Diseases and medicine of birds of prey (Proceedings)

Apr 01, 2010

Historically, birds of prey have fascinated man, and are commonly held in captivity for conservation education, falconry, captive breeding, or rehabilitation purposes. Although the classification of birds of prey is somewhat controversial, in general these birds are divided into two major orders: Strigiformes (owls) and Falconiformes (hawks, eagles, kites, falcons, caracaras, condors, and vultures).

Nutritional Diseases

Metabolic Bone Disease

Raptors, like many other animals, require vitamin D3 for the normal absorption and utilization of calcium. Metabolic bone disease can result from the deficiency of vitamin D, an absolute calcium deficiency, or an inappropriate calcium:phosphorus ratio. The consequence of any of these dietary problems is nutritional secondary hyperparathyroidism.

There are several effects of prolonged hypocalcemia or calcium deficiency. The excessive production of PTH works to maintain serum calcium levels at the expense of the bone matrix. The skeletal effects are due to the improper mineralization of osteoid, which can lead to skeletal fragility, pathologic fractures, bone deformities, and fibrous osteodystrophy. Insufficient parathyroid response or prolonged hypoparathyroidism can result in serious declines of serum calcium levels leading to hypocalcemic tetany. Treatment of hypocalcemic tetany requires parenteral administration of calcium gluconate. Diazepam may be used if needed, although most birds generally respond to parenteral calcium gluconate. These birds should not be given corticosteroids as these drugs adversely affect calcium absorption and renal excretion. The bird should be placed in a quiet dark area during recovery. Correction of the underlying dietary deficiencies is paramount (i.e., whole carcasses of a variety of adult prey items closely resembling the birds natural diet should be fed).

Infectious Diseases

Avian tuberculosis

Mycobacterium avium or avian tuberculosis is a relatively important bacterial disease of raptors. Clinical signs associated with avian mycobacteriosis are highly variable and depend on the organ systems affected. M. avium can affect almost any system but is most commonly seen in the gastrointestinal tract, liver, and spleen in raptor species. The disease is usually chronic, resulting in a very thin, depressed bird. Although clinical signs can be variable, other consistent clinical signs in raptors infected with M. avian include wasting despite an excellent appetite, recurrent diarrhea, polyuria, anemia, and dull plumage. Lesions may also appear within bone marrow, joints, or muscle (particularly the muscles of the legs) resulting in shifting lameness, decreased use of a limb, and arthritis of affected joints. The organism is ingested by the host, causing lesions in the intestinal wall. These lesions then disseminate organisms to other organs, resulting in tubercles or granulomas. The intestinal tubercles may continuously shed organisms into the feces. Visceral tubercles, especially on the liver, are a primary finding at necropsy. Definitive diagnosis of M. avium infection can be difficult antemortem, although a markedly elevated WBC count, elevated liver enzymes, and radiographic evidence of masses or enlarged organs in a thin and depressed bird is suggestive. Presumptive diagnosis of avian tuberculosis can also be based on cytological evaluation of tissues/feces, with acid-fast stain. However, the presence of acid-fast organisms in the feces is not necessarily diagnostic of a mycobacterial infection. Liver biopsy (i.e., via endoscopic examination) of suspected birds may reveal early changes associated with M. avium. Diagnosis is confirmed by culturing of feces or affected organs, a procedure that requires a special medium and takes 4-6 weeks to grow. The presence of acid-fast rods in tissues or feces is highly suggestive of avian tuberculosis.

Treatment of raptors with human antituberculosis drugs is rarely successful and is seldom recommended. Birds with mycobacteriosis are generally euthanized because of the zoonotic potential in immunosuppressed people and to reduce or eliminate exposure to M. avium in the environment. In cases where treatment is attempted, a combination of drugs including isoniazid (30 mg/kg PO q24h), ethambutol (30 mg/kg PO q24h), rifampin (45 mg/kg PO q24h) may be used long term. Although the progression of the disease may be stopped, the infection usually remains and may be reactivated at any time.