There are many presenting signs of FeLV, none of which are pathognomonic.  Most cats brought to the veterinarian present with weight loss, anorexia, depression, anemia, renal failure with polyuria/polydipsia, and immunosuppression.

Death of persistently viremic cats occurs within 3 years in 80% of cases (4).  FeLV causes oncogenesis to give rise to tumors, as well as degenerative, proliferative and immunologic diseases.  Co-infection is common with diseases such as Feline Infectious Peritonitis (FIP), upper respiratory diseases, Feline Immunodeficiency Virus (FIV), hemotrophic mycoplasmosis, stomatitis, anemia, lymphoma, leukopenia, thrombocytopenia, and leukemia (4).


Age is the most important host factor for developing clinical signs (3).  FeLV in neonatal kittens causes viremia if they fail to clear the virus, resulting in thymic atrophy, which can lead to fading kitten syndrome.  Kittens fail to nurse, become dehydrated, are found hypothermic, and die within two weeks.  Older cats exposed to FeLV have a progressive resistance against infection (2).  Usually cats that become infected develop mild clinical signs, which disappear once the cat has cleared the virus.

Viral factors such as exposure to a specific subgroup are also responsible for clinical manifestation.  FeLV-B usually causes tumor formation while FeLV-C causes non-regenerative anemia (4).

Major clinical signs of FeLV fall under five major categories: (3)

1.      TUMORS

When the provirus inserts randomly into the host cell DNA, it alters expression of neighbouring genes and removes the regulation of protein synthesis.  Non-regulated gene expression leads to malignancy if there is no proper immune response directed at the excessive cellular proliferation.

i)                     Lymphoma                                                                                                            

A neolastic disorder of lymphoid tissue which is also known as malignant lymphoma.  They are classified based on the predominant cell type affected and degree of differentiation.  Lymphomas can develop in the absence of FeLV, but most
cases are associated with FeLV.  Lymphoma affects the T-cells only; B-cells remain functional.  There are three major types of lymphoma associated with FeLV:

a)     Mediastinal lymphoma - lymphoma that develops in the thymus usually occurs in cats less than 3 years of age.  80 to 90% of all cases are associated with FeLV.  It causes malignant pleural effusion.  Clinical signs include dyspnea, occasional                                                    Thymic Lymphoma
regurgitation (from esophageal pressure), and sometimes         
Horner’s syndrome (pressure on the sympathetic nerves).

b)     Alimentary lymphoma - only 25-30% of cases are associated with FeLV, mainly because this is a lymphoma of older cats.  Clinical signs include vomiting, diarrhea, anorexia, and weight loss.  Mesenteric lymph nodes are affected.

c)      Multicentric lymphoma - half of all cases are associated with FeLV.  Several sites are affected such as the nasal cavity, liver, kidneys, bladder, brain.

ii)                  Leukemia

A progressive, malignant disease of the blood-forming organs, marked by proliferation and development of leukocytes and their precursors in the bone marrow and blood.  It causes enlarged liver, spleen, and lymph nodes.
Half of all cases are associated with FeLV.  Leukemia affects all hematopoietic cell lines and causes myeloproliferative diseases such as erythremic myelosis (low hematocrit, non-regenerative anemia, abnormal red blood cells).  Hematopoiesis can be suppressed, resulting in clinical signs such as lethargy from anemia, sepsis from granulocytopenia, bleeding tendencies from thrombocytopenia, hepatomegaly from cellular infiltration and splenomegaly.

iii)                Myelofibrosis

Abnormal fibroblast proliferation due to chronic bone marrow stimulation.  It causes replacement of bone marrow with fibrous tissue and occurs in the end-stage of myeloid metaplasia.  

iv)                Fibrosarcoma

A neoplasia arising from fibroblasts, fibrosarcoma results from infection with a recombinant FeLV virus, called FeSV.  FeSV contains the genome from FeLV-A in addition to incorporated cellular oncogenes.  It causes multicentric tumors.



These disorders result from bone marrow suppression.  They require actively replicating virus.

i)                    Anemia - a reduction in the normal number or volume of circulating red blood cells, or a decrease in quantity of                         hemoglobin in the blood.  Some clinical signs associated with anemia are weakness, exercise                                     intolerance, moderate tachypnea, pale mucous membranes, tachycardia, and increased heart                                         sound intensity.

a)     hemolytic anemia - 10% of anemias are regenerative with a high reticulocyte count.  Clinical signs include lethargy, anorexia, depression, pallor, icterus, dehydration, and splenomegaly.

b)     Non-regenerative anemia - most anemias resulting from FeLV are non-regenerative and result from bone marrow suppression.  Reticulocytosis is not observed.  Clinical signs include weakness, pallor, tachycardia, hyperpnea, and increased heart sound intensity.

c)      Pure erythrocyte aplasia—results from FeLV-C infection.  There is no regeneration of red blood cells and hematocrit drops to  less than 15%.  It is not treatable.

d)     Pancytopenia (aplastic anemia)—involves all cell lines and is usually associated with FeLV cats.

ii)                  Platelet Function

FeLV suppresses bone marrow and can infect megakaryocytes.  Platelet production is suppressed, resulting in thrombocytopenia.  Platelets have an altered shape, size, function, and vary in number.  FeLV cats have a decreased platelet lifespan.  This can lead to bleeding tendencies.

iii)                Leukocyte abnormalities

FeLV decreases the number of normal granulocytes and lymphocytes to create a lymphopenia.

iv)                Feline panleukopenia-like syndrome (FPLS)

Causes a severe leukopenia and enteritis with anemia.  This is one way to differentiate it from feline panleukopenia virus.



Immunosuppressed cats are predisposed to a variety of insults.  Different cats have varying degrees of immunosuppression.  Thymic atrophy and lymph nodes are usually affected.  Lymphopenia and
neutropenia occur.  Concurrent infections (bacterial, viral, protazoal, and fungal) arise from a lack of immune



These diseases arise from an over-active dysregulated immune system.  Some FeLV associated diseases are:

i)                    autoimmune hemolytic anemia - an immune-mediated disease resulting from autoantibodies directed at                                                                                    erythrocytes with concurrent lysis

ii)                  glomerulonephritis - inflammation of the capillary loops in the glomeruli of the kidney which can be in acute,                                                     subacute, or chronic forms.

iii)                uveitis  - inflammation of the uvea (iris, ciliary body, and choroid), followed with immune complex deposition in                          the eye

iv)                polyarthritis - inflammation of several joints due to deposition of immune complexes

            Immune complexes form because T-suppressor cells are dysfunctional.



i)                    Reproductive problems

The virus can infect kittens transplacentally and kill them, causing embryonic death and fetal resorption.  Abortion is also common, as well as neonatal death if the kittens survive birth.

                                      Aborted kitten fetuses due to Feline Leukemia

ii)                  Neuronopathy

This problem occurs from white matter degeneration, swollen axons, and dilated myelin in the brainstem and spinal cord.  Clinical signs include abnormal vocalization, hyperesthesia, paresis/paralysis, anisocoria, and urinary incontinence.

iii)                Hepatopathy

A liver problem that usually results in icterus, inflammatory, and degenerative liver diseases such as hepatic lipidosis and liver necrosis.

iv)                FeLV-associated Enteritis (FAE)

This inflammatory disease results from degeneration of the intestinal epithelium.  Clinical signs such as hemorrhagic diarrhea, vomiting, oral ulceration, gingivitis, anorexia, and weight loss may be present.

v)                  FeLV-Feline Acquired Immunodeficiency Syndrome

This syndrome develops clinical signs such as intractable diarrhea, weight loss, and lymphopenia.

vi)                Skin Diseases

These may arise from immunodeficiency.  FeLV cats have an increase in mucosal and bacterial microflora, which are commonly associated with secondary infections.  Abscessation may occur following trauma.

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