Anatomical, histological and computed tomography comparisons of the eye and adnexa of crab-eating fox (Cerdocyon thous) to domestic dogs

Abstract

An understanding of species' morphological and physiological parameters is crucial to developing conservation strategies for wild animals kept in human care. Detailed information is lacking for crab-eating fox (Cerdocyon thous) eyes and adnexa. Therefore, the aim of this study was to describe anatomical, histological and computed tomography (CT) features of the eye and adnexa in crab-eating fox, compared to domestic dogs. CT of the eye and adnexa of one live animal and a frozen specimen was performed for anatomical identification. In addition, the heads of five animals of each species were fixed in 10% buffered formalin for gross anatomical description of the eye and adnexa using topographic dissection and exenteration techniques. All steps were photographed and features such as location, shape, and distances and relationships between structures were described. For histological evaluation, two eyes of each species were fixed in 10% buffered formalin, processed by routine paraffin inclusion technique and stained with hematoxylin and eosin. The CT scan was difficult to evaluate, mainly that of the frozen head, which did not provide good definition of the soft tissues; nevertheless, it demonstrated the potential for structure visualization and description. The gross anatomical and histological evaluations showed the presence of eyelashes on the upper eyelid and of upper and lower lacrimal points, an incomplete orbit with supraorbital ligament, slightly exposed sclera with discretely pigmented limbus and pigmentation throughout the conjunctiva, and a slit-shaped pupil. Hematoxylin and eosin staining demonstrated structural similarities between the crab-eating fox and domestic dog. Thus, the possibility of using the domestic dog as a study model for the preventive and therapeutic management of wild dogs kept in human care is demonstrated.

Computed tomography and cross‐sectional anatomy of the head in the red fox (Vulpes vulpes)

“…Therefore, infection may spread from these cavities to the orbital structures (Singh, 2018). In agreement to our results, the orbital rim of the crab‐eating fox is incomplete dorsolaterally (Lantyer‐Araujo et al., 2019). Consequently, the orbit is accessible for surgery without bone resection (Singh, 2018).…”

Morphology and Histology of the Orbital Region and Eye of the Asiatic Black Bear (Ursus thibetanus)—Similarities and Differences within the Caniformia Suborder

“…The obtained results of the eyeball macroscopic measurements performed in the examined Asiatic black bear were similar to the brown bear, American black bear Ursus americanus , grizzly bear Ursus arctos horribilis and sloth bear Melursus ursinus [ 26 , 66 ], but also they were similar to the measurements made on some representatives of the Canidae family (crab-eating fox Cerdocyon thous , Arctic fox Alopex lagopus and bush dog Speothos venaticus ) compared to mongrel dogs, South African painted dog Lycaon pictus , wolves Canis lupus , gray fox Urocyon cinereoargenteus and red fox Vulpes , where the eyeball measurements were characterized by much higher parameters [ 25 , 66 , 67 , 68 ]. However, some species of Mustelidae and the only representative of Ailuridae red panda Ailurus fulgens [ 25 , 66 , 69 ] are characterized by small and medium dimensions of the eyeball compared to Ursidae and Canidae.…”

“…In the examined terrestrial caniforms, the pupil was characterized by different shapes and for example, the pupil in our examined Asiatic black bear was big and round in shape, similar to that of other bear species subfamily Tremarctinae and Ursinae and also in the domestic dog and in some wild Canidae ( Chrysocyon brachyurus , Canis lupus , Lycaon pictus pictus ) [ 68 , 96 ], and according to Kirbas Dogan et al, 2020 [ 26 ], the brown bear Ursus arctos horribilis the pupil had a wider temporonasal length, while vertically slit-shaped in the giant panda, Cerdocyon thous and Vulpes [ 3 , 67 , 86 , 96 , 97 ] and in North American racoon Procyon lotor as nocturnal animals the pupil is wide [ 98 ]. Johnson, 1901 [ 24 ] reported that the pupil in Ursidae is round except Ursus americanus , in which it is vertically oval, dilatating to a full circle.…”

“…Murphy et al, 2012 [ 105 ], reported that in the domestic dogs, the tapetal coloration, as well as the degree of pigmentation present in the non-tapetal region of the fundus, is correlated with coat color (e.g., dogs with brown and red coat colors had a more orange-tinted tapetal color while dogs with white or gray coats more often had a green-colored tapetal region) but also the tapetal size was in some cases correlated with breed with the smaller breeds often having proportionally smaller areas of the fundus occupied by tapetum. In the case of wild Canidae, a large color variation was also observed of this tapetum lucidum, not only between species, but also within the same species (green or yellow with a green border in the crab-eating fox, yellow with a green border in the maned wolf, or milky in the South African painted dog) [ 67 , 68 , 97 ] and, for example, on the kinkajou Cercoleptes caudivolvulus (Procyonidae), the tapetum lucidum is a yellowish-green color, while on the ringtailed coati Nasua rufa (Procyonidae) it is bright green with yellowish patches [ 24 ]. The variation in the color of the tapetum lucidum can also be observed in semiaquatic mammals; for example, in the sea otter, it has a semicircular shape and the central tapetal area is blue-green and lightly mottled with yellow, while the intermedia area is yellow heavily mottled with orange [ 99 ], while in the California sea lion it was green [ 91 ].…”

“…The lens of the eye is soft, transparent and was protein-rich [ 105 ]. In the examined Asiatic black bear, the lens was biconvex round like in the sloth bear and in domestic and wild Canidae structure, where it is suspended in contact with the posterior face of the iris and the anterior face of the vitreous body [ 33 , 67 , 68 , 105 ]. Its function is to bring images into critical focus on the photoreceptor layer of the retina.…”