Papillomaviruses (PV) have been isolated in humans, and a number of domestic and nondomestic species of animals.14,19 These viruses are relatively species specific and are associated with proliferative lesions of the skin and the mucous membranes. In most species, viral papillomas (VP) are benign, but in some the papillomas have been shown to transform to malignant tumors such as squamous cell carcinomas (SCC).10,18,19
Viral papillomas in felids are uncommon. Recently papillomas were restricted to the oral cavity of domestic cats (Felis domesticus), Asiatic lions (Panthera leo persica), snow leopards (U. uncia), Florida panthers (F. concolor), bobcats (F. rufus), clouded leopard (Neofelis nebulosa), but the lesions also have been found on the skin of domestic cats and snow leopards.2,3,8,11,16,17 Until recently, only in the domestic cat were these papillomas shown to be associated with malignant transformation.10
At Woodland Park Zoological Gardens (WPZ), five individual snow leopards have been identified with raised, thickened, irregular, black lesions on the skin, mainly on the face and forelimbs, ranging in size from 2–15 mm in diameter. In 1989, the first lesions were identified on a breeding male during a routine physical exam. Biopsy results identified the lesions as papillomas of likely viral etiology, and papillomavirus antigen was identified by immunohistochemistry (IHC). Viral induced papillomas were identified on the mate of this animal during a routine physical exam in 1992. In 1998, the breeding male developed multiple SCC on its feet and died due to septicemia that likely originated in the infected areas in the neoplastic foci. Papillomavirus antigen was also identified by IHC in one SCC from this male.
In 1996, two offspring of this pair that were sent to another institution developed SCC and were euthanatized. Additionally, another male snow leopard also housed at WPZ died of oral SCC in 1996.
All five of the snow leopards from WPZ that had VP, eventually developed SCC. Of these five, only one, a 16-year-old female mate, was diagnosed early enough to attempt a significant treatment based on treatment regimens used in domestic cats.1,12,13 On 20 November 1999 this female, which had papillomas identified in 1992, was diagnosed with early signs of SSC on her feet. These lesions appeared to be papillomas but on closer examination, the black, rough irregular surface of several lesions could be easily scraped off leaving a raw sore. In the usual papilloma lesions, the black surface was firmly adhered to the dermis and could not be scraped off. Biopsies of these lesions showed transition zones of SCC arising from VP.
Our treatment of this animal consisted of laser surgery for the SCC on 15 December 1999 with follow up examinations to check for SCC on 14 January and 4 April 2000. At these exams, there was only one small area suspicious of an early SCC, which on histology was confirmed to be a papilloma. Any papilloma lesion that was found was removed by electrocautery or by surgical excision. On 14 May the animal was examined for anorexia and lethargy and was euthanatized due to kidney failure. There was no evidence of SSC found on necropsy and there were a few small, 2–5 mm, papilloma lesions found on the forearms and head of this animal.
Concurrently, in 1998, as a follow up to a survey done in 1987, a request for medical records for snow leopards for the last 10 years was sent to all holders of Snow Leopard Species Survival Plan (SSP) animals (76 institutions), as part of the SSP medical review. Medical records were reviewed from 66 institutions (an 87% return) for 424 animals of which 230 had died. Twenty-one individuals died with SCC. This accounted for 9% of the mortalities in the survey. In addition, there were two snow leopards identified with oral papillomas, one of these leopards is a new animal at WPZ that has not developed skin lesions. Oral papillomas were similar to those described in other exotic felids16,17 and are pale, small (1–5 mm) nodules most commonly found under the tongue. In addition, oral papillomas have been documented in two snow leopards in a zoo in Russia.16
Cutaneous papillomas have repeatedly been found by immunohistochemistry to be positive for papilloma group specific antigen. In addition, the epidemiology of the infection is being evaluated. Papillomas from the treated affected snow leopard are being probed for viral DNA.
Presently, the authors recommend that all snow leopards be examined opportunistically for evidence of papilloma or SCC on the skin and in the oral cavity. Larger lesions should be removed by surgical excision, laser surgery, or cryosurgery. Smaller lesions can be ablated by cryosurgery. If laser surgery is used to remove larger lesions, care should be taken that the plume from the surgical site is vacuumed off, otherwise the viral particles in this smoke are still viable and will seed the surrounding skin with PV causing further papillomas.4 Cauterizing the papillomas with electrocautery is not recommended since the papillomas, which were ablated with electrocautery, appeared in this case to also have seeded the surrounding skin causing the production of more papillomas circling the site of the ablation. Any non-healing wounds and oral swellings should be biopsied to rule out SCC. One should be especially suspicious of an early transformation of the papillomas to a SCC if the raised thickened pigmented tissue can be easily scraped off leaving an ulcer, since with the typical papilloma, the raised area is firmly attached to the underlying dermis. Surgical biopsies of these early lesions will confirm the diagnosis of SCC. Once a diagnosis of SCC is made, aggressive surgical excision should be performed on all suspicious lesions. Follow up examinations should be made at 6-week intervals to evaluate for further tumor development. Unfortunately, as indicated in the medical survey, the prognosis for these cases is poor.
It is because of this poor prognosis that the authors are investigating the possibility of developing a recombinant vaccine for snow leopards in order to control the spread of this virus in the captive population worldwide. Recombinant papilloma virus vaccines have been developed by two of the authors, Jenson and Ghim, against carcinogenic and non-carcinogenic human papillomavirus, bovine fibropapilloma virus, equine papillomas, and canine oral papillomavirus.5-7,9,15,19-21
Identification of papillomavirus in Asiatic lions and in pygmy chimpanzees (Pan paniscus) have excluded them from being brought into the United States and being represented in the Species Survival Plans for these species (A.B. Jenson, pers. comm.). This study could be used as a model for viral infection in other cats and thus ultimately help these other species, some of which are also highly endangered.
This work was supported by a grant from the Woodland Park Zoological Society. We thank all of the veterinarians at institutions holding snow leopards who have supplied us with copies of their medical records and biopsy specimens, Histology Consulting Service for histopathology slide preparation, Dan Wharton and Evon Hekkala for their assistance with the epidemiology analysis, Barbara Powers, DVM, PhD, DACVP from Colorado State College of Veterinary Medicine who supplied pathology specimens, Susan Walls for her assistance with this manuscript and the following staff at WPZ who assisted in the care of these animals: Helen Shewman, Peter McLane, Heidi Frohring, Debbera Stecher, James Scott, Amy Brandt, Roz Sealy, Linda Shipe, Harmony Frazier, Carol Strickland, Kathe Rasler, Neal Duncan, Cheryl Clarke, Mike Waller, Bruce Bohmke, and Lee Werle.
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