Page 45 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

24 PART I The Biology and Pathogenesis of Cancer

the time of experimental infection to tumor production is 1 to polyionic dye used to treat filariasis in humans), nucleoside ana-
23 months (mean, 5.3 months). 355,356 The younger the cat when logs (AZT, DDC, DDA, and PNEA), glucose homopolymers,
371–374
dextran sulfate, phosphonate, and others.
More detailed
infected with FeLV, the shorter the time to the development of
VetBooks.ir neoplastic disease. Some cats infected with FeLV die of immuno- descriptions of these therapies are provided elsewhere. 371,374 In
general, most of these agents have shown efficacy in vitro against
suppressive disease before tumors have a chance to develop.
Treatment of Feline Leukemia Virus Infections FeLV, the human immunodeficiency virus (HIV), and in some
cases, FIV. Most of these drugs result in some reduction in vire-
Although no effective treatment exists to eliminate established mia in vivo, but none are capable of reversing established viremia,
FeLV infection in cats, a variety of antiviral drugs and biologic although some may prevent viremia if administered prophylacti-
response modifiers (BRMs) have been used to manage retroviral cally. Most of these drugs cause significant adverse effects at the
infections in cats and humans. The mainstay of therapy for cats dosages needed to produce antiviral effects and therefore have not
infected with FeLV or other retroviruses is supportive care. 357–359 gained popularity in clinical practice. Azidothymidine (AZT, zid-
Maintaining hydration and nutritional status not only prolongs ovudine) is the most widely studied RT inhibitor. 375 AZT inhibits
life, but also enhances quality of life. The cat should be kept in FeLV reverse transcriptase when administered at a dosage of 10 to
a humid environment to reduce the chance of water loss. Appe- 20 mg/kg in daily divided doses. AZT prevents viremia if given
tite stimulants and placement of gastrostomy tubes may facilitate within 72 hours of exposure to FeLV. The antiviral effects of AZT
nutritional therapy. The cat’s requirement for B vitamins is eight appear to be synergistic with human recombinant interferon-
times that of the dog, and dietary concentrations must be main- α-2a. 376,377 Reversal of established experimental FeLV viremia
tained to maintain appetite. Semimoist cat foods often contain through adoptive transfer of lectin/IL-2–activated lymphocytes,
propylene glycol, which can shorten red blood cell survival. These interferon-α, and AZT has been reported.
foods, although often quite palatable, should not be used for the
nutritional management of cats infected with FeLV. 358 Many cats Prevention and Control
with FeLV are anemic, but administering erythropoietin is not The most effective means of preventing FeLV infection is to elimi-
helpful because endogenous erythropoietin levels usually are 20 nate contact with viremic cats. The “test and removal” program
times normal. 360 is the most effective means of controlling FeLV in multiple-cat
households. 378 The program consists of closing the household or
Biologic Response Modifiers cattery to new cats, testing the remaining cats every 3 months, and
A variety of BRMs have been used in cats infected with FeLV, 361– removing all animals that test positive. When all cats test nega-
369 but none has shown benefit in controlled trials. A few of the tive for two consecutive sessions, the facility is determined to be
most popular are discussed here. FeLV free. New cats may enter the household or facility only after
Human recombinant, bovine, and feline interferons have been a 3-month quarantine and two negative FeLV tests. The test and
studied extensively for the management of FeLV infection, but removal system has been shown to reduce the incidence of FeLV
the results have been mixed. However, some uncontrolled stud- in a variety of settings and geographic locations. 378
ies have shown improvement in the clinical status of cats treated
with oral human recombinant interferon-α, 368 and one study in Prevention by Vaccination
a limited number of cats using feline interferon subcutaneously Vaccinations help control or eliminate many infectious diseases
showed improvement compared with placebo-treated controls. 410 in veterinary medicine. The first commercial FeLV vaccine was
Larger controlled trials are needed to establish the true efficacy introduced in 1985. Since then many FeLV vaccine products have
of interferon products. Orally administered interferon probably is been licensed for sale to veterinarians in the United States. Despite
inactivated by gastric acid in the stomach. Parenterally adminis- the fact that FeLV vaccines have been available for decades, they
tered interferons from other species (i.e., bovine and human) are come with the risk of injection site tumors. The frequency of use
likely to have only temporary activity because of the production of FeLV vaccines continues to be debated, especially in adult cats,
of neutralizing antibodies. most of which develop age-acquired immunity. In addition to
Carrisyn (Acemannan) is a BRM designed to enhance mac- age-acquired immunity and the risk of injection site tumors, most
rophage phagocytosis and cell killing. Viremic cats treated with currently available FeLV vaccines, although effective in prevent-
Carrisyn have been reported to improve clinically; however, the ing persistent viremia, fail to prevent latent infections of the bone
studies reporting these results have not been well controlled, and marrow after challenge. Some studies of available vaccines have
the observed benefit may be due to the natural waxing and waning reported efficacies ranging from 0% to 100%. 379,380
of the clinical course commonly observed in FeLV-positive cats. 369 FeLV vaccination issues are discussed elsewhere. 341 However,
A lymphocyte T-cell immunomodulator (LTCI) recently has several comments about FeLV vaccines should help practitio-
become commercially available for the treatment of cats infected ners decide whether to use FeLV vaccines in their practice. FeLV
with FeLV and FIV. The true efficacy of this product, if any, awaits vaccines may contain two or three subgroup antigens. Because
the results of controlled trials in cats. 370 only subgroup A is transmitted contagiously between cats, vac-
The apparent positive effect of many of the BRMs, which, in cines need only to contain subgroup A. Their primary means of
fact, may be due to the anabolic effect observed with some of these protecting against tumor development is preventing persistent
cytokines, is thought to be based on endorphin release rather than viremia. If a vaccine protects against persistent FeLV infection,
a direct effect on the viral infection. it need not contain FOCMA. The value of FOCMA in FeLV
vaccines has yet to be proven. Vaccines should protect against a
Reverse Transcriptase Inhibitors variety of strains of subgroup A, and none of the available vac-
Drugs that inhibit RT and retrovirus integration into the host cines contain more than one strain of subgroup A. Differences
cell have been evaluated for their potential use in the treatment in published comparative studies of vaccine efficacy probably
of FeLV-positive cats. The drugs evaluated include suramin (a are related to differences in vaccine strains and to the challenge