness. On physical examination, both elk were thin and showed diffuse swelling of all lower limbs. Radiographs of the lower limbs showed periosteal thickening of the distal extremities, consistent with hypertrophic osteopathy. Thoracic radiographs indicated the presence of pulmonary nodules. Cytologic evaluations of tracheal washes on both elk were consistent with inflammation. Acid-fast stains on both samples were negative. Because of the poor prognosis, both elk were euthanized. At necropsy, the carpal, metacarpal, tarsal, and metatarsal bones, as well as the radius, ulna, and tibia had thickening of cortical bone. There were multiple encapsulated nodules throughout the lungs, lymph nodes, and kidney, and smaller nodules in the myocardium. On microscopic examination, these nodules contained myriads of hyphae, and immunohistochemistry for Aspergillus sp. was strongly positive. Aspergillus fumigatus was isolated from affected tissue in 1 elk. Necropsy findings in both elk were consistent with disseminated fungal granulomas and periosteal hyperostosis. This case presents the first description of hypertrophic osteopathy in elk. The source of infection was undetermined, but inhalation of spores from contaminated feed or bedding was suspected.
Two yearling bull elk (Cervus elaphus) were presented to the Purdue University Veterinary Teaching Hospital (West Lafayette, IN) for a primary complaint of decreased appetite, weight loss, and lameness. The elk were born on a commercial farm, with approximately 385 elk raised for meat, velvet, and breeding. Both elk were genetically related and had been in the same pasture since weaning. There was no prior history of elk with similar clinical signs on the farm. Elk no. 1 had an acute onset of lameness of the left thoracic limb 3.5 months before presentation and eventually became recumbent. A few months before, both elk had a history of pelvic limb lameness, attributed to trauma in elk no. 1 and osteomyelitis and arthritis in elk no. 2. At the request of the owner, no treatment was pursued.
On presentation, elk no. 1 was in thin body condition. Severe lameness was noted in the left thoracic limb and left pelvic limb. Elk no. 2 was in fair body condition, with no observable lameness. Firm swelling was present over the dorsal aspect of all distal limbs in both animals. Results of a complete blood cell count and chemistry panel showed a mild anemia, increased serum total protein with hyper-globulinemia and hyperfibrinogenemia in both elk, consistent with chronic inflammation. Radiographic findings were similar in both animals and consisted of well organized, palisade-like periosteal proliferation along the metatarsal bones and the calcaneum of both hind limbs. Similar radiographic changes were also present in both metacarpi and distal radius. None of the joint spaces were affected. These changes were consistent with hypertrophic osteopathy. Thoracic radiographs revealed multiple discrete nodules throughout the lungs. Differential diagnoses included pulmonary abscesses, granulomas, and cysts. Metastatic neoplasia was possible but was deemed less likely because of the young age and number of animals affected. Bacterial and fungal agents, as well as mycobacteria (Mycobacterium bovis), were suspected.
Cytologic evaluation of the tracheal wash fluid from elk no. 1 revealed a predominance of degenerate neutrophils. Phagocytized bacteria were frequently noted. Cocci, rods, chain-forming rods, and linear arrays of rod-shaped bacteria were observed. These findings were consistent with septic suppurative inflammation. Cytologic evaluation of the tracheal wash fluid from elk no. 2 revealed a predominance of nondegenerate neutrophils, without clear evidence of sepsis, consistent with mild neutrophilic inflammation. Acid-fast stains on both tracheal wash samples were negative. Because of the poor prognosis for recovery, both elk were euthanized and submitted for necropsy.
At necropsy, one retropharyngeal and the left prescapular lymph node of elk no. 1 were markedly enlarged and contained caseous material (Fig. 1). Both elk had multiple variably sized encapsulated nodules throughout the pulmonary parenchyma and multifocal fibrous pleural adhesions in the thoracic cavity (Fig. 2). On cut section, the nodules were filled with caseous or liquefied necrotic material. Smaller nodules, but similar in appearance, were observed in the left ventricular myocardium of elk no. 1; there were also yellow nodules and irregular tracts in the renal parenchyma of elk no. 1. Other noticeable lesions in elk no. 1 were ulcerative glossitis, rumenitis, and interstitial nephritis. Major long bones from all 4 extremities were dissected and cut longitudinally and transversally. Two different types of bone lesions were present in both elk. Extensive thickening of the radius, ulna, metacarpal, and metatarsal bones was from expansion of periosteal bone in both elk and was consistent with hyperostosis. The dorsal portion of the bones was more severely affected than the palmar portion (Fig. 3). The second type of bone lesion was characterized by tan and softened areas in the left metatarsus, left tarsal bone, left hind third phalanx (P3; elk no. 1) and metatarsus and rear left P3 (elk no. 2), associated with loss of cortical bone.
Histopathologic examination of formalin-fixed, paraffin-embedded tissues revealed a pyogranulomatous reaction in the nodules present in the lung, lymph node, kidney, and heart (elk no. 1) and in lung (elk no. 2). The center of these nodules had extensive and partially mineralized case-oliquefactive necrosis surrounded by a thick and irregular band of degenerate neutrophils, eosinophils, epithelioid macrophages, and fewer multinucleate giant cells, and an outer capsule of dense collagen (Fig. 4). In the lungs, heart, and kidney, these granulomas contained myriads of tangled hyphae. Hyphae had homogeneous thickness of 4–6 μm and were septate with dichotomous branching (Fig. 5). The center of lymph-node granulomas contained 20 μm in diameter spherules with thick refringent walls were present within these necrotic centers and multinucleate cells. These spherules were filled with pale or brightly eosinophilic material separated from the cell wall by a clear halo. Caseogranulomas were also present in the rumen and tongue, but fungal hyphae were not apparent. Bone lesions consistent with hyperostosis were characterized by expansion of the periosteum by deposition of irregular trabeculae of woven bone perpendicularly oriented to the surface (Fig. 6). The microscopic appearance of caseogranulomas in the bone was similar to other organs and also contained hyphae that was morphologically similar to those in the lung. The morphologic diagnosis was disseminated granulomatous inflammation with intralesional fungal organisms.
Bacteriology of the lung revealed no significant growth by aerobic culture and was negative by Haemophilus and Actinobacillus culture. Mycobacterial cultures performed at the National Veterinary Service Laboratory (Ames, IA) were negative. Bovine viral diarrhea virus and Bovine herpesvirus 1 (also known as Infectious bovine rhinotracheitis virus) were not detected by fluorescent antibody test, and no virus was isolated from the tongue and spleen pool. The hyphae in the lung and fungal structures in the lymph nodes were strongly labeled with antibodies to Aspergillus spp. a by immunohistochemistry (Fig. 7). 14 Based on histologic, microbiologic, and immunohistochemical results, a diagnosis of disseminated aspergillosis with hypertrophic osteopathy (HO) was made in both elk.
To the authors' knowledge, these are the first reported cases in the literature of HO in farmed elk. Hypertrophic osteopathy, or Marie-Bamberger's disease, is a symmetric periosteal proliferation of new bone, primarily localized to the phalanges and terminal epiphyses of long bones.
Prescapular lymph node, elk 1. This node is expanded by caseous exudate (C). Bar = 2 cm.
Lung, elk 2. Multiple nodules are present throughout the pulmonary field. Bar = 5 cm.
Metatarsus, elk 1. Hyperostosis (H) decreases in severity from proximal (upper left corner) to distal (lower right corner). M = marrow cavity; C = cortical bone. Bar = 2 cm.
Lung, elk 1. Caseous granuloma. N = necrotic center; I = band of leukocytes admixed with fungal hyphae; C = capsule. Hematoxylin and eosin. Bar = 700 μm.
Lung, elk 1. Detail of hyphae with parallel walls, septation, and dichotomous branching. Gomori methenamine silver stain. Bar = 70 μm.
Metatarsus, elk 2. Immature trabeculae of bone are oriented perpendicularly to the periosteal surface (P). Hematoxylin and eosin. Bar = 280 μm.
Lung, elk 1. Most hyphae are labeled with antibody to Aspergillus spp. Immunoperoxidase-diaminobenzidine. Bar = 70 μm.
Arthropathy is seldom observed, because the bony proliferation rarely involves articular surfaces. Numerous case reports of HO in large animals have been published, most commonly in equids. 10 Most cases in horses were associated with intrathoracic pathology; however, HO secondary to ovarian neoplasia, pituitary adenoma, and pregnancy 9 also were identified. Hypertrophic osteopathy secondary to intrathoracic disease was reported in a cow, 11 a steer, 12 an alpaca, 2 and a roe deer. 15 Intrathoracic lesions associated with HO include tuberculosis, primary lung tumors, metastatic lung tumors, pulmonary abscessation, pneumonia, and pulmonary infarction. 10,13 As in the present report, affected animals present with bilaterally symmetrical limb swellings that usually involves all limbs. Signs commonly associated with these changes include stiffness and/or lameness and weight loss. On radiographic examination, periosteal new bone formation over the diaphyses and metaphyses of the affected limbs is observed. Several hypotheses have been proposed as to the cause of the periosteal reaction. The primary disease process may result in an increase in peripheral blood flow and periosteal proliferation. This excess peripheral blood flow appears to be poorly oxygenated, passing through arteriovenous shunts and bypassing the capillary bed. This type of flow results in local passive congestion and poor tissue oxygenation, which stimulates the proliferation of various connective tissues. Soft-tissue proliferation, which is initially noted, is followed by the production of osteophytes that radiate from the cortices at 90°. 13 Because the majority of cases appear to occur in animals with pulmonary lesions, irritation of the periosteum and synovial membranes by circulating toxic products from the pulmonary lesion has been proposed as an alternative hypothesis. 7 A neurovascular mechanism that involves a reflex emanating from thoracic tumors and the nearby pleura, and is carried through efferent vagal fibers has also been postulated. Extrapulmonary lesions are thought to follow the distribution of the vasopharyngeal and vagus nerves. 1 These nerves carry fibers that innervate vascular tissues and have a common nucleus of termination in the gray matter that is closely associated with their efferent dorsal nuclei. This may account for a common afferent arc that forms a basic part of the mechanism responsible for osteoarthropathy. 1 Other factors proposed to be involved include hyperestrogenism, deficient oxygenation, and increased blood flow.4–6 The presence of severe pulmonary aspergillosis in these 2 elk most likely contributed to the development of HO. Aspergillosis has been found in wild and captive animals, usually associated with A. fumigatus. 3 Aspergillus spp. are opportunistic pathogens that cause serious disease in debilitated or immunocompromised animals and animals on prolonged antibiotic treatment. Transmission of Aspergillus occurs by inhalation of spores, which results in pneumonia. The lung is usually the site of primary infection, but invasion of blood vessels is common, which results in hematogenous spread to other organs. 8 In animals with clinical fungal pneumonia, compromise of cell-mediated immunity, usually secondary to an immuno-suppressive state, may allow the inhaled conidia to germinate and form hyphae that invade locally. In this situation, because neither of the animals was on prolonged antibiotic therapy, it is possible that an immunocompromised state resulted in the development of fungal pneumonia. A survey of 8 North American veterinary diagnostic pathology laboratories determined that pneumonia in general is the second leading known cause of death in all ages of elk. Furthermore, for elk between the ages of 30 to 365 days, pneumonia was the most common primary diagnosis. 16 In this retrospective study, pneumonia was associated with Aspergillus spp. in 6 of 43 primary diagnoses (13.9%) in the infectious disease category. However, none of these cases had signs or lesions of HO.
A unique feature of the cases reported in the present study is the simultaneous occurrence of 2 juvenile elk with HO. Case reports in the literature commonly describe single cases of HO. To the authors' knowledge, multiples cases at the same facility have not been reported. These 2 elk were most likely immunocompromised because of stress. In addition, because they were related, they may have been genetically immunodepressed. Inhalation of A. fumigatus from the hay source is the most likely cause of disseminated aspergillosis. This dissemination further suggests that the animals were in an immunocompromised state, combined with high exposure to fungal spores.
Footnotes
a.
Dako North America Inc., Carpinteria, CA.