Feline Leukemia Virus Infection

Feline leukemia virus (FeLV) is a retrovirus belonging to the family Retroviridae, genus Gammaretrovirus, that causes a persistent, immunosuppressive, and oncogenic infection in domestic and wild felids. The disease is characterized by a spectrum of outcomes ranging from transient, self-limiting infection to progressive, lifelong viremia with associated clinical syndromes including lymphoma, leukemia, anemia, immunosuppression, and secondary infections. FeLV infection is a major cause of morbidity and mortality in cats, with a global distribution. The clinical course is influenced by the age of the cat, the viral dose, the route of exposure, and the host's immune response. Infection can be classified into regressive, progressive, abortive, and focal (atypical) forms based on the presence of viral antigen and proviral DNA in blood and tissues. The disease is a leading cause of vaccine-preventable infectious disease in cats, and management focuses on prevention through vaccination and testing, as well as supportive care for infected individuals.

The causative agent is Feline leukemia virus (FeLV), an enveloped, single-stranded RNA virus of the genus Gammaretrovirus. The virus is approximately 80-110 nm in diameter and contains a diploid RNA genome. FeLV is classified into several subgroups (A, B, C, D, and T) based on envelope glycoprotein (gp70) antigenicity and receptor usage. FeLV-A is the ecotropic, horizontally transmitted form found in all naturally infected cats; FeLV-B arises from recombination with endogenous retroviral sequences and is associated with oncogenicity; FeLV-C is a mutant form causing pure red cell aplasia; FeLV-T is T-cell tropic and causes immunosuppression. The virus is transmitted primarily through saliva, but also via nasal secretions, urine, feces, and milk. Transmission occurs through mutual grooming, sharing food/water bowls, bite wounds, and from queen to kitten (transplacental or transmammary). The virus is labile in the environment and is inactivated by common disinfectants, heat, and desiccation. The virus infects feline cells by binding to specific receptors: FeLV-A uses the thiamine transporter THTR1, FeLV-B uses Pit1/Pit2, and FeLV-C uses heme exporter FLVCR. After entry, the viral RNA is reverse-transcribed into DNA, which integrates into the host genome as a provirus. The virus replicates in bone marrow, lymphoid tissues, and epithelial cells, leading to viremia and shedding.

FeLV infection occurs worldwide, with prevalence varying by population. In the United States, approximately 2-3% of healthy cats are persistently viremic, but prevalence is higher in sick cats (up to 15%) and in high-risk populations such as free-roaming, multi-cat households, and cats with outdoor access. The infection is more common in male cats, particularly intact males, due to fighting and biting behaviors. Age is a significant factor: kittens are highly susceptible, with infection rates declining with age; cats older than 1 year are relatively resistant. Breed predilections are not well-defined, but certain breeds may have genetic resistance. The virus is endemic in feral cat populations. Geographic variation exists, with higher prevalence in warmer climates and areas with high cat density. The incidence of FeLV has decreased in recent decades due to testing and vaccination programs, but it remains a significant cause of disease in unvaccinated or untested populations. Co-infection with Feline Immunodeficiency Virus (FIV) is common, and co-infected cats have poorer outcomes.

The pathogenesis of FeLV infection begins with oronasal exposure, followed by viral replication in local lymphoid tissues (tonsils, pharyngeal lymph nodes). Within 2-4 days, the virus spreads to regional lymph nodes and then to the bloodstream, causing a primary viremia. The virus then targets bone marrow and other hematopoietic tissues, where it infects precursor cells. The outcome depends on the host's immune response. In regressive infection, the immune system mounts a strong humoral and cell-mediated response, clearing the viremia within 4-8 weeks, but proviral DNA persists in bone marrow and other tissues, and the cat remains latently infected. In progressive infection, the immune response is inadequate, leading to persistent viremia and shedding. The virus causes immunosuppression by infecting and depleting T lymphocytes, particularly CD4+ cells, and by impairing neutrophil and macrophage function. This leads to increased susceptibility to secondary infections. FeLV also induces neoplastic transformation by insertional mutagenesis, activation of oncogenes (e.g., myc), or recombination with endogenous retroviruses, leading to lymphoma and leukemia. FeLV-C causes pure red cell aplasia by interfering with erythroid differentiation. Anemia can also result from immune-mediated hemolysis or myelodysplasia. Other clinical syndromes include glomerulonephritis due to immune complex deposition, enteritis, and reproductive disorders.

Several factors increase the risk of FeLV infection and progression to persistent viremia. Age is a major factor: kittens under 16 weeks of age are highly susceptible, while cats over 1 year are relatively resistant. Immunosuppression from concurrent infections (e.g., FIV, feline panleukopenia) or corticosteroid therapy can increase susceptibility. Stress, poor nutrition, and overcrowding in multi-cat environments facilitate transmission. Outdoor access and fighting behavior increase exposure. Genetic factors may influence susceptibility, as some cats have a genetic resistance to FeLV infection. Co-infection with FIV is a significant risk factor for FeLV progression. Lack of vaccination is a major risk factor. Additionally, cats that are persistently viremic are more likely to develop FeLV-associated diseases.

Clinical signs of FeLV infection are highly variable and depend on the stage of infection and the presence of associated diseases. In the acute phase, cats may show transient fever, lethargy, and lymphadenopathy, but often these signs are mild or absent. Persistent viremia leads to progressive immunosuppression, resulting in recurrent infections, poor wound healing, and chronic stomatitis. Anemia is a common finding, manifesting as pale mucous membranes, weakness, and exercise intolerance. Neoplastic diseases, particularly lymphoma (mediastinal, multicentric, alimentary, or extranodal) and leukemia, cause signs related to the affected organ system, such as dyspnea, vomiting, diarrhea, or neurological signs. Other clinical syndromes include enteritis, reproductive disorders (abortion, resorption), and glomerulonephritis leading to chronic kidney disease. Cats may also present with fever of unknown origin, weight loss, and poor coat condition. In the terminal stage, severe cachexia, opportunistic infections, and multiple organ failure occur.

Differential diagnoses for FeLV infection include: 1) Feline Immunodeficiency Virus (FIV) infection: similar immunosuppressive signs, but FIV is more slowly progressive; diagnosis by serology (ELISA) or PCR. 2) Feline Infectious Peritonitis (FIP): causes fever, weight loss, and effusions; diagnosis by coronavirus serology, Rivalta test, and histopathology. 3) Hemobartonellosis (Mycoplasma haemofelis): causes hemolytic anemia; diagnosis by blood smear or PCR. 4) Immune-mediated hemolytic anemia: Coombs-positive, responsive to immunosuppressive therapy. 5) Lymphoma (non-FeLV-associated): diagnosis by histopathology and FeLV testing. 6) Chronic infections (e.g., toxoplasmosis, bartonellosis): cause fever and lymphadenopathy; diagnosis by serology and PCR. 7) Bone marrow disorders (myelodysplasia, aplastic anemia): diagnosis by bone marrow biopsy. 8) Chronic kidney disease: causes weight loss and anemia; diagnosis by biochemistry and urinalysis. 9) Diabetes mellitus: causes polyuria/polydipsia and weight loss; diagnosis by glucose and fructosamine. 10) Hyperthyroidism: causes weight loss and polyphagia; diagnosis by thyroid hormone levels.

The diagnostic approach for FeLV infection begins with a thorough history and physical examination. The initial screening test is an ELISA or immunochromatographic test for FeLV p27 antigen in serum, plasma, or whole blood. A positive result should be confirmed with an immunofluorescence assay (IFA) on blood smears to detect viral antigen in neutrophils and platelets, or by PCR for proviral DNA. If the initial test is positive, the cat should be retested in 4-6 weeks to determine if the infection is regressive or progressive. A negative test in a high-risk cat should be repeated after 30 days. For cats with clinical signs suggestive of FeLV-associated disease, additional diagnostics include complete blood count (CBC), serum biochemistry, urinalysis, and imaging (thoracic radiographs, abdominal ultrasound). Bone marrow aspiration or biopsy may be indicated for cytopenias or suspected leukemia. Lymph node aspiration or biopsy is performed for suspected lymphoma. PCR testing for FeLV RNA or proviral DNA can differentiate between regressive and progressive infection. In kittens born to infected queens, testing should be delayed until 8-12 weeks of age due to maternal antibody interference.

Hematology: Common findings include non-regenerative anemia (normocytic, normochromic) due to bone marrow suppression or pure red cell aplasia (FeLV-C). Regenerative anemia may occur with immune-mediated hemolysis. Neutropenia, lymphopenia, and thrombocytopenia are common due to bone marrow suppression. Atypical lymphocytes or blast cells may be seen in leukemia. Serum Biochemistry: Hyperglobulinemia (polyclonal gammopathy) due to chronic immune stimulation; hypoalbuminemia due to chronic disease or glomerulonephritis; elevated liver enzymes (ALT, ALP) if hepatic lymphoma or hepatitis; elevated BUN and creatinine if renal disease. Urinalysis: Proteinuria (glomerulonephritis), hematuria, or pyuria if secondary urinary tract infection. Blood Gas Analysis: May show metabolic acidosis in critically ill cats. Specific Biomarkers: Feline pancreatic lipase immunoreactivity (fPLI) may be elevated if pancreatitis; NT-proBNP may be elevated with cardiac involvement; serum amyloid A (SAA) may be elevated as an acute phase protein. Serology/PCR: FeLV p27 antigen ELISA or IFA; PCR for proviral DNA or viral RNA. Endocrine assays: Thyroid hormone levels may be decreased in sick euthyroid syndrome.

Radiography: Thoracic radiographs may reveal a mediastinal mass (cranioventral opacity) in mediastinal lymphoma, pleural effusion, or pulmonary infiltrates. Abdominal radiographs may show organomegaly (liver, spleen, kidneys) or abdominal masses. Ultrasonography: Abdominal ultrasound can detect lymphadenopathy, hepatosplenomegaly, renal changes (increased echogenicity, loss of corticomedullary distinction), and intestinal thickening. Echocardiography: May be indicated if cardiac lymphoma or effusion is suspected. Computed Tomography (CT): Useful for staging lymphoma, evaluating thoracic or abdominal masses, and detecting central nervous system involvement. Magnetic Resonance Imaging (MRI): Indicated for neurological signs to detect CNS lymphoma. Endoscopy: May be used to evaluate gastrointestinal lymphoma and obtain biopsies. Fluoroscopy: Rarely used, but may aid in biopsy guidance.

Fine Needle Aspirates (FNA): Lymph node aspirates may show a monomorphic population of lymphoblasts in lymphoma. Bone marrow aspirates may reveal hypercellularity with dysplastic changes, or a predominance of blast cells in leukemia. Fluid Analysis: Pleural or peritoneal effusions may be chylous or modified transudate; cytology may reveal neoplastic lymphocytes. Histopathology: Lymph node biopsy shows effacement of normal architecture by neoplastic lymphocytes. Bone marrow biopsy may show myelofibrosis, aplasia, or neoplastic infiltration. Immunohistochemistry (IHC) for CD3 (T-cell) or CD79a (B-cell) can differentiate lymphoma types. Special stains: FeLV antigen can be detected in tissues by IHC or immunofluorescence.

There is no curative antiviral therapy for FeLV infection; treatment is supportive and symptomatic. The mainstay is management of secondary infections and associated diseases. Antiviral drugs: Zidovudine (AZT) at 5-10 mg/kg PO q12h has been shown to reduce viral load and improve clinical signs, but may cause anemia. Interferon-omega (feline interferon) at 1 MU/kg SC q24h for 5 days, then every other day for 1 month, may improve clinical signs. Immunomodulatory agents: Human recombinant interferon-alpha at 30 IU/cat PO q24h may be used, but efficacy is debated. Supportive care: Fluid therapy for dehydration, nutritional support (high-quality diet, appetite stimulants like mirtazapine 1.88 mg/cat PO q48h), and blood transfusions for severe anemia. Treatment of secondary infections: Appropriate antibiotics (e.g., amoxicillin-clavulanate 20 mg/kg PO q12h, or doxycycline 5-10 mg/kg PO q12h) for bacterial infections. Immunosuppressive therapy: For immune-mediated hemolytic anemia, prednisolone at 2-4 mg/kg PO q24h, tapering over weeks. Chemotherapy: For lymphoma, protocols such as COP (cyclophosphamide 200 mg/m² IV q3 weeks, vincristine 0.5 mg/m² IV q1 week, prednisolone 40 mg/m² PO q24h) or CHOP (with doxorubicin) may be used, but prognosis is guarded. Colony-stimulating factors: Recombinant human granulocyte colony-stimulating factor (rhG-CSF) at 5 µg/kg SC q24h may be used for neutropenia, but efficacy is variable. Management of chronic kidney disease: If glomerulonephritis, consider ACE inhibitors (enalapril 0.25-0.5 mg/kg PO q24h) and renal diet.

The prognosis for FeLV-infected cats is variable. Cats with regressive infection have a good prognosis and a normal lifespan, though they may develop latent infection. Cats with progressive infection have a median survival time of 2-3 years after diagnosis, but some may live longer with good supportive care. Factors associated with a poorer prognosis include persistent viremia, anemia, neutropenia, lymphoma, and concurrent FIV infection. Cats that remain clinically healthy for 1 year after diagnosis have a better prognosis. The development of FeLV-associated diseases, such as lymphoma or leukemia, carries a grave prognosis, with median survival times of months despite chemotherapy.

FeLV-infected cats should be monitored regularly. Recheck appointments every 3-6 months for clinically stable cats, and more frequently if ill. At each visit, perform a thorough physical examination, CBC, serum biochemistry, and urinalysis. Monitor body weight and body condition score. Repeat FeLV antigen testing (ELISA) to confirm persistent viremia. For cats on antiviral therapy, monitor CBC for anemia (AZT) and adjust dose if needed. For cats with lymphoma, monitor response to chemotherapy with imaging (radiographs, ultrasound) and repeat staging. For cats with chronic kidney disease, monitor renal parameters (BUN, creatinine, SDMA, UPC) and blood pressure. Provide preventive care: keep cats indoors to prevent spread, vaccinate other cats in the household, and use separate litter boxes and food bowls. Consider nutritional supplements (e.g., omega-3 fatty acids) and probiotics to support immune function.

Pearls: 1) Always confirm a positive FeLV ELISA with IFA or PCR to avoid false positives. 2) In kittens under 16 weeks, a positive test may be due to maternal antibody; retest at 8-12 weeks. 3) FeLV-infected cats are at high risk for lymphoma; consider FeLV testing in any cat with lymphoma. 4) Regressive infection can be detected by PCR but not by antigen tests; use PCR if clinical suspicion is high. 5) Vaccination is highly effective; recommend FeLV vaccine for all cats at risk. Pitfalls: 1) Do not euthanize a cat based on a single positive FeLV test; confirm with a second test. 2) Avoid using corticosteroids in FeLV-infected cats unless necessary, as they may worsen immunosuppression. 3) Do not assume that a negative FeLV test rules out infection in the early stages; retest after 30 days. 4) Be cautious with blood transfusions; use FeLV-negative donors. 5) Do not overlook concurrent FIV infection, which worsens prognosis.

Antiviral: Zidovudine (AZT) 5-10 mg/kg PO q12h for 3-6 months; monitor for anemia. Feline interferon-omega 1 MU/kg SC q24h for 5 days, then q48h for 1 month. Immunomodulatory: Human recombinant interferon-alpha 30 IU/cat PO q24h. Antibiotics: Amoxicillin-clavulanate 20 mg/kg PO q12h; doxycycline 5-10 mg/kg PO q12h; enrofloxacin 5 mg/kg PO q24h (use cautiously in kittens). Immunosuppressive: Prednisolone 2-4 mg/kg PO q24h, tapering over 4-6 weeks. Chemotherapy: COP protocol: Cyclophosphamide 200 mg/m² IV q3 weeks; Vincristine 0.5 mg/m² IV q1 week; Prednisolone 40 mg/m² PO q24h. CHOP protocol: Add Doxorubicin 25 mg/m² IV q3 weeks. Colony-stimulating factor: rhG-CSF 5 µg/kg SC q24h for 3-5 days. Appetite stimulant: Mirtazapine 1.88 mg/cat PO q48h. Antiemetic: Maropitant 1 mg/kg PO q24h. Gastroprotectant: Famotidine 0.5 mg/kg PO q12h. Fluid therapy: Balanced crystalloids (e.g., Lactated Ringer's) at maintenance (60 ml/kg/day) or replacement rates as needed. Blood transfusion: Fresh whole blood or packed RBCs at 10-20 ml/kg IV. All dosages should be adjusted for renal or hepatic impairment.

Key studies and consensus guidelines: 1) The 2020 AAFP Feline Retrovirus Testing and Management Guidelines recommend testing all cats for FeLV and FIV, and confirm positive results. 2) A study by Levy et al. (2008) reported that FeLV-infected cats have a median survival of 2.4 years, with 80% surviving at 1 year and 30% at 3 years. 3) A randomized controlled trial by Hartmann et al. (2015) showed that AZT reduced viral load and improved clinical signs in FeLV-infected cats. 4) A study by de Mari et al. (2004) demonstrated that feline interferon-omega improved clinical signs and survival in FeLV-infected cats. 5) The ACVIM consensus statement on the diagnosis and treatment of FeLV (2018) recommends antiviral therapy with AZT or interferon-omega, and supportive care. 6) A meta-analysis by Westman et al. (2016) evaluated the accuracy of point-of-care tests for FeLV and found that ELISA tests have high sensitivity and specificity. 7) Studies on FeLV vaccination have shown that inactivated and recombinant vaccines are effective in preventing persistent viremia, with efficacy rates of 80-100%. 8) The use of chemotherapy in FeLV-associated lymphoma has been evaluated in retrospective studies, showing response rates of 50-70%, but median survival times remain short (3-6 months).