Page 46 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 1 The Etiology of Cancer 25

strains used. Vaccines that contain adjuvants enhance immunity positive; this helps differentiate them from vaccine-associated sar-
but at the expense of producing local inflammatory reactions at comas, which may have growth characteristics similar to those
381
of virally induced tumors. Cats with multicentric FeSV-induced
the injection site.
These local reactions may lead to the devel-
VetBooks.ir opment of soft tissue sarcomas; however, the development of tumors have a very poor prognosis.
soft tissue sarcomas after vaccination, either with rabies or FeLV
vaccines, is thought to occur in only 1 in 1000 to 1 in 10,000 Feline Immunodeficiency Virus
vaccinates. 380 Nonadjuvanted FeLV vaccines have shown little
or no inflammatory reaction 21 days after administration. 381 FIV, which is classified as a retrovirus in the subfamily Lentiviri-
A canarypox-vectored FeLV nonadjuvanted vaccine is available nae, is distinct from other retroviruses that infect cats. As are other
that stimulates both cellular and humoral immunity without retroviruses, FIV is an enveloped, single-stranded RNA virus in
significant injection site inflammation. Clinical discretion, con- which the RNA is copied into the DNA in the infected host by
sidering the potential risk and benefit, should be used when rec- RT in the virus.
ommending FeLV vaccines. The reader is advised to consider the The nucleotide sequence of several FIV isolates has been
current recommendations of the American Association of Feline determined, and genetic homology falls between 36% and 97%.
Practitioners (AAFP) (www.catvets.com). Despite this homology, significant differences in pathogenicity
and infectivity exist between FIV strains. 389,390 Although lentivi-
Feline Sarcoma Virus ruses are known to infect wild felids, they are antigenically distinct
from domestic cat isolates; they also are well adapted to their host
FeSVs are true hybrids that result from the rare recombination and seldom cause clinical disease.
of FeLV DNA provirus with cat proto-oncogenes. Cats have at
least 30 proto-oncogenes. 306,382,383 Proto-oncogenes have many Transmission
biologic functions; when they are altered and activated inappro- FIV is present in all bodily fluids of infected cats, similar to FeLV,
priately, they are called oncogenes, which can play a key role in but at much lower concentrations. FIV is mainly cell associ-
the development of cancerous phenotypes. Proto-oncogenes can ated and is present in relatively low concentrations in the blood,
be activated by mutations that produce chromosomal transloca- although high amounts can be found in the saliva. 391,392 FIV is
tions, such as those that may be associated with inflammation not thought to be highly infectious and is mainly transmitted
and vaccine-associated sarcomas, or by incorporation into a ret- through biting during cat fights. 393,394
rovirus, such as FeLV. 382–384 When FeLV-derived DNA inserts
near a proto-oncogene and takes up the proto-oncogene into the Feline Immunodeficiency Virus–Associated Neoplasms
FeLV provirus, formation of FeSV results. In the process, part The prevalence of neoplasms in FIV-positive cats ranges from
of the FeLV GAG gene, most of the FeLV envelope gene, and all 1% to 62%. 352,395,396 Lymphomas and myeloid tumors (myelog-
of the pol genes are lost. 383 The loss of these vital components enous leukemia, myeloproliferative disease) and a few carcino-
makes FeSV dependent on FeLV as a helper virus for replica- mas and sarcomas are the neoplasms most commonly linked to
tion. Cats that have FeSV always test FeLV positive. Because FIV infection. One study found that cats infected with FIV and
several different recombinations may recur with several differ- FeLV are 5.6 times more likely to develop lymphoma or leuke-
ent proto-oncogenes, each recombination is a unique event, and mia than if they had been infected with either virus alone. Cats
each isolate is distinct. 384 Despite this phenotypic heterogene- with combined infections had a 77% greater likelihood of devel-
ity, the recombinations transform fibroblasts, and all produce oping lymphoma or leukemia than noninfected cats. 352 In con-
fibrosarcomas. trast to FeLV-associated lymphomas, FIV-associated lymphomas
Natural transmission of FeSV between cats has not been most often develop in extranodal sites and occur in older cats
described, and as with other FeLV recombinants (e.g., FeLV-B), (mean age, 8.7 years). 352 Myeloproliferative disease also has been
transmission of the recombinant product is not thought to occur observed in cats naturally and in cats experimentally infected
in nature. Some cats are capable of rejecting transformed cells and with FIV. 352,397,398
producing FOCMA antibody. 359,360 FOCMA is important in the Although lentiviruses such as FIV have not been thought to
experimental response of cats to FeSV because it has been associ- be oncogenic in themselves, they are markedly immunosuppres-
ated with tumor regression and failure to develop tumors. 385 Cats sive and affect normal immunosurveillance of neoplastic cells.
that fail to develop antibodies against FOCMA die quickly of fast- FIV-positive cats with lymphoma have extremely low CD4 lym-
growing sarcomas. 386 phocyte counts. 388 SCCs of the skin have been linked to FIV
infection in two geographic areas, California and Colorado, but
Clinical Features of Feline Sarcoma Virus– this association is believed to be due to a co-risk behavior (out-
Induced Fibrosarcomas door cats) rather than to any direct viral contribution to tumor
Only 2% of fibrosarcomas of cats are virally induced. 312 In contrast development. 399,400 Other reports have linked FIV infection to
to the solitary, slow-growing, nonvirally induced sarcomas seen in oral SCC, mammary carcinoma, fibrosarcoma, myeloproliferative
older cats, FeSV-induced tumors are multicentric and are found disease, Bowen disease and histiocytic mast cell disease. 395,401,402
most frequently in young cats. 387 They are characterized by rapid The nature of these associations awaits further investigation.
growth, including doubling times as short as 12 to 72 hours. 306
This rapid growth often is accompanied by superficial ulceration. Treatment
Lesions frequently occur at sites of previous bite wounds. 306 The same treatment considerations in the management of cats
Metastasis to the lungs or other organs occurs with approximately with FeLV can be applied to the treatment of FIV-positive cats.
30% of virally induced fibrosarcomas in cats. Hypercalcemia was The most widely applied treatments have been the RT inhibi-
observed in association with multicentric fibrosarcomas in one tors and human recombinant interferon-α (see the earlier dis-
cat with FeSV. 306 Virally induced fibrosarcomas are always FeLV cussion on the treatment of FeLV). As in the treatment of FeLV,