Pathology Case of the Month - Red Wolf

Gross Findings: On external examination, the carcass was heavily scavenged with open holes in the ventral abdomen and anal area. The teeth were heavily worn and discolored pink. There were solitary, irregular, hard, nodular swellings on the medial surface of the distal front legs proximal to the carpal-metacarpal joint and on the medial surface of the distal hind legs at the level of the first proximal metatarsal. On internal examination, the abdominal organs were missing, and the diaphragm was torn. There was adequate subcutaneous, visceral, and epicardial fat. Moderate numbers of adult heartworms (Dirofilaria immitis) were present in the caudal vena cava, the right ventricle, and atria of the heart (Fig. 1A); the pulmonary artery and the left middle lung lobe. The hard, nodular swellings on the legs (Fig. 1B) were composed of bony proliferations as seen on radiographs (Fig. 1C) and gross examination (Figs. 1D). Histopathologic findings were not useful due to the autolyzed state of the carcass.

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Morphologic Diagnosis: Multifocal, moderate, subacute to chronic periosteal bone proliferation.

Disease: Hypertrophic osteopathy

Etiology: Hypertrophic osteopathy (HO) is an unusual and poorly understood condition in which bilaterally symmetrical proliferation of fibrous tissue and periosteal bone, usually on the appendicular skeleton, occurs secondary to other disease processes. Most commonly, the inciting condition involves a mass lesion in the thoracic cavity, often a malignant tumor of the lung. Tumors in the abdominal cavity may also be associated with HO, as well as a number of non-cancerous conditions such as abscesses, granulomas, heart disease, and even pregnancy. Cases of HO associated with heartworm disease appear to be uncommon as they are rarely described in the literature.

Distribution: Worldwide

Seasonality: Any time of year, as the underlying causes of this disease are highly variable and many have no seasonal distribution.    

Host range: Hypertrophic osteopathy has been reported in a wide range of mammals as well as in humans.

Clinical signs: Shifting leg lameness, swelling of the legs, and reluctance to move. Other signs may be present that are related to the primary cause.

Pathology: Bilaterally symmetric foci of periosteal and fibrous tissue proliferation on long bones of the appendicular skeleton. Proliferative bone formation starts distally on affected bones and progresses proximally over time. Periosteal bone proliferation occurs perpendicular to the shaft of the long bone.

Diagnosis: Compatible clinical findings along with typical radiographic findings diagnose HO in a live animal. At necropsy, confirmation of bilaterally symmetric periosteal bone formation on gross and histopathologic examination. A search for a primary cause of HO should be undertaken.

Public health concerns: None.

Wildlife population impacts: Hypertrophic osteopathy is an uncommon condition affecting individual animals and as such is not expected to have population impacts.

Management: Not applicable.

References:

Bergman PJ. 2013. Paraneoplastic syndromes. In: Withrow and MacEwen’s Small Animal Clinical Oncology, (Fifth Edition), Withrow SJ, Vail DM, Page RL, editor. Elsevier Publishing, St. Louis, Missouri, pp 83-97.

Allan G, Davies S. 2018. Radiographic signs of joint disease in dogs and cats. In: Textbook of Veterinary Diagnostic Radiology (Seventh Edition), Thrall DE, editor. Elsevier Publishing, St. Louis, Missouri, pp. 403-433.