Macroscopic, Histologic, and Ultrastructural Lesions Associated With Avian Keratin Disorder in Black-Capped Chickadees (Poecile atricapillus)

Abstract

An epizootic of beak abnormalities (avian keratin disorder) was recently detected among wild birds in Alaska. Here we describe the gross, histologic, and ultrastructural features of the disease in 30 affected adult black-capped chickadees (Poecile atricapillus). Grossly, there was elongation of the rhamphotheca, with varying degrees of lateral deviation, crossing, and gapping between the upper and lower beak. Not uncommonly, the claws were overgrown and there was alopecia, scaling, and crusting of the skin. The most prominent histopathologic features in the beak included epidermal hyperplasia, hyperkeratosis, and core-like intrusions of necrotic debris. In affected birds, particularly those with moderate to severe beak overgrowth, there was remodeling of premaxillary and mandibular bones and various dermal lesions. Lesions analogous to those found in beaks were present in affected claws, indicating that this disorder may target both of these similar tissues. Mild to moderate hyperkeratosis occurred in other keratinized tissues, including skin, feather follicles, and, occasionally, sinus epithelium, but typically only in the presence of microbes. We did not find consistent evidence of a bacterial, fungal, or viral etiology for the beak lesions. The changes observed in affected birds did not correspond with any known avian diseases, suggesting a potentially novel hyperkeratotic disorder in wild birds.

Environmental contaminants and chromosomal damage associated with beak deformities in a resident North American passerine

“…In addition, the lack of microscopic lesions characteristic of toxic exposure also suggests that alternative hypotheses should be considered. Detailed histology of the liver and other organs commonly affected by some toxicants did not identify consistent abnormalities in these tissues .…”

“…Applied directly to eggs, PAHs have been shown to cause teratogenic effects, including beak defects, in aquatic birds, and they can have mutagenic and carcinogenic action, including alteration of genes involved in growth regulation (for review, see Albers [42] and Walker [43]). Such regulatory disruption is of particular interest because of the abnormally accelerated beak epidermal growth that we have experimentally and histologically confirmed in chickadees affected by avian keratin disorder [6,9].…”

“…Detailed histology of the liver and other organs commonly affected by some toxicants did not identify consistent abnormalities in these tissues [9].…”

“…Iron is an important micronutrient not only for avian immune response to pathogens but also a requirement for the pathogens themselves [28]. Thus, it is plausible that the reduced level of iron in livers of affected adults could have been related to mobilization of stores from the liver in a general elevated immune response of the affected host or to the depletion of stores by secondary infectious pathogens, which we commonly documented among chickadees with beak deformities [9]. Concentrations of selenium in liver tissues of adults and nestlings ranged from <0.26 mg/g to 6.7 mg/g dry weight, all less than 10 mg/g, which is considered the background level for this essential trace element [3].…”

“…These deformities compromise birds' ability to feed and preen and have important fitness consequences for affected individuals . Despite extensive testing for infectious diseases, genetic abnormalities, and dietary changes, the cause of this debilitating condition is currently unknown .…”