Page 43 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

22 PART I The Biology and Pathogenesis of Cancer

17 of 19 and 9 of 12 fibropapillomas in cats. 303,304 Although a of the feline leukemia vaccine in 1985, the incidence of lymphoma
cause and effect relationship has yet to be proved for carcinoma in cats in the United States and Europe has markedly declined;
however, along with the reduction in FeLV-related disease, there
in situ, Bowen disease, fibropapillomas, and papillomaviruses, the
VetBooks.ir evidence is compelling. 302–304 has been a rise in FeLV vaccine injection site tumors. 407 The cat is
believed to be affected by the largest number of retroviruses of any
Bovine papillomaviruses may also play a role in the pathogen-
esis of feline cutaneous fibropapilloma (sarcoid). In a study of 20 companion animal, and these viruses produce a wide spectrum of
cats with fibropapillomas, more than half were known to have diseases, including cancer. 308–310
exposure to cattle, and all were within an area with dairy farms. 303 The cat has both endogenous and exogenous retroviruses. The
In one isolate the nucleotide sequence was similar to that of the endogenous retroviruses generally are considered nonpathogenic,
bovine papillomavirus. Injection of that isolate back into bovine are present in the host DNA, and are passed from generation
skin resulted in asymptomatic infection. to generation genetically, as are other chromosomal genes. The
exogenous retroviruses include both pathogenic and nonpatho-
Clinical Features genic viruses and are passed horizontally and vertically between
Lesions in the cat differ from those in the dog because they are cats. Pathogenic exogenous retroviruses include FeLV and FIV. 311
more like plaques than warts. 406 The plaques are several milli- The exogenous RNA sequences of FeLV play the most important
meters in diameter, may be white or pigmented, and are scaly or role in tumorigenesis in the cat. 310 Another pathogenic retrovirus,
greasy. Lesions in the cat usually affect haired skin. Thought to be the feline sarcoma virus (FeSV), arises from the recombination
associated with grooming activity, lesions on the ventral tongue of exogenous FeLV and proto-oncogenes in the cat’s genome. 312
surface that are sessile in character also occur. 298,302 Cutaneous Feline syncytium-forming virus (FeSFV), also called the feline
viral plaques and bowenoid in situ carcinomas (BISCs) are caused foamy virus, is a nonpathogenic exogenous retrovirus. 306
by FcaPV-2; the former are a precursor to BISC. Some of these FeLV is believed to have been contracted from the ancestral
lesions progress to SCC, with spread to lymph nodes and lungs. rat approximately 10 million years ago. 313 The ancestral source of
Sarcoids occur in young cats and appear on the head, neck and other retroviruses is unknown.
digits.
Diagnosis Feline Leukemia Virus
Definitive diagnosis depends on histopathologic, immunohis- The retrovirus FeLV belongs to the subfamily Oncornavirinae, or
tochemical (IHC), or electron microscopic (EM) examination tumor-producing RNA viruses. As do other retroviruses, it has a
of excised lesions. Histologic features include proliferation of single strand of RNA and an enzyme, reverse transcriptase (RT),
all cell layers with little or no inflammation. Typically, epider- which synthesizes DNA from the virus RNA template. Nondo-
mal hyperplasia is accompanied by acanthosis, hypergranulosis, mestic felids, including the cheetah and bobcat, can be infected
hyperkeratosis, and ballooning degeneration of cells of the stra- by FeLV; however, it is considered enzootic in wild felids, and in
tum spinosum and stratum granulosum. Amphiphilic cytoplasmic European wild cats in France and Scotland. It has been reported
inclusion structures may be present in cells of the upper stratum in mountain lions in the United States. 314,315
granulosum. EM findings in the lesions include intranuclear par- The basic FeLV proteins include the envelope proteins and the
ticles within keratinized cells in the superficial epithelial strata of core proteins, several of which are important clinically. Two enve-
the plaques. 403 IHC can be performed on tissue sections using lope proteins, the P15E and the GP70 glycoproteins, have par-
band-reactive, genus-specific antisera. Interestingly, the histologic ticular clinical significance. 316–318 P15E is thought to be one of
features of the feline fibropapilloma are very similar to those of the mediators of immunosuppression in FeLV-infected cats. 319 The
equine sarcoids, with characteristic fibroblastic proliferation, glycoprotein of the envelope GP70 may contain three subgroup
hyperplasia of epidermis, and rete ridges. 303 PCR also has dem- antigens, A, B, and C. 320–321 An individual cat may have combina-
onstrated papillomavirus DNA in 40% of SCCs, but whether it is tions of viruses with these subgroup viral antigens. Considerable
responsible for these tumors in all cases is unknown. 404 antigenic variation exists within subgroups, which can affect the
biologic properties of the individual isolates or strains of FeLV. 310,320
Treatment These subgroup antigens bind the virion to receptors on the surface
Surgical excision generally is used; however, parenteral human of cells. The specific characters of these proteins also predict the
recombinant interferon-α-2a has been suggested as an alterna- pathogenicity, host range, infectivity, and other biologic properties
tive. Medications containing 5-FU that are used in humans and of the virus. 310,320 Antibodies produced against envelope proteins
dogs should not be used in cats. 409 Imiquimod 5% cream is a can be neutralizing and thereby can prevent infection. Envelope
novel immune-response modifier (IRM) that has been used in proteins thus are important components of FeLV vaccines.
humans with Bowen disease and recently has been used in cats Core proteins (capsids) include P15C, P12, P10, and P27. P27
with the same disease. Although 41% of cats treated with imiqui- is quite soluble and can be found in large amounts in the cyto-
mod developed some level of toxicity, most adverse events were plasm of cells and bodily fluids, such as tears and serum. 316–318
manageable. 305 P27 is the antigen that is detected in immunofluorescent assay
(IFA) tests and enzyme-linked immunosorbent assays (ELISAs),
Retroviruses which are commonly used in the diagnosis of FeLV infection. 321

Retroviral infections are considered the number one infectious Transmission
cause of morbidity and mortality in the domestic cat. Before the FeLV is an enveloped virus and is considered very fragile. Desicca-
vaccine was developed and routine testing and control measures tion rapidly reduces the amount of viable virus in saliva, and inac-
became widespread, the feline leukemia virus (FeLV) was associ- tivation occurs in 1 to 2 hours. In exudates or blood the virus may
ated with one-third of deaths in cats. 306,307 Since the introduction be viable for only 48 hours (at 37°C) or 1 to 2 weeks (at 22°C). 322