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Overview of Feline Leukemia Virus and Related Diseases

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Feline leukemia virus (FeLV) remains one of the most important infectious diseases of cats globally. It manifests primarily through profound anemia, malignancies, and immunosuppression and infects domestic cats and other species of Felidae. In the laboratory, cells from a much wider range of species can be infected by some strains of the virus.

The Feline Retrovirus Management Guidelines published by the American Association of Feline Practitioners is a key resource for expert consensus on prevention, diagnosis, and management of FeLV for veterinary practitioners in private practice, animal shelters, and catteries (see Table 1: Summary of Clinical Management of Feline RetrovirusTables).

Table 1

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FeLV is a retrovirus in the family Oncovirinae. Other oncoviruses include feline sarcoma virus, mouse leukemia viruses, and two human T-lymphotropic viruses. Although oncogenesis is one of their more dramatic effects, oncoviruses cause many other conditions, including degenerative, proliferative, and immunologic disorders.

There are four FeLV subgroups of clinical importance. Almost all naturally infected cats are originally infected by FeLV-A, the original, archetypical form of the virus. Additional mutated forms of the original FeLV-A subtype as well as FeLV-B, FeLV-C, or FeLV-T may develop in infected cats. FeLV-B increases the frequency of neoplastic diseases; FeLV-C is strongly associated with development of erythroid hypoplasia and consequent severe anemia; and FeLV-T has the propensity to infect and destroy T lymphocytes, leading to lymphoid depletion and immunodeficiency. Viruses of all four subgroups are detected (but cannot be distinguished) by commonly used FeLV diagnostic test kits.

The prevalence of FeLV infection documented in cross-sectional surveys in North America has been declining throughout the past three decades and is attributed to testing and vaccination efforts. In the USA, 3.1% of cats in a large, nation-wide data set tested positive for FeLV in 2010, with increased risk among outdoor cats, unneutered males, and cats with other disease conditions (particularly respiratory disease, oral disease, and abscessation). Prevalence was highest in the midwestern and western regions of the USA and lowest in the northeast. Seroprevalence surveys of varying statistical power have found rates of positive test results to range from 3.6% in Germany and Canada to 4.6% in Egypt and 24.5% in Thailand.

Persistently infected, healthy cats serve as reservoirs of FeLV for both vertical and horizontal viral transmission. Oronasal contact with infectious saliva or urine represents the most likely mode of horizontal transmission; vertical transmission in utero or through nursing is also common. Tears and feces may contain virus but are not considered to be clinically significant in disease transmission or diagnostic detection. Although direct contact, mutual grooming, and shared litter trays and food dishes are the primary methods of horizontal transmission, infection through bite wounds is possible. In a national (USA) study, FeLV infection was diagnosed in 9% of cats undergoing treatment for bite wounds, approximately three times the rate for cats in general. Because FeLV is a fragile, enveloped virus, horizontal transmission between adults usually requires prolonged, intimate contact. In addition, the dose required for oronasal transmission of the virus is relatively high.

FeLV is considered to be an age-dependent disease; young kittens are at higher risk of progressive infection and more rapid disease progression, whereas adults display some degree of age resistance. However, transmission can occur at any age, and factors affecting clinical course of disease are complex and incompletely understood.

After oronasal inoculation, the virus first replicates in oropharyngeal lymphoid tissue. From there, virus is carried in blood mononuclear cells to spleen, lymph nodes, epithelial cells of the intestine and bladder, salivary glands, and bone marrow. Virus also appears in secretions and excretions of these tissues and in peripheral blood leukocytes and platelets. Viremia is usually evident 2–4 wk after infection. The acute stage of FeLV infection occurring 2–6 wk after infection is rarely detected but typically characterized by mild fever, malaise, lymphadenopathy, and blood cytopenias. Cats unable to mount an adequate immune response become persistently viremic and develop a progressive infection, often leading to fatal disease. Oncogenesis occurs when FeLV virus inserts into the host cellular genome, either in proximity to an oncogene resulting in activation or directly into the oncogene itself to form a recombinant subgroup virus such as FeLV-B that can induce new neoplastic activity in any cell the recombinant virus enters.

The most recent classification system for FeLV labels infections as progressive, abortive, regressive, and focal. Progressive infections are defined by uninhibited viral replication with subsequent persistent viremia and probable eventual manifestation of clinical disease. Previously, most adult cats were thought to have abortive infections in which transient viremia was followed by complete clearance of viral infection. However, improved sensitivity of PCR testing has revealed that antigen-negative cats may still harbor FeLV provirus in tissues; this is termed regressive infection. It is believed that cats with regressive infections generally are aviremic, do not shed infectious virus, and do not develop FeLV-associated diseases; however, they are considered carriers with the potential for reactivation and future shedding. It is likely that FeLV can be transmitted by blood transfusion from cats with regressive infection, so feline blood donors should have PCR testing performed as a screening test before donating. The incidence of regressive infections and the causes and frequency of reactivation of viral shedding among these cats are incompletely understood. Focal infections, rare in natural infections, involve viral replication in specific tissues, such as the eyes or bladder, releasing low levels of viral antigen and, therefore, variable results on diagnostic testing.

FeLV-related disorders are numerous and include anemia, neoplasia, immunosuppression, immune-mediated diseases, reproductive problems, enteritis, neurologic dysfunction, and stomatitis.

The anemia caused by FeLV is typically nonregenerative and normochromic. Less commonly, macrocytosis or regenerative hemolytic anemia is seen in only 10% of FeLV-induced anemia cases. The cause of nonregenerative anemia is usually bone marrow suppression due to viral infection of the hematopoietic stem cells and the supporting stromal cells. Platelet dysfunction, thrombocytopenia, and neutropenia are all possible sequelae as well.

Lymphoma is the most frequently diagnosed malignancy of cats. Tumors such as lymphoma and lymphoid leukemia develop in as many as 30% of cats with progressive FeLV infections. Regressive infections are also implicated in the occurrence of these tumors in the absence of viremia, but cats with progressive infections may face an increased risk of lymphoma development as high as 60-fold. Most American cats with mediastinal, multicentric, or spinal forms of lymphoma are FeLV-positive. However, these forms of lymphoma are becoming less common as the prevalence of FeLV decreases. Diffuse GI lymphoma is now more likely to be found in FeLV-negative cats of middle or older age and can be difficult to differentiate from inflammatory bowel disease. Fibrosarcomas and quasi-neoplastic disorders such as multiple cartilaginous exostoses (osteochondromatosis) can be FeLV-associated. Other types of tumors share a suspected but not yet clearly defined link with the FeLV virus.

Leukemia is characterized by the neoplastic proliferation of hematopoietic cells originating in the bone marrow, including neutrophils, basophils, eosinophils, monocytes, lymphocytes, megakaryocytes, and erythrocytes. Feline leukemias are strongly associated with FeLV infection and typically involve neoplastic cells circulating in the blood. Lymphoid leukemias are further classified as acute and chronic. Acute lymphocytic leukemia is characterized by lymphoblasts circulating in the blood, whereas chronic lymphocytic leukemias have an increased number of circulating lymphocytes with mature morphology.

The immunosuppression caused by FeLV creates increased susceptibility to bacterial, fungal, protozoal, and viral infections. Numbers of neutrophils and lymphocytes in the peripheral blood of affected cats may be reduced, and those cells that are present may be dysfunctional. Many FeLV-positive cats have low blood concentrations of complement; this contributes to FeLV-associated immunodeficiency and oncogenicity, because complement is vital for some forms of antibody-mediated tumor cell lysis.

Immune complexes formed in the presence of moderate antigen excess can cause systemic vasculitis, glomerulonephritis, polyarthritis, and a variety of other immune disorders. In FeLV-infected cats, immune complexes form under conditions in which FeLV antigens are abundant and anti-FeLV IgG antibodies are sparse, a situation ideal for the development of immune-mediated disease.

Reproductive problems are commonly associated with FeLV infection. Fetal death, resorption, and placental involution may occur in the middle trimester of pregnancy, presumably as a result of in utero infection of fetuses by virus transported across the placenta in maternal leukocytes. Abortion typically occurs in late gestation accompanied by risk of bacterial endometritis, especially in neutropenic queens. Transmission during birth and nursing constitutes the greatest risk of producing live, viremic kittens. There is some evidence that regressively infected queens may pass virus on to their kittens either in utero or in milk. Neonatal kittens are at risk of rapidly progressive infection with clinical manifestations of hypothermia, dehydration, failure to nurse, and early mortality collectively termed “fading kitten syndrome.” It is likely that transmission from infected queens to their kittens is the single greatest source of FeLV infections.

Coinfection with FeLV and feline panleukopenia virus (FPV) has been implicated in feline panleukopenia-like syndrome (FPLS), which is also termed FeLV-associated enteritis. FPLS resembles feline panleukopenia both clinically and histopathologically and is characterized clinically by progressive anorexia, depression, vomiting, hemorrhagic diarrhea, weight loss, gingivitis, oral ulceration, severe neutropenia, and septicemia. FPV antigen is inconsistently present on diagnostic testing in these cases, and the pathogenesis and exact role of each virus in the development of this syndrome are incompletely understood.

Although neurologic disorders associated with FeLV are most often caused by compression of the brain and spinal cord by lymphoma tumor tissue, a mechanism for neuropathology is also suspected to result in peripheral neuropathies, urinary incontinence, and ocular pathology, including anisocoria, mydriasis, Horner syndrome, and central blindness even in the absence of visible compressive lesions on diagnostic imaging. If antineoplastic therapy is planned, it is important to distinguish neoplasia from neuropathy.

Stomatitis is more classically associated with FIV infection, but FeLV infection can also predispose cats to chronic ulcerative proliferative gingivostomatitis. Clinical sequelae include pain, anorexia, and tooth loss. An immune-mediated mechanism is likely, particularly in combination with coinfections such as feline calicivirus.

Testing for FeLV infection is recommended when cats are first acquired, before vaccination against FeLV, if there has been potential exposure or bite wound from a cat of unknown or positive retroviral status; annually if they live in a household with FeLV-positive cats; before blood donation; and regularly if they have outdoor access. For cats entering a new home or known to be at high risk of exposure, testing should be repeated 30 days after the first test in case of recent infection that has not yet resulted in detectable circulating antigen. Documentation of a previous negative test does not negate the need for repeat testing in the above situations. Prior vaccination does not interfere with diagnostic testing unless performed immediately before blood collection for antigen testing.

Three types of tests are now readily available for clinical use: immunochromatography (such as ELISA), immunofluorescent assay (IFA), and PCR. Virus isolation is considered the gold standard diagnostic test but is not generally available to private practitioners. ELISA or other point-of care antigen test kits can be used in the veterinary clinic to detect the presence of soluble FeLV p27 antigen in whole blood or serum using a lateral flow test kit or a multi-well plate. Saliva and tears are not considered to be reliable samples for testing purposes. Several different test kits are available; most have sensitivities and specificities of ~98%.

IFA tests for the presence of FeLV p27 and other structural core antigens in the cytoplasm of cells. In clinical practice, peripheral blood smears are usually used for IFA, but cytologic preparations of bone marrow or other tissues can also be used. IFA requires submission to a diagnostic laboratory and cannot detect infection until bone marrow involvement occurs. False-negative test results may occur because of leukopenia or lack of bone marrow involvement, whereas technical error is most often the cause of false-positive results. Like ELISA, IFA cannot detect regressive infection because of the lack of sufficient viral antigen production. Historically, cats with nonregenerative anemia or other cytopenias and negative FeLV antigen tests on blood samples were subjected to bone marrow testing in search of occult FeLV infections. Several recent studies have indicated this is unnecessary, because cats with FeLV-associated bone marrow suppression invariably have positive results on blood tests.

Discordant results between tests, often a positive initial ELISA followed by negative results on either repeat ELISA or IFA, may reflect the inconsistent antigen circulation during various stages of FeLV infection, technical error, or possibly regressive infection status. These cats are generally considered presumptively infected and potential sources of infection until further clarification is possible. Standard recommendations to resolve discordant testing dictate repeating both tests in 30–60 days using serum instead of whole blood. It is not uncommon for cats, especially kittens, to test negative on a subsequent test. This could indicate a false-positive on the first test, a false-negative on the second test, or development of a regressive or abortive infection status. Once a single positive test result has been obtained, it can be difficult to ever know the true status of the cat, even if subsequent tests are negative.

PCR testing on whole blood, bone marrow, and other tissues is increasingly available through diagnostic laboratories, although validated sensitivity and specificity studies are often lacking. Real-time PCR offers great potential to provide extremely sensitive detection of FeLV rapidly after infection and can be useful to detect regressive infections and resolve conflicting test results if a positive result is obtained.

Diagnosis of FeLV-induced neoplasia is similar to that of other tumors. Cytologic examination of fine-needle aspirates of masses, lymph nodes, body cavity fluids (eg, pleural effusion), and affected organs may reveal malignant lymphocytes. Bone marrow examination may reveal leukemic involvement, even when the peripheral blood appears normal. Biopsy with histopathologic examination of abnormal tissues is often necessary for diagnostic confirmation. Cellular phenotyping via flow cytometry, immunocytochemistry, or other techniques can provide additional diagnostic information.

Unfortunately, no curative treatment currently exists to eradicate retroviral infection.

In vitro studies have yielded cautiously promising results suggesting virus-suppressing activity of FDA-approved drugs used to treat HIV and other myelodysplastic syndromes against FeLV virus (eg, raltegravir, tenofovir, gemcitabine, decitabine). Further research is needed to demonstrate efficacy and safety in vivo and in field trials, as well as to address affordability of these drugs for most cat owners. Feline interferon omega and human interferon alpha have been associated with improved survival, but concerns surrounding availability, cost, and absence of strong evidence in controlled field studies have limited their widespread integration into standard treatment protocols for FeLV.

Anecdotal reports of various antiviral and immunotherapeutic agents to reverse viremia, improve clinical signs, and prolong survival are abundant. Controlled studies using naturally infected cats have either not been performed or have not confirmed anecdotal observations. Treatment efficacy must be demonstrated in controlled clinical trials, because spontaneous reversion to seronegative status or prolonged survival is not uncommon, even in the absence of medical treatment.

Some FeLV-positive cats can live without major disease complications for years with routine prophylactic care, good husbandry, minimal stress, and avoidance of secondary infections. Infected cats should be kept strictly indoors to reduce the risk of exposure to infectious agents and to prevent transmission of the virus to other cats. Routine vaccinations should be administered based on individual risk assessment and in compliance with local laws. Use of inactivated vaccines could be considered because of concerns regarding use of live vaccines reverting to virulence in immunocompromised animals, although this does not appear to be common. FeLV vaccinations should not be administered, because there is no evidence to suggest a benefit after infection. Physical examinations focusing on external parasites, skin infections, dental disease, lymph node size, and body weight should be performed semiannually, along with a routine program for parasite control and annual fecal, CBC, chemistry panel, and urinalysis testing. All infected cats should be neutered. Owners should be advised to watch for signs of FeLV-related disease, particularly secondary infections. Although FeLV-positive cats often respond well to treatment, therapy for such infections or other illnesses should be early and aggressive because of immunocompromise.

Because FeLV is historically associated with rapid and grave disease, the modern prognosis varies considerably depending on husbandry, veterinary care, and individual immune system variation. Large-scale studies have demonstrated an average survival of 2.4 yr after diagnosis among positive cats (versus 6 yr after testing for negative control cats), with 50% mortality in 2 yr and 80% mortality by 3 yr after diagnosis. Progression of disease is much more rapid in kittens, whereas some adult cats remain healthy for many years and may succumb to conditions unrelated to their retroviral status.

Lymphoma Treatment:

Feline lymphoma can be treated with cytotoxic drugs. These drugs may cause significant toxicities if not dosed and administered properly. Most cytotoxic drugs are also carcinogens and must be handled properly. Before administering these drugs, veterinarians should familiarize themselves with proper dosing and administration procedures, appropriate monitoring of the patient, toxicities and complications, and safe handling to prevent exposure of veterinary personnel and owners to the agents and their metabolites.

Approximately 50% of cats with lymphoma that are treated obtain a complete remission, defined as no clinical evidence of disease. FeLV-negative cats that attain a complete remission live an average of 9 mo, whereas survival among FeLV-positive cats averages 6 mo. Cats not treated or those not responding to treatment survive an average of 2–6 wk.

Many protocols for treatment of feline lymphoma have been published; most use similar drugs with differing schedules of administration. One widely used protocol consists of an intensive induction phase (vincristine weekly for 4 wk, cyclophosphamide every 3 wk on the same day as vincristine, and prednisolone daily), followed by a less intensive maintenance phase (vincristine and cyclophosphamide given every 3 wk on the same day, and prednisolone continued daily). Treatment is continued for 1 yr or until relapse. With this protocol, 79% of cats attained remission, and average survival was 150 days. Changing the maintenance protocol to doxorubicin every 3 wk provided an average remission of 281 days. When relapse occurs, the drug regimen can be changed and a second remission achieved; however, second remissions seldom last as long as the first.

Another popular chemotherapy protocol involves an initial treatment with L-asparaginase and vincristine. Treatment is continued with daily prednisolone and alternating doses of cyclophosphamide, vincristine, and doxorubicin for a total of three cycles. When relapse occurs, the protocol is started again. Using this protocol, the median survival has been reported to be 210 days. Other protocols incorporating alkylating agents such as mustargen and procarbazine have demonstrated efficacy as well, sometimes even after other combination therapies have not achieved remission.

Most lymphomas have an intermediate or high histopathologic grade and are clinically aggressive, except for a subset identified as small-cell lymphoma or lymphocytic lymphoma. Small-cell lymphomas are characterized by a diffuse infiltration of malignant lymphocytes throughout affected organs, typically intestines, and can often be successfully treated with less aggressive chemotherapy. Administration of prednisolone and chlorambucil orally daily for 4 consecutive days every 3 wk has been used. Using these drugs in clinical trials to treat small-cell lymphoma involving the GI tract produced a median survival of 963 days. If other sites were involved, with or without GI disease, the median survival was 636 days.

In addition to small-cell lymphoma, large granular lymphocyte lymphoma also affects the intestinal tract. This is an extremely aggressive disease, with a response to chemotherapy of ~30% and a median survival of 57 days. An intestinal mass is usually present and may cause intestinal obstruction.

Acute lymphocytic leukemia is treated with the same protocol as lymphoma, but only ~25% of cats obtain remission for an average of 7 mo. Chronic lymphocytic leukemia typically carries a much better prognosis than the acute form and is best treated with chlorambucil and prednisolone given every other day on alternating days. Leukemias other than lymphocytic are rarely treated because of severity of illness at diagnosis and poor response to therapy.

FeLV virus is unstable in the environment and is susceptible to all common detergents and disinfectants. In a hospital or boarding setting, infected cats may be kept in the general population as long as they are housed in separate cages. Medical and surgical equipment contaminated with body fluids, even when dried, can be fomites for infection. Thorough cleaning and sterilization of equipment, strict attention to washing contaminated hands, and avoiding reuse and sharing of single use and consumable supplies between patients are critical practices to prevent iatrogenic transmission.

FeLV vaccines are non-core and are intended to protect cats against FeLV infection or to reduce the likelihood of persistent viremia. Types of vaccines include killed whole virus, subunit, and genetically engineered. Vaccines may vary in protective effect, and manufacturers' claims and independent comparative studies should be carefully noted. Vaccines are indicated only for uninfected cats, because there is no benefit in vaccinating an FeLV-positive cat.

The AAFP Feline Retrovirus Management Guidelines include the recommendation that all kittens should receive the two-dose FeLV vaccination as a component of the routine initial vaccination series and should also receive a booster vaccination 1 yr later. This is prudent, because rehoming and lifestyle changes such as outdoor access frequently occur as cats mature. Annual revaccination after maturity would depend on the cat's risk of FeLV exposure.

The adult cat's risk of exposure to FeLV-positive cats should be assessed and vaccines used only for those cats at risk. FeLV vaccines have been associated with development of sarcomas at the vaccination site, although the risk of tumor development is very low. Uninfected cats in a household with infected cats should be vaccinated; however, vaccination is not universally protective, and other means of reducing transmission to uninfected cats, such as physical separation, should also be used.

While testing of cats in an animal shelter environment is considered optional for individual housing, FeLV status should be determined before placement in group housing and is recommended at the time of adoption or foster home placement. Because tests are not 100% accurate, shelter cats placed in group housing should be vaccinated against FeLV, especially in longterm conditions such as sanctuaries. Because of the equivalent prevalence of FeLV among feral and free-roaming pet cats and the role of neutering in decreasing the spread of infection, expending resources on FeLV testing is not considered a mandatory component of community cat trap-neuter-return programs.

Some strains of FeLV can be experimentally grown in human tissue cultures, leading to concerns of potential for transmission to people. Studies addressing this concern have shown no evidence that any zoonotic risk exists, and there are no known cases of zoonotic transmission.

Last full review/revision December 2014 by Julie K. Levy, DVM, PhD, DACVIM; Amanda Burling, DVM

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