Lysosomal storage diseases are generally due to decreased enzyme activity due to a defect in the gene coding for a specific lysosomal enzyme. Other defects have been discovered which include: synthesis of a catalytically inactive enzyme, failure to attach a mannose-6-phosphate “marker,” lack of an enzyme activator or protector protein, lack of a substrate activator protein, or a lack of a transport protein required for removal of the degradation product from the lysosome.2 The mode of inheritance is usually autosomal recessive. Decreased activity of the lysosomal enzyme causes an accumulation of cellular or extracellular degradation products that impair cellular structure and function. The central nervous system (CNS) is often affected and patients exhibit neurologic signs. Lysosomal storage diseases are classified by the primary stored material which can be determined by assay of lysosomal enzyme activity in white blood cells, serum, and tissues. Post-mortem diagnosis is by characteristic histologic changes, special stains, and ultrastructural analysis. Lysosomal storage diseases have been described in humans, non-human primates, dogs, cats, cattle, pigs, sheep, goats, shell parakeets, Hawaiian geese, emu, muntjac deer, and black bear.1
This report describes three cases of storage disease in captive-bred Costa’s hummingbirds (Calypte costae) at the Indianapolis Zoo. From 1997–1999 a breeding pair produced 10 offspring. The first two female offspring born in 1997 were transferred to another institution at 5 months of age. Three offspring of unknown sex disappeared while on exhibit and were presumed dead. Two offspring (one male and one female) are currently housed at the Indianapolis Zoo. Two males and one female died and had histologic changes consistent with a lysosomal storage disease. Hummingbird #1 was a 4.5-month-old male, which had been housed, in a holding enclosure for 3.5 months prior to death. Two weeks prior to death the bird was observed to be flying backwards, having trouble elevating, and swaying back and forth rhythmically while perching. The keepers reported that the bird had a history of flying as if in a wind tunnel. Physical exam was unremarkable. The neurologic signs progressed; the bird developed a head tilt and died despite antibiotic therapy. No gross lesions were found on necropsy. The whole carcass was placed in formalin. Histopathology revealed large numbers of cytoplasmic microvesicles associated with neuroaxonal degeneration and spheroid formation in the neurons of the brain, spinal cord, root ganglia, and optic tract. Microvesicular changes were also noted in the harderian and uropygial glands, and the inner nuclear layer of the retina. Some vacuoles stained faintly positive with Luxol fast blue stain suggesting the presence of some membrane material or myelin. Hummingbird #2 was a 2-week-old female that was found dead on a public pathway. It was suspected that the bird flew into a glass-ceiling panel. Gross necropsy revealed an area of hemorrhage over the cranium. Histologic changes consisted of acute hemorrhage of the liver and spinal cord as might occur with blunt trauma. There were microvesicles in the cytoplasm of the neurons in the spinal cord and brain. Some of the vacuoles stained faintly positive with Luxol fast blue stain. Hummingbird #3 was a 5-month-old male sibling of #2. At 4 months of age this bird was observed to be flying backwards and in circles. While perching the bird moved his head back and forth in a rhythmic motion. A video recorder documented the birds’ unusual flight pattern and head tremors. This bird was euthanatized 1 month following clinical presentation. Gross necropsy was unremarkable, and tissues were preserved in Trump’s medium and formalin. Histopathology findings included multiple clear microvesicles in the cytoplasm of several neuron cell bodies in the CNS, cells of the bone marrow, glandular epithelium of the esophagus, and macrophages of the spleen. Transmission electron microscopy disclosed neuronal intracytoplasmic bodies that were membrane-limited, of varying sizes, and containing material of differing structure, consistency, and appearance. Some bodies consisted primarily of lamellar material, sometimes in concentric myelin figure-type whorls and sometimes in lamellar stacks. Other bodies contained primarily electro-dense, granular material, whereas many of the bodies contained heterogenous mixtures of these two components. The ultrastructural appearance of these bodies is consistent with that observed in lysosomal storage diseases.3 Lysosomal storage disease should be considered in the differential diagnosis for neurologic disease in hummingbirds.
We acknowledge Histology Consulting Service for histology slide preparation.
1. King NW, Alroy J. Intracellular and extracellular depositions; degenerations. In: Jones TC, Hunt RD, King NW, eds. Veterinary Pathology. 6th ed. Baltimore, MD: Williams and Wilkins; 1997:6–7, 30–41.
2. Tager JM. Lysosomal storage diseases. In: Cotran RS, Kumar V, Robbins SL, eds. Robbins Pathologic Basis of Disease. 4th ed. Philadelphia, PA: W.B. Saunders Company; 1989:144.
3. McDowell EM. Fixation and processing. In: Trump BF, Jones RT, eds. Diagnostic Electron Microscopy. Vol. 1. John Wiley & Sons. New York and Toronto. 1978.