Malignant Neoplasia of the Sex Skin in 2 Chimpanzees (Pan troglodytes)

This report describes 2 cases of spontaneous malignant neoplasia within the sex skin of aged female chimpanzees. In both cases, the initial presentation resembled nonhealing traumatic wounds to the sex skin, with different degrees of infection, ulceration, and tissue necrosis. Histopathology of the lesions confirmed the diagnosis of squamous cell carcinoma in one case and of adenocarcinoma with metastasis in the other. Advanced age and previous trauma likely contributed to the development of the neoplasias in both cases; long-term sun exposure may also have contributed to the development of the squamous cell carcinoma. To our knowledge, these 2 cases represent the first reports of sex skin neoplasia in chimpanzees.Abbreviation: SCC, squamous cell carcinomaThe term ‘sexual skin’ was first used in 1891 to describe the cyclically swelling skin on the buttocks and thighs of rhesus macaques.⁶ Since then, prominent sexual swellings have been described in multiple NHP species, including many cercopithecines, some colobines, and chimpanzees.^9-11,30 In chimpanzees, the sex skin is a thin, distensible, hairless, and hypopigmented region of skin that surrounds the vagina and extends dorsally around the anus.^27,34 Sex skin swelling during the menstrual cycle function to increase female attractiveness and stimulate male sexual arousal.^10,30,34 The onset of cyclical sex skin tumescence also is used to detect puberty in chimpanzees, which generally occurs in females between the ages of 8 and 11 y.²¹In NHP species that exhibit pronounced swellings, such as chimpanzees, there is often extensive variation in swelling size both among females and between the cycles of individual females.¹¹ Swelling increases progressively during the follicular phase of the estrous cycle, as the sex skin becomes smooth, shiny, and turgid and the color changes from dull pink to bright red.^6,34 Histologic examination of sex skin during the follicular phase reveals a thin stratum corneum and dilated dermal blood vessels.⁶ The skin remains at maximal turgescence for 5 to 6 d before the swelling begins to decrease during the luteal phase.³⁴ The bright color rapidly fades, the skin becomes flaccid and wrinkly and attains a scaly appearance due to a thickened stratum corneum.^6,34 There is then a resting phase before menstruation begins again, and during this stage, some females slough areas of epithelium.³² Traditionally, maximal sexual swelling (1 to 6 d prior to detumescence) has been an accepted marker to signal ovulation in chimpanzees.^2,21,30,34 However, this association is not certain, given that ovulation has been documented to occur at other times during the estrous cycle and in view of reports of sexual swelling during infertile times, such as adolescence and pregnancy.^2,9,26,35Although spontaneous neoplasia was once considered to be uncommon in NHP and especially in chimpanzees, cases are increasingly reported as captive primate colonies age.^3,828,31 A report of all neoplasia documented at 2 major chimpanzee facilities from inception through April 2008 and including both spontaneous and experimental tumors revealed that neoplasia was not uncommon in chimpanzees and that, although most tumors were benign, malignancies did occur.³ The incidence was much higher in female than male chimpanzees due to the high incidence of neoplasia in the female urogenital system, particularly uterine leiomyoma.³ However, in that report of 117 tumors, no sex skin or perineal neoplasms were documented.³ Sporadic spontaneous tumors of the perineal skin and subcutis in female baboons have been reported and include cases of squamous cell carcinoma, lipoma, spindle cell sarcoma, myxoma, myxofibroma, myxosarcoma, and benign angiomyxoid proliferation.^7,8Here we describe 2 cases of spontaneous malignant neoplasia within the sex skin of aged female chimpanzees. To our knowledge, this report represents the first documentation of sex skin neoplasia in chimpanzees.