Feline Leukemia Virus (FeLV)

Overview of Feline Leukemia Virus (FeLV)

Feline leukemia virus, commonly abbreviated as FeLV and called “fe-leuk”, is a viral disease of domestic cats that impairs immune system function and causes some types of cancer. FeLV is transmitted by bite wounds, shared dishes or litter pans, or close contact (grooming). Kittens can be born with FeLV or acquire the virus through their mother’s milk.

Below is an overview on Feline Leukemia Virus (FeLV) followed by in-depth detailed information on the diagnosis and treatment of this condition. 

The average age of infection is 3 years and male cats may have a higher prevalence of infection than female cats. The rate of FeLV infection is higher in multi-cat households, catteries and urban areas with high cat populations. The FeLV infection rate ranges from 2 to 13 percent in the general cat population.

FeLV is destroyed in the environment within minutes. There is no evidence of transmission of FeLV from cats to humans. The course of FeLV infection is variable, and there are 3 main stages of infection.

Approximately 33 percent of FeLV-infected cats are infected briefly but are able to eliminate the virus within 4 to 6 weeks. Such transiently infected cats do not become ill or develop FeLV-related disorders.

  • Another 33 percent of FeLV-infected cats develop FeLV-related disease with approximately 60 percent mortality within 2 years.
  • The remaining 33 percent of infected cats cannot completely eliminate the virus, but do not usually develop FeLV-related disease. Such infections cannot be detected with routine blood tests, and require special diagnostic tests such as bone marrow culture of the virus or identification by a special molecular biology technique called polymerase chain reaction (PCR). In a small percentage of these cats, reactivation of the virus is possible. Latent infection also may explain development of certain types of cancers which develop in older cats.
  • FeLV-associated diseases usually are categorized as neoplastic (cancer) or non-neoplastic (include impairment of immune function and development of secondary infections).
  • What to Watch For

  • Secondary infections
  • Anemia
  • Difficulty swallowing or eating
  • Weight Loss
  • Difficulty breathing
  • Eye problems
  • Coughing
  • Lethargy
  • Poor wound healing
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    Diagnosis of Feline Leukemia Virus (FeLV)

    Veterinary care should include diagnostic tests and subsequent treatments. Diagnostic tests are needed to recognize FeLV, and exclude other diseases. Tests may include:

  • A complete medical history and a thorough physical examination.
  • A screening blood test for FeLV. The most commonly performed screening tests are the ELISA (enzyme-linked immunosorbent assay) and IFA (indirect immunofluoresence) tests. Healthy cats should not be euthanized based on the results of one screening blood test.

    Treatment of Feline Leukemia Virus (FeLV)

  •  There is no specific treatment that will reliably eliminate FeLV infection. However, treatments for FeLV may include any of the following:
  • Antibiotics for secondary bacterial infections
  • Nutritional support (force feeding, tube feeding)
  • Parasite control
  • Biopsy to diagnose cancer
  • Surgery to remove tumors
  • Chemotherapy for Lymphosarcoma (cancer of the lymph nodes and tissues that produce lymphocytes)
  • Home Care

    Administer as directed any medications prescribed by your veterinarian. Provide high quality nutrition and schedule regular follow-up visits with your veterinarian to monitor your cat’s condition.

    All FeLV-infected cats should remain indoors to decrease exposure to other cats. Isolate FeLV-infected cats from FeLV-negative cats.

    Preventative Care

    Feline leukemia virus is an infectious disease that can be prevented primarily by eliminating interactions with infected cats. Recommendations include:

  • Keep your cat indoors.
  • Prevent fighting with other cats.
  • Neuter males to help decrease roaming and fighting.
  • Test other cats in the household and isolate FeLV-negative cats from FeLV-positive cats.
  • Vaccinate “at-risk” cats against FeLV. “At-risk” cats include cats that spend any time outdoors, breeding cats, or cattery cats.

    Vaccination against other diseases should be discussed with your veterinarian. If yearly vaccinations are given, only killed vaccine products (those composed of killed virus rather modified live virus) should be used to avoid complications in FeLV-infected cats with potentially weakened immune systems.

    Antibiotics should be administered to FeLV-infected cats before preventative dentistry to minimize the risk of secondary bacterial infection.

  • In-depth Information on Feline Leukemia Virus (FeLV)

    Other medical problems can lead to symptoms similar to those encountered in FeLV-infected cats. It is important to exclude these conditions before establishing a diagnosis of FeLV infection.

  • Feline ehrlichiosis (a disease caused by bacteria called rickettsia which live inside the cells of the animal).
  • Feline infectious peritonitis (FIP) (a viral infection of cats that causes fever and abnormal function of the nervous system, eye, liver, and kidneys).
  • Feline immunodeficiency virus (FIV) (a viral infection of cats similar to AIDS in humans).
  • Feline infectious anemia, also known as Hemobartonella felis (a parasite that infects the red blood cells of cats).
  • Hemolytic anemia (a disease characterized by destruction of the cat’s red blood cells).
  • Histoplasmosis (a disease caused by a fungus in the environment that can infect the lungs and other organs of pets).
  • Leukemia (cancer of the blood-forming tissues of the body).
  • Lymphoid hyperplasia syndrome (a non-cancerous abnormal proliferation of lymphocytes in the lymph nodes of the body).
  • Lymphosarcoma (Lymphoma) (cancerous proliferation of lymphocytes in the lymph nodes and other lymphoid tissues of the body).
  • Mast Cell Tumors (a tumor arising in the skin or occasionally in the spleen or intestinal tract of cats).
  • Multiple myeloma (a tumor arising from the antibody-producing cells of the body called plasma cells; also called plasma cell tumor or plasma celly dyscrasia).
  • Myeloproliferative disease (cancerous proliferation of white blood cell precursor cells in the bone marrow).
  • Cancers of tissues other than lymph nodes and bone marrow.
  • Rocky Mountain spotted fever (another rickettsial disease).
  • Sepsis (bodywide bacterial infection).
  • Systemic lupus erythematosus (a systemic autoimmune disorder affecting the skin, kidneys, joints, and blood-forming tissues).
  • Thrombocytopenia (a decrease in blood platelet concentration).
  • Thymoma (cancer of the thymus, a blood cell-producing organ found in the chest of young animals).
  • Toxoplasmosis (a disease caused by a protozoan parasite that affects the nervous system, eye, and other organs).
  • Veterinary care should include diagnostic tests and subsequent treatments.

    Diagnosis In-depth

    Certain diagnostic tests must be performed to confirm the diagnosis of FeLV infection and exclude other diseases that may cause similar symptoms. Tests may include:

  • A complete medical history and a thorough physical examination. Special attention will be paid to the color of the mucous membranes including the gums, lymph node size and the presence of any signs of bacterial infection.
  • Blood tests for other infectious diseases of cats such as feline immunodeficiency virus (FIV). The symptoms of FIV infection are very similar to those of FeLV infection.
  • Tests for feline infectious peritonitis (FIP),feline toxoplasmosis, and ehrlichiosis. These diseases of cats also have symptoms that may resemble those found in FeLV-infected cats.
  • A complete blood count (CBC or hemogram) to evaluate your cat for anemia, low platelet count, inflammatory or infectious diseases, and cancer of blood cells.
  • A biopsy or fine needle aspirate on enlarged lymph nodes or solid tumors to obtain a sample for microscopic examination.

    Your veterinarian may recommend additional diagnostic tests to exclude or diagnose other conditions, or to better understand the impact of FeLV on your pet. These tests insure optimal medical care and are selected on a case-by-case basis. Examples include:

  • Serum biochemistry tests. These tests evaluate the impact of FeLV infection on other organ systems (e.g. liver, kidney) and evaluate the general health of your pet. They usually are normal in cats infected with FeLV.
  • Urinalysis to evaluate kidney function, identify protein loss in the urine and determine if urinary tract infection is present. Urinalysis usually is normal in cats infected with FeLV.
  • Thoracocentesis is a procedure in which fluid is withdrawn from the chest cavity using a syringe and needle. It is used to relieve difficult breathing in cats with accumulation of fluid in the chest and to obtain a fluid sample for cyotologic analysis. Cats with FeLV infection and a form of cancer called “thymic lymphosarcoma.” Other diseases that may cause accumulation of fluid in the chest in cats include pyothorax (an infection causing accumulation of purulent material in the chest), chylothorax (an accumulation of fluid that is high in fat content in the chest), heart failure, and feline infectious peritonitis (FIP).
  • Abdominal paracentesis is a procedure in which fluid is withdrawn from the abdomen using a syringe and needle. This procedure is used to obtain fluid for microscopic analysis, which may facilitate diagnosis.
  • X-rays of the chest may be recommended in cats with difficulty breathing (“dyspnea”) or abnormal findings that your veterinarian may hear by listening to the cat’s chest with a stethoscope. Chest x-rays may show fluid accumulation or pneumonia caused by secondary bacterial infection.
  • Microscopic evaluation of bone marrow samples may be indicated in cats with severe anemia, low white blood cell count, or low platelet count. In some instances, FeLV infection can cause decreased production of red cells by the bone marrow and occasionally tumor cells may invade the bone marrow and crowd out the blood cell precursors normally found in the marrow.
  • Cerebrospinal fluid analysis may be recommended in FeLV-infected cats that have symptoms of neurologic disease. Microscopic and chemical analysis of cerebrospinal fluid may identify infectious agents, inflammatory cells, or cancer cells.
  • Treatment In-depth

    Treatment of FeLV infection must be individualized based on the severity of the condition and other factors that must be analyzed by your veterinarian. If your cat has no clinical signs, no treatment may be recommended. In this situation, regular follow-up visits to your veterinarian are important to evaluate your cat for possible progression of disease. If FeLV-induced disease is present, additional treatments may be necessary.

    There is no effective treatment that will eradicate established FeLV infection. Supportive care is important, and may include:

  • Antibiotics for secondary bacterial infections
  • Nutritional support
  • Fluid therapy in dehydrated FeLV-infected cats
  • Parasite control
  • Topical medications to treat inflammation of the mouth (stomatitis) if present
  • FeLV-infected cats should be kept inside and isolated from other cats
  • Masses found in FeLV-infected cats should be biopsied and removed as necessary
  • Blood transfusions should be administered on an emergency basis as necessary in FeLV-infected cats with severe anemia
  • Suspected concurrent infection by the red blood cell parasite Hemobartonella felis should be treated in FeLV-infected cats suffering from hemolytic anemia. Tetracycline antibiotics are used against this organism and cortisone-like drugs (e.g. prednisone) also may be used if immune-mediated destruction of red blood cells is suspected.
  • Manage lymphosarcoma (Lymphoma), if present, using cancer chemotherapy drugs. Cancer chemotherapy should be supervised by a veterinary oncologist or a veterinarian who has experience using anti-cancer drugs.
  • Drug Therapy for Feline Leukemia Virus (FeLV)

  • Interferon (Roferon-A®) may be used in an attempt to limit viral replication. It is not licensed for use in cats, but has been used by some veterinarians to treat FeLV-infected cats. It may prevent disease development and prolong survival.
  • Other agents that stimulate the immune system potentially could be beneficial in FeLV-infected cats. Examples include diethycarbamazine, Staphylococcal protein A (SPA), Propionbacterium acnes (Immunoregulin) and acemannan (Carrisyn). The effectiveness of these agents is unknown.
  • Zidovudine or azidothymidine (commonly called AZT) is a nucleoside analog that is used to treat human patients with AIDS. AZT may limit virus replication and may prolong survival of FeLV-infected cats, but is was most effective in experimental cats when given very soon after infection. AZT has the potential to cause serious adverse effects (including bone marrow suppression) and should only be administered to cats under the supervision of a veterinary oncologist or a veterinarian experienced in the use of anti-viral medications.
  • AZT) and PMEA have been reported to reduce the severity of chronic mouth infections in cats with FeLV. These drugs have the potential for serious adverse effects and should only be administered to cats under the supervision of a veterinary oncologist or a veterinarian experienced in the use of anti-viral medications.
  • Bone marrow transplantation has been performed in some affected cats and may correct low white cell counts, but cats remain infected with FeLV. This experimental procedure would only be available at a small number of veterinary research institutes.
  • Follow-up Care for Cats with Feline Leukemia Virus 

    Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be crucial and may include the following:

  • Administer as directed all medications prescribed by your veterinarian.
  • Observe your cat’s appetite and general activity level. Examine and monitor mucous membrane color (the pink color of the gums), urinations, defections, lymph node size and presence of any masses.

    The “Test and Remove” strategy is a method to eliminate FeLV-positive cats from catteries or multiple cat households and includes the following:

  • Do not bring any new cats of unknown FeLV status into the cattery or household.
  • Remove all FeLV-positive sick cats from the cattery or household.
  • Test all remaining cats Feline in the cattery or household for FeLV infection.
  • Disinfect all bedding, food and water dishes, litter pans, and other objects that have been in contact with infected cats.
  • Confine all FeLV-negative cats in one area. Re-test these cats in 3 months. These cats are considered free of FeLV if they are negative on re-testing.
  • Quarantine all new cats and isolate them from other cats in the cattery or household for 3 months.
  • Newly added cats are considered free of FeLV infection after 2 negative test results 3 months apart.
  • Re-test any FeLV-positive cat.
  • Cats that re-test positive on the second test may be latently infected or persistently infected.
  • Cats that re-test negative after testing positive on the first test should continue to be isolated. Retest these cats after 3 more months. They may be transiently infected but are not considered negative until they have 2 negative test results 3 months apart
  • The actual follow-up procedure will depend on the severity of your cat’s disease, his response to therapy, your veterinarian’s recommendations and your own wishes for your cat.