Abstract

Cownose stingrays (Rhinoptera bonasus) are commonly housed in public aquaria. Little is known, however, about their reproductive cycle and reproductive diseases.^1–3

An adult, female, cownose stingray presented for a routine monthly examination including a coelomic ultrasound. A large, heterogeneously hyperechoic mass with many variably sized and shaped anechoic cavitations was detected on ultrasound. The mass started on the left dorsum of the cranial to mid-coelom extending caudo-ventrally, filling approximately 40% of the coelomic cavity. Approximately one month prior, the animal gave birth to a healthy stingray pup. The animal had no associated clinical signs and follow-up ultrasounds every two to three weeks showed progressive enlargement of the mass over a five-to-six-week period. Six weeks after the initial diagnosis of the mass, the animal became acutely anorexic and profoundly lethargic with disseminated, patchy discoloration of the skin. The animal was humanely euthanized, and a necropsy was performed.

The necropsy revealed two to three, small, erosive skin lesions on the ventrum as well as monogeneans on skin scrape. The coelomic cavity contained approximately 600 mL of yellow-red-tinged, clear fluid, and an approximately 20.0 cm x 12.0 cm x 3.5 cm cylindrically shaped, pink to dark purple, edematous, polycystic mass that filled approximately 75% of the coelomic cavity. The liver was small and dark brown. The reproductive tract consisted of bilateral nidamental glands, and bilateral uteruses. A mummified fetus was identified at the opening to the left uterus. The kidneys were adhered to the dorsal aspect of the reproductive tract and dorsally underlying tissues were severely edematous.

Histologically, the coelomic mass was well demarcated, thinly encapsulated, and composed of numerous, variably sized cystic structures filled with clear fluid admixed with various amounts of fibrin. The cystic structures were primarily lined by a single layer of thin, attenuated epithelial cells but occasionally were lined by tall columnar epithelial cells that contained a moderate amount of foamy cytoplasm and eosinophilic globules. A large oocyte was identified embedded among the cysts with multiple small oocytes seen along the periphery. Based on the histological characteristics, the mass was diagnosed as polycystic ovary. Additional microscopic findings included moderate conus arteriosus mineralization, moderate nephrocalcinosis, severe hepatic and pancreatic atrophy with hepatic iron accumulation, and moderate to severe necrotizing and histiocytic dermatitis.

Reproductive diseases in stingrays under managed care are poorly understood with few reports in the literature. Reported conditions include dystocia, pyometra, mucometra, and polycystic ovary.^1–3 Dystocia is more commonly noted in cownose stingrays because physical signs of labor are not always evident.³ Polycystic ovary and mucometra are reported most in southern stingrays (Dasyatis americana). It is thought that southern stingrays are induced ovulators, thus contributing to the potential for polycystic ovary and mucometra when copulation does not occur.³ Current treatment for polycystic ovary is limited to ovariectomy. It is not known if cownose stingrays are induced ovulators, and thus, more information is needed to understand the etiopathogenesis of polycystic ovary formation and for the development of treatment and preventive methods.

Literature Cited

1.  Garner MM. 2013. A retrospective study of disease in elasmobranchs. Vet Pathol 50:377–389.

2.  Mylniczenko ND. 2011. Medical Management of Rays. In: Miller RE, Fowler M, editors. Fowler’s zoo and wild animal medicine current therapy, volume 7. St. Louis (MO): Saunders. p 170–176.

3.  George RH, Steeil J, Baine K. 2017. Diagnosis and treatment of common reproductive problems in elasmobranchs. In: Smith M, Warmolts D, Thoney D, Hueter R, Murray M, Ezcurra J, editors. The elasmobranch husbandry manual II: recent advances in the care of sharks, rays and their relatives. Columbus (OH): Ohio Biological Survey. p 357–362.