Gross Findings: Doves have anisodactyl feet with four digits with digit I (hallux) facing backward and digits II–IV facing forward (Fig. 1A). In birds, the number of phalanges in digits I–IV is generally two, three, four, and five, respectively.  The last phalange on each toe is the claw or nail. On external examination of the Eurasian Collared-Dove, on both the left and right feet, the distal phalanges of digits I-IV are absent and the remaining distal tissue is bulbous (Figs. 1B-D).  Skin is dry and the tips of right and left digit II are pale.

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Condition: Frostbite.

Host range: Occurs more commonly in North America in non-native species in cold environments (e.g., European starlings [Sturnus vulgaris]).  Risk factors in wild birds include unseasonable weather, missed migration, and previous injury.

Distribution: Occurs especially in colder climates, but may occur with cold weather in southern climates.

Pathogenesis: When temperatures drop and a bird is unable to maintain its internal temperature, vasoconstriction occurs in the extremities to conserve heat in the internal organs.  Cycles of vasoconstriction and vasodilation known as the “hunting reflex” occur at first to prevent tissue death, but this reflex ceases as cooling continues. Tissue damage results from freezing and ischemia.  Freezing causes ice crystal formation in extracellular spaces which alters the osmotic gradient and causes intracellular dehydration.  Ice crystals also cause mechanical damage to cells.  Tissue ischemia occurs with vasoconstriction and is exacerbated by the release of inflammatory mediators that cause further vasoconstriction, platelet aggregation and thrombosis.  Ischemia can progress to necrosis and sloughing. Resulting pain from tissue injury may result in self-mutilation.

Gross findings: Frostbite in birds most often involves the feet and toes but can involve the wings.  Edema may be observed in the first 24 hours.  With recent injury, the tissue may be pale, dry, and avascular.  As time progresses, the affected tissue becomes necrotic with a line of demarcation separating injured from viable tissue.  Eventually, digits may self-amputate. 

Histologic findings: In experimental frostbite lesions in swine, the earliest reported change was vacuolation of keratinocytes in the epidermis, which was followed by spongiosis, necrosis and then separation of the necrotic tissue from the dermis.  Other early lesions included hemorrhage and hyperemia. Inflammation occurred within 6–48 hours. Within a week, arteriolar thrombosis occurred.  Regeneration of the epithelium was observed within two weeks.

Diagnosis: History, clinical signs, gross, and microscopic findings.

Wildlife population impacts: If a bird recovers, even when toes are lost to frostbite, walking and perching may still be possible.  Superficial frostbite affecting only skin and subcutaneous tissues has a better prognosis than deep frostbite that affects bones, joints, and tendons.

References:

• Abourachid A, Fabre A-C, Cornette R & Höfling E. 2017. Foot shape in arboreal birds: two morphological patterns for the same pincer-like tool. J Anat 231(1): 1–11. DOI:10.1111/joa.12614
• Martel-Arquette A, Mans C, & Sladky, K. 2016. Management of severe frostbite in a grey-headed parrot (Poicephalus fuscicollis suahelicus). J Avian Med Surg 30(1): 39–45. DOI:10.1647/2015-100
• Mauldin EA & Peters-Kennedy J. 2016. Integumentary System. In: Jubb, Kennedy & Palmer’s Pathology of Domestic Animals, 6th edition. Maxie, MG, editor. Elsevier, St. Louis, pp. 509–736.
• Wellehan, J. 2003. Frostbite in birds: pathophysiology and treatment. Compendium 25(10): 776–781.