Page 42 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 1 The Etiology of Cancer 21

beneficial, as has imiquimod cream (Aldara) applied topically
every 24 to 48 hours. 406 A new recombinant canine oral papil-
lomavirus vaccine (COPV) produced by Georgetown University
VetBooks.ir Medical center has been reported effective. The vaccine presents
the capsid protein L1 of CPV-1 and was given in six injections;
after the initial vaccination, three booster vaccines were given
every 2 weeks followed by two more boosters given monthly. 406
Corticosteroids should be avoided because they are thought to
contribute to papilloma dissemination. 293,409
Most systemic chemotherapeutic agents (e.g., bleomycin, vin-
cristine, cyclophosphamide, and doxorubicin) have failed to cause
regression of papillomas. However, the retinoid drug etretinate (1
mg/kg PO every 24 hours) has been effective in some dogs with
persistent papillomas. 294 Topical or intralesional compounds con-
taining 5-fluorouracil (5-FU) have been used in both humans and
• Fig. 1.11 Multiple cutaneous papillomatosis in the inguinal region of a dog. dogs to treat papillomas. 406 In the past, autologous wart vaccines
have been recommended, but they have proved of little value in
the treatment of resistant papillomatosis in the dog. 287 In at least
Canine genital papillomas are considered uncommon but one study papillomavirus vaccines have been shown to prevent the
venereal in nature and are characterized by raised papillomatous development of oral papillomas in the dog; however, cutaneous
plaques on the penis or vaginal mucosa. neoplasms at the injection sites have been attributed to adminis-
Canine footpad papilloma was first noted in young greyhounds tration of the vaccines. 285,295,296
but now is recognized in adult dogs and other breeds. It may be
present on multiple footpads, interdigital spaces, and on multiple
limbs. 289,406 Tumor-Causing Viruses of the Cat
Ocular papillomas, which are less numerous and less common Papillomaviruses
than the oral type, appear on the conjunctiva, cornea, and eyelid
margins (see Fig. 1.10). 287 Experimentally, viruses isolated from Feline viral papillomatosis is a less commonly recognized clinical
ocular lesions can produce oral papillomatosis, although whether condition caused by a papillomavirus specific to the cat. 409 The
this occurs in nature is unknown. 284 Ocular papillomatosis most virus is widespread in the cat population, and most infections
frequently occurs in dogs 6 months to 4 years of age, but it occa- are asymptomatic. 408 At least three papilloma viruses FcaPV-1,
sionally is reported in older dogs. -2, -3, have been recently sequenced for the domestic cat and
Although papillomatosis generally should be considered a are responsible for cutaneous plaques, bowenoid in situ carci-
benign disease, in rare cases oral and corneal papillomas have noma (Bowen disease), and cutaneous SCC. 403 These viruses
transformed into SCC. 281,282 are thought to be genomically similar to the canine isolates
but are considered species specific. 297 Bovine papillomavirus
Treatment BPV-14 is thought to be responsible for feline sarcoid. 403 Pap-
Most clinicians elect not to treat papillomatosis because of the lack illomavirus-associated lesions have been reported in six species
of proven efficacy of recommended treatments and the expected of felids besides the domestic cat: the mountain lion, Florida
spontaneous regression of these tumors; however, if the number panther, bobcat, Asian lion, snow leopard, and clouded leop-
of papillomas increases or if the animal has significant dysphagia, ard. 298,299 Unlike in the domestic cat, in which the lesions com-
owners often request treatment. Surgical excision, cryotherapy, monly affect areas of haired skin, papillomas in exotic species
laser surgery, radiation therapy, electrocautery, or surgery for just a most often are found in the oral cavity, similar to those in the
few lesions has resulted in regression of the remaining papillomas, dog. 298,302 Despite the clinical similarities, genetic and antigenic
presumably through immunologic mechanisms. 287,290,291 studies indicate that each species of felid is infected by a unique
If surgery is performed, seeding of the virus is a possible papillomavirus.
complication, resulting in lesion recurrence. The exact mecha-
nism by which regression of papillomas occurs is unknown. Pathogenesis
Serum from dogs in which papillomas have undergone spon- In cats, as in other species, papillomas are believed to develop
taneous regression not only fails to produce tumor regression after the virus is introduced through lesions or abrasions in the
when administered to infected animals, but may enhance exist- skin. Unlike in the dog most feline case reports involve older cats
ing tumor growth. This effect may be a result of induction of (6 to 13 years of age), although papillomavirus lesions have been
blocking antigen-antibody factors, which may impede cytotoxic reported in kittens as young as 6 to 7 months old. 300,301 As in
lymphocyte action on target cells. CD4 T cells activate macro- other species, impaired T-cell function likely plays a significant
phages and have been shown to inhibit the virus in dogs, and role in lesion formation. Papillomas in cats that are receiving
administration of immune lymphocytes from dogs in which immunosuppressive therapy or are infected with the feline immu-
tumors have regressed has been shown to enhance regression. 283 nodeficiency virus (FIV) support this hypothesis.
Interferon-α also has been tried therapeutically, with some Although papillomas most frequently are benign lesions, recent
2
success (1–3 million IU/m given subcutaneously three times studies have associated the papillomavirus with SCCs and other
a week [Monday-Wednesday-Friday]), and chemotherapy of malignant neoplasms in cats 302 ; specifically, papillomavirus has
resistant lesions has produced variable results. 292 Azithromycin been isolated from 30 of 63 SCC in situ skin lesions. Through
(5–10 mg/kg PO every 12 to 48 hours) has been reported to be the use of PCR techniques, papillomaviruses have been found in