A case of polymelia associated with syndactyly in Didelphis aurita (Wied-Neuwied,

2020 (With 7 figures) Abstract In January 2018, a young male of Didelphis aurita (Wied-Neuwied, 1826), the Brazilian common opossum, was admitted for veterinary evaluation at the Recovery Clinic of Wild Animals of Estácio de Sá University, CRAS, in Rio de Janeiro, Brazil, due to the presence of a partially duplicated right hind limb. After x-ray evaluation, polymelia associated with syndactyly was diagnosed. The images revealed the following alterations on the right limb: single femur with increased diameter, two tibiae, two fibulae and absence of hallux. The observed malformations were restricted to the hind right limb, which remains functional. This is the first report of such anomalies in an individual


Introduction
Rio de Janeiro is a city particularly famous for its natural beauties. Considering the environment of its metropolitan region, many officially protected areas are continuously threatened by the expanding surrounding limits of densely populated urban areas. This condition predisposes the frequent contact of many wildlife species with the local population (Ditchkoff et al., 2006). Some of these encounters are harmful for the local fauna because many people do not know how to deal with such animals. For this reason, the local veterinary practice should be aware of the most common diseases afflicting wildlife, as well as any emerging pathology. The knowledge of such conditions can be essential to mitigate its effects, aiming at the welfare of both wildlife individuals and local human population. Finally, the understanding of the etiology of these pathologies can help planning reintroduction of rehabilitated individuals in the wild, contributing to the preservation of local biodiversity (Gonçalves et al., 2016).
The Brazilian common opossum, Didelphis aurita (Wied-Neuwied, 1826), is part of the wildlife biodiversity of Brazil. It occurs in the Araucaria and Atlantic Forests, the latter forest being the predominant original vegetation in the State of Rio de Janeiro. This species is listed as Least Concern by the IUCN classification (Astua de Moraes et al., 2015), the main reasons being mainly due to its wide distribution, presumed large population, tolerance to habitat modification, occurrence in several protected areas and because it is unlikely to be declining at nearly the rate required to qualify for listing in a threatened category (Astua de Moraes et al., 2015). This species is known to be locally hunted or trapped for food, sport and as predators of poultry. However, it does not appear to have been adversely affected by human settlement. Moreover, commercial hunting for the fur trade does not appear to have much impact in Didelphis aurita population (Astua de Moraes et al., 2015). Considering the wildlife in Rio de Janeiro, this species is one of the most frequently encountered by the local inhabitants (Bonvicino et al., 2002).
The aim or the present work is to report a case of polymelia associated with syndactyly in an individual of Didelphis aurita found in the State of Rio de Janeiro, Brazil. These pathologies are congenital defects (Retnam et al., 2016). To the knowledge of the authors, this is the first report of such anomalies in that species.

Case History
In January 2018, a young male of Didelphis aurita was admitted at the Rehabilitation Centre of University Estácio de Sá, CRAS, for veterinary evaluation due to the presence of a partially duplicated right hind limb (Figures 1 and 2). Case history included the death of the female parent and the release of the other youngsters, as they appeared to be normal. Regarding opossums, the existence of an extra limb could supposedly impair the animal's normal activities for survival. At the time of admission, this individual weighed 62 g and was 27 cm long.
The occurrence of one or more supernumerary limbs in one individual is a congenital defect designated as polymelia. It is quite common in animals although rarely reported in humans (Retnam et al., 2016). The extra limb may appear in different positions of the body and can be deformed or shrunken (Retnam et al., 2016). There are cases in which the limb is partially duplicated. According to the region of the body to which the extra limb is attached, a specific term is used to designate the location. In this way, polymelia is called cephalomelia if the limb is attached to the head, notomelia if attached to the backbone, thoracomelia if attached to the thorax and pygomelia if attached to the pelvis (Retnam et al., 2016).
The individual of Didelphis aurita affected by polymelia and syndactyly from this report was found in São Gonçalo, a county of the metropolitan region of the State of Rio de Janeiro. This area is particularly densely populated and human occupation is ever expanding (Santos and Pinto, 2006).
In São Gonçalo County, there is an official conservation unit called Área de Proteção Ambiental do Engenho Pequeno (APAEP), where opossums are found (Santos and Pinto, 2006). For this reason, though not informed, this animal with polymelia and syndactyly might have come from this conservation unit.
After a thorough physical examination of the accessory limb, however, two digits were notably fused, characterizing also a case of syndactyly associated with polymelia. Apart from the limb malformations, no other pathological condition was observed in the individual after clinical examination. The extra limb demonstrated to be quite functional (Figure 3). Complete blood counts were normal. The animal demonstrated normal appetite and activity.

Results
The patient was submitted to radiographic examination for bone development assessment and the occurrence of other malformations apart from the right hind limb duplication.
Results show (see Figures 4, 5 and 6) that bone malformation is apparently restricted to the right limb. The skull and other parts of the body seem to be normal. The following alterations were revealed in the x-ray evaluation of the right limb: single femur with increased diameter, two tibiae and two fibulae, two metatarsal bones of the caudal right toe are fused and complete absence of hallux. The cranial duplicated part of the right limb       articulates with the single femur. The caudal tibia and fibula, although functional, seem not to be part of the joint to the femur.
After three months, the animal has grown considerably. Its length has increased to 60 cm, including the tail, and its weight is presently 879 g. Another x-ray examination of the extra limb evaluated any change in bone development (Figure 7). No significant changes appeared when compared to the first images.

Discussion
Opossums have a relatively short lifespan. When living in the wild, their average survival time is 2 years, whereas in captivity this lifespan increases to 4 years (Gentile et al., 2000). For this reason, opossums offer an opportunity to study physiological alterations due to aging.
In the present case, in the near future, the double femur-tibial articulation will predispose the occurrence of osteoarthritis due to constant trauma. If osteoarthritis is present, pain will impair movement, condition with poor prognosis. Since the animal is clinically healthy up to now, sequential radiographic evaluations will enable detection of any alterations in bone or joint.
Opossums are very prolific, yielding average litters of above five individuals. Their gestation period is quite short, of approximately 15 days. In this way, they are frequently breeding which is also an explanation for their keeping a constant population in urban areas (Gentile et al., 2000). On the other hand, wildlife is vulnerable to many teratogenic substances produced by human activities. The occurrence of congenital defects, such as polymelia, may have a genetic or an environmental cause (Ditchkoff et al., 2006;Hussein et al., 2012). Gene disorders investigated through chromosome analysis will determine if the anomaly in the present case has a genetic origin. Chemical products from the environment can produce congenital anomalies (Hussein et al., 2012). This condition will certainly affect not only the local population, but also the coexisting wildlife in such places.
Surgical removal of the extra limb aims at improving the well-being of the patient and its cosmetic appearance. There are cases where the position of the accessory limb may result in constant injury by the animal's normal activities, being the indication of limb excision. However, surgery of animals with non-disturbing supernumerary digits is optional (Hussein et al., 2012). Since the individual of case hereby reported has not shown any sign of discomfort yet, the removal of the supernumerary limb is not considered. In this respect, the follow-up of bone development by sequenced diagnostic imaging is important in case this condition changes.

Concluding Remarks
This case is the first report of polymelia associated with syndactyly in Didelphis aurita. It may help understand how such pathologies progress and assess prognosis for patients carrying these anomalies.
Due to the short lifespan of the species, the study of polymelia and syndactyly in Didelphis aurita can bring insight to bone development and ageing in mammals.