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History: An adult female 7.57-g Little Brown Bat (Myotis lucifugus) was found dead in May in Montana, USA and was collected for screening for white-nose syndrome (WNS). 

Gross Findings

There were multiple foci of depigmentation on the patagia, the largest of which measured 3 mm in diameter (Fig. 1A).  Under ultraviolet light, there was a single pinpoint focus of orange fluorescence and multiple pinpoint foci of blue fluorescence on the left plagiopatagium that were not consistent with Pseudogymnoascus destructans, the causative agent of WNS. (Fig. 1B).  Compared to an age-matched control with long, rounded pinnae with smooth margins (Fig. 1C), pinnae in this case were truncated and the outer margins of both pinnae were irregular (Fig. 1D).  There were no palpable fractures.

Photographs from a Little Brown Bat (Myotis lucifugus) found dead in Montana, USA.
Figure 1. Photographs from a Little Brown Bat (Myotis lucifugus) found dead in Montana, USA (A, B, D). (A) Multiple foci of depigmentation (arrowheads) measuring up to 3 mm in diameter are present on the patagia. (B) Under ultraviolet light, there is a single pinpoint focus of orange fluorescence (arrowhead) and multiple pinpoint foci of blue fluorescence (arrow). (C) Unaffected age-matched control bats with long, rounded pinnae with smooth margins. (D) Affected bat with truncated pinnae with irregular margins.

Histopathological Findings

Pinnae of Little Brown Bats (M. lucifugus) are composed of the epidermis, dermis, and a central plate of elastic cartilage (Fig. 2A).  The epidermis is composed of multiple layers of keratinocytes and the stratum basale contains large numbers of melanocytes (Fig. 2B).  The dermis is composed of loose and dense irregular connective tissue and contains blood and lymphatic vessels, nerves, hairs, and sebaceous and apocrine glands (Fig 2B).  Hairs are more abundant on the convex or outer surface of the pinnae.  The matrix of the central plate of elastic cartilage is basophilic and contains chondrocytes in lacunae surrounded by the perichondrium (Fig. 2B).  The inner layer of the perichondrium is chondrogenic with cells capable of becoming chondroblasts.  In the bat with the square-eared anomaly, the epidermis is lightly pigmented, dermal collagen is replaced by edema fluid, hairs and glands are reduced, and the cartilage plate is eosinophilic (Fig. 2C).  Compared to the unaffected bat, there are far fewer melanocytes in the stratum basale of the affected bat (Fig. 2D).  Connective tissue fibers are not evident in the dermis, which is expanded by eosinophilic, amorphous material (edema fluid) and low numbers of mononuclear inflammatory cells (Fig. 2D).  The elastic collagen plate is eosinophilic and lacunae are mostly devoid of chondrocytes or contain degenerating chondrocytes with vacuolated cytoplasm (Fig. 2E).

Photomicrographs from little brown bats, one with square-eared anomaly.
Figure 2. Photomicrographs from an unaffected Little Brown Bat (M. lucifugus; A-B) and Little Brown Bat found dead in Montana, USA with the square-eared anomaly (C-E). Periodic Acid Schiff-Hematoxylin. (A) Pinna of an unaffected Little Brown Bat showing the epidermis (e and arrow), dermis (d) and central elastic cartilage plate (c). (B) Higher magnification of the pinna showing the large number of melanocytes in the stratum basale (arrowhead), the dermis with adnexa (d), and central basophilic elastic cartilage plate composed of chondrocytes (caret) in lacunae (arrow) surrounded by the perichondrium (p). (C) Pinna of a Little Brown Bat with square-eared anomaly showing lightly pigmented epidermis (e and arrow), dermis with no evident collagen fibers and reduced adnexa (d), low numbers of adipocytes (arrow), and eosinophilic elastic collagen plate (c). (D) Higher magnification of the epidermis and dermis showing low numbers of melanocytes in the epidermis (arrowhead) and eosinophilic, amorphous material (edema fluid, asterisk) in the dermis along with low numbers of mononuclear inflammatory cells (arrow). (E) Higher magnification of the eosinophilic elastic cartilage plate showing empty lacunae (arrow) or lacunae with degenerating chondrocytes with vacuolated cytoplasm (arrowhead).

Disease: “Square-eared anomaly.”

Cause: Suspect frostbite. 

Host range: The square-eared anomaly has been reported in Little Brown Bats (Myotis lucifugus), Indiana Bats (M. sodalis), Keen’s Myotis (M. keenii), and Grey Bats (M. grisescens).

Distribution: Worldwide, in cold areas when temperatures are <32F.

Pathogenesis: Damage from frostbite occurs by direct cellular damage and from the effects of vascular inflammation, thrombosis, and ischemia.  Cycles of vasoconstriction and vasodilation occur when a tissue is first exposed to cold.  As heat loss continues and ice crystals form in extracellular spaces, the osmotic pressure increases and causes water to move out of cellular spaces which results in dehydration, hyperosmolality, decreased pH, and the denaturation of lipids and proteins.  Ultimately, this results in damage to cell membranes and leads to necrosis.  Cell membranes can also be mechanically damaged by ice crystals.  Release of inflammatory mediators occurs and leads to thrombosis, ischemia and cell death.

Gross findings: Symmetrical or asymmetrical up to 50% reduction or abbreviation of pinnae in one or both ears giving a “square-eared” appearance.

Microscopic findings: There is hyperplasia and dysplasia of the auricular cartilage.  Chondrocyte lacunae are often empty or filled with debris. Chondrocytes are degenerate with vacuolization or pyknosis.  The matrix of the cartilage is eosinophilic with reduced or absent elastic fibers. Mature dermal collagen is replaced with short, interwoven collagen fibers.  Lymphocytes and macrophages are present in the dermis.  Adipose tissue is reduced.  Hair follicles and sebaceous glands may be absent.  Melanocytes are reduced.

Diagnosis: Gross and microscopic findings of square-eared anomaly coupled with a history of freezing temperatures are suggestive of a diagnosis of frostbite.

Wildlife population impacts: The reported incidence (1%) in Little Brown Bats is low and the anomaly does not result in mortality.


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