A CBC can be performed on whole blood, revealing a non-regenerative anemia, often severe, together with a lymphopenia.  A neutropenia and thrombocytopenia can also be seen from bone marrow suppression or immune mediated destruction.  Red cells reveal increased nucleated cells in circulation and a macrocytosis, with or without an appropriate reticulocytosis.  Chemistry results can reveal azotemia, hyperbilirubinemia, bilirubinuria, and increased liver enzymes. A proteinuria with golmerulonephritis can be found on urinalysis.  Arrest of the bone marrow occurs causing erythrodysplasia (2).  A bone marrow aspiration or biopsy can be performed to check if hypocellular bone marrow exists(8). 



Testing for the FeLV antigen is the most effective way to prevent the spread of the virus.  Vaccines should not be a substitute for testing.  Always test before giving a vaccination (3), to ensure that an infected cat is not being vaccinated.  An infected cat will not respond to the vaccine.  If the tested cat does reveal a FeLV positive test, then appropriate steps can be taken to minimize disease progression.

There are 2 main tests that can be done to detect FeLV positive cats.  They detect the FeLV antigens, not the anti-FeLV antibodies produced by the cat.  Maternal antibodies and vaccines also do not interfere with testing (3).
1.    IFA (ImmunoFluorescent Antibody) – IFA detects the structural core antigen p27 within the cytoplasm of blood cells (leukocytes and platelets) (3).  This cannot be done in house and must be sent to a commercial laboratory (4).  Peripheral blood smears are made using whole blood or the buffy coat and then analyzed.  Cytologic preparations of bone marrow can also be used (8).  IFA is not found positive until AFTER the bone marrow is affected (2).  Therefore, this test is used for prognostic reasons or to confirm a positive result with the ELISA test and suspicious samples (3).  97% of IFA positive cats remain persistently infected, contagious, and viremic for life (8).
Immunofluorescence Antibody Identification                                             Method of Detecting Feline Leukemia        www.ihcworld.com/.../10001/normal_RBL.jpg                                                        using IFA Diagnosis                                                                                                                                                            www.dshs.state.tx.us/lab/images/ifa.jpg

      2.    ELISA (Enzyme Linked ImmunoSorbent Assay) – ELISA detects the core p27                     antigen as well as free soluble FeLV p27 in plasma or serum (3).  It is usually found in             the form of in-house test kits.  Most test kits have a sensitivity and specificity of 98%             (4).  A colour change from the immunoglobulin reaction results in a positive sample.              Whole blood, plasma, serum, saliva, or tears can be used, although the latter two are             not reliable (2).  ELISA tests can be run to diagnose infections PRIOR to the infection of         bone marrow.  Therefore, FeLV can be detected in the early stages of infection and                 self-limiting infections even if the IFA test is negative.  A single positive result cannot             predict which cats will be persistently viremic; the cat must be retested in 12 weeks (8). 
        If ELISA positive cats revert to negative they develop neutralizing antibodies, latent                 infections, or a sequestered infection.
                        ELISA Snap Test


-    Accuracy can be improved by running both ELISA and IFA tests together.  
-    A positive ELISA cat should be retested 6 weeks after the initial test.  If still positive, then a retest is performed on the      cat after 10 weeks.  If positive on the third test then the cat is most likely persistently viremic and positive for life.  
-    A positive ELISA cat can be immediately tested with the IFA test and if also found positive a transient infection is               least likely (3-9%).  
-    A positive ELISA is more likely to convert to negative than cats with a positive IFA test.  
-    If a repeated ELISA test is positive and the IFA test is negative there is most likely a focal infection kept localized by          the immune system.  
-    A negative ELISA and positive IFA is always false.  
-    Blood smears being too thick can create false positives (2).
-    False negatives occur with a leukopenia or thrombocytopenia.  This is due to the evaluation of an inadequate                  number of cells (2). 
-    Virus isolation and/or PCR can be performed as confirmatory tests for latent or sequestered infections.  

All sick cats, new cats to be introduced to a household, exposed cats, and high risk groups should be tested for FeLV.  Keep in mind that no test is 100% accurate; all results should be interpreted in light of the patient’s health and prior exposure to FeLV (3).  


Cytologic exam of fine needle aspirates of masses, lymph nodes, body cavity fluids, or affected organs can reveal malignant lymphocytes.  A bone marrow exam can reveal lymphocytic involvement even when peripheral blood appears normal.  A biopsy and histopathology is often necessary for a diagnostic confirmation of neoplasia (4). 

Neoplasia in a cat

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