Feline Leukemia Virus (FeLV): Symptoms, Progression, Transmission, Diagnosis, Prevention, Treatment

First discovered in the 1960s, feline leukemia virus (FeLV) is a transmittable RNA retrovirus that can severely inhibit a cat’s immune and causes certain types of cancer. This virus infection is responsible for a majority of deaths in household cats, affecting all breeds. Males are more likely to contract the infection than females, and it is usually seen between the ages of one to six years old.

Feline leukemia virus is one of the most common infectious diseases in cats, affecting between 2 and 3% of all cats in the United States. Infection rates are significantly higher (up to 30%) in cats that are ill or otherwise at high risk. Fortunately, the prevalence of FeLV in cats has decreased significantly in the past 25 years since the development of an effective vaccine and accurate testing procedures.

Feline leukemia is a cancerous disease caused by feline leukemia virus. FeLV causes diseases other than leukemia including other cancers and immunodeficiency. Cats may not start to show signs of disease for months or years after being infected with FeLV. Infection with FeLV is a major cause of illness and death in domestic cats. Approximately 2.3% of cats in the United States are infected with FeLV.

There are three popular types of feline leukemia infections, FeLV-A occurs in all cats infected with the virus. It causes the suppression of the immune system that makes the disease so dangerous. FeLV-B occurs in about half of infected cats, and it causes tumors and other abnormal tissue growth. FeLV-C is by far the rarest, found in only about one percent of infected cats, and causes severe anemia. FeLV-positive cats can have one, two or all three types of infections.

Signs and symptoms

feline leukemia

The signs and symptoms of infection with feline leukemia virus are quite varied and include loss of appetite, poor coat condition, anisocoria (uneven pupils), infections of the skin, bladder, and respiratory tract, oral disease, seizures, lymphadenopathy (swollen lymph nodes), skin lesions, fatigue, fever, weight loss, stomatitis, gingivitis, litter box avoidance, pancytopenia, recurring bacterial and viral illnesses, anemia, diarrhea and jaundice.

Asymptomatic carriers will show no signs of disease, often for many years.


feline leukemia

The disease has a wide range of effects. The cat can fight off the infection and become totally immune, can become a healthy carrier that never gets sick itself but can infect other cats, or a mid-level case in which the cat has a compromised immune system. Nevertheless, the development of lymphomas is considered the final stage of the disease. Although it is thought that virus protein has to be present to induce lymphomas in cats, newer evidence shows that a high percentage of FeLV-Antigen negative lymphomas contain FeLV-DNA, indicating a “hit-and-run” mechanism of virus-induced tumor development.

Once the virus has entered the cat, there are six stages to a FeLV infection.

  • Stage One: The virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsilar B-lymphocytes and macrophages. These white blood cells then filter down to the lymph nodes and begin to replicate.
  • Stage Two: The virus enters the blood stream and begins to distribute throughout the body.
  • Stage Three: The lymphoid system (which produces antibodies to attack infected and cancerous cells) becomes infected, with further distribution throughout the body.
  • Stage Four: The main point in the infection- where the virus can take over the body’s immune system and cause viremia. During this stage the hemolymphatic system and intestines become infected.

If the cat’s immune system does not fight off the virus, then it progresses to:

  • Stage Five: The bone marrow becomes infected. At this point, the virus will stay with the cat for the rest of its life. In this phase, the virus replicates and is released four to seven days later in infected neutrophils, and sometimes lymphocytes, monocytes, and eosinophils (all white blood cells formed in the bone marrow).
  • Stage Six: The cat’s body is overwhelmed by infection and mucosal and glandular epithelial cells (tissue that forms a thin protective layer on exposed bodily surfaces and forms the lining of internal cavities, ducts, and organs) become infected. The virus replicates in epithelial tissues including salivary glands, oropharynx, stomach, esophagus, intestines, trachea, nasopharynx, renal tubules, bladder, pancreas, alveolar ducts, and sebaceous ducts from the muzzle.


feline leukemia

Cats persistently infected with FeLV serve as sources of infection for other cats. The virus is shed in saliva, nasal secretions, urine, feces, and milk of infected cats. Cat-to-cat transfer of the virus may occur from a bite wound, during mutual grooming, and (rarely) through the shared use of litter boxes and feeding dishes. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing. FeLV does not survive long outside a cat’s body – probably less than a few hours under normal household conditions.

Cats at greatest risk of FeLV infection are those that may be exposed to infected cats, either via prolonged close contact or through bite wounds. Such cats include cats living with infected cats or with cats of unknown infection status, cats allowed outdoors unsupervised where they may be bitten by an infected cat, and kittens born to infected mothers.

Kittens are much more susceptible to FeLV infection than are adult cats, and therefore are at the greatest risk of infection if exposed. However, even healthy adult cats can become infected if sufficiently exposed.

Overwhelming epidemiologic evidence suggests FeLV is not transmissible to either humans or dogs. This statement is based on the fact that approximately one pet dog in five lives with a cat, and many domestic cats live with humans (some 60 million pet cats in the United States). It is species-specific, and does not infect other animals, such as dogs (in fact, there is apparently no canine version of this disease at all).

Diagnosis and prognosis

feline leukemia

There are several types of tests available to diagnose FeLV. Most veterinarians and shelter professionals use the ELISA (enzyme-linked immunosorbent assay) test, which detects antigen to the FELV virus in the bloodstream. Other test like the IFA (indirect fluorescent antibody) test is recommended to confirm positive ELISA test results. The IFA test is performed at a laboratory, rather than in your vet’s clinic. In general, cats that are IFA-positive have a poor long-term prognosis.

In some cases, isolating the whole virus or detecting DNA of the virus using a test called a polymerase chain reaction (PCR) may be recommended to determine whether FeLV has infected the bone marrow. Always consult with your veterinarian to determine which tests are appropriate for your cat.

A complete blood count is done to determine if the cat has anemia or other blood disorders. Diagnosis may also be determined by conducting a urinalysis, or through a bone marrow biopsy or bone marrow aspiration (removing a small amount of marrow fluid for study).

In 2008, The American Association of Feline Practitioners (AAFP) revised their guidelines for FeLV testing. They recommend that all cats be tested for FeLV. In addition, cats should be (re)tested:

  • During sickness: Regardless of previous negative results. While many signs (such as fever, stomatitis, vomiting, and diarrhea) are obvious indicators of illness, other signs are subtle and may include changes in behavior, grooming, and eating habits.
  • When being adopted/entering new home: Regardless of age and whether or not they will be entering a household with other cats. They should be tested prior to being introduced into the household.
  • When living in multi-cat households in which another cat is infected with FeLV, or are otherwise at high risk (e.g., cats that go outdoors unsupervised).
  • After potential exposures: When cats have had known or possible exposure to other cats who are infected or are of unknown infection status. If a negative test is obtained, the test should be repeated after a minimum of 30 days.
  • Prior to initial FeLV vaccination: FeLV vaccine should not be given to FeLV infected cats.

In addition, it is important to note that:

  • No test is 100% accurate. In populations of cats with low FeLV infection rates, many positive tests may be false positive.
  • Any positive ELISA test should be repeated.
  • In most cases, the ELISA test should be used as the screening test, and IFA as the confirmatory test.
  • ELISA tests using tears or saliva are not recommended.
  • All cats with a negative result but with the potential of having been exposed to FeLV should be retested after a minimum of 30 days.
  • All cats with a suspected or known exposure should be tested.
  • If the ELISA is positive but the IFA test is negative, the cat should be retested in 60 days and then annually until test results agree.
  • The vaccine will not interfere with FeLV testing.
  • The vaccine will not affect the course of disease in already infected cats. A “carrier,” though, who is vaccinated and develops FeLV-related disease will appear to be a “vaccine failure”.

Finally, a negative FeLV test does not imply the cat is immune to FeLV or was never exposed to it. A negative test could mean the cat:

  • Has never been exposed to FeLV
  • Is infected, but the number of virus particles is too low to detect. The cat may test positive at a later date.
  • Has overcome a previous infection.


feline leukemia

Vaccines for FeLV are available, though no currently available vaccine offers 100% protection from the virus. Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received FeLV vaccines subsequently developed vaccine-associated sarcomas, an aggressive tumour, at the injection site. The development of sarcomas with the use of the old FeLV and other vaccines may be due to the inflammation caused by aluminium adjuvants in the vaccines.

Marial produces a recombinant vaccine consisting of canarypox virus carrying FeLV gag and env genes (sold as PUREVAX FeLV in the USA and Eurifel FeLV in Europe). This is thought to be safer than the old vaccine as it does not require an adjuvant to be effective. Although this is a live virus, it originates from a bird host and so does not replicate in mammals.

Keeping your cat indoors and away from infected cats is a sure way to prevent him from contracting FeLV.

New cats or kittens over eight weeks of age should be tested for the virus before being introduced to a multi-cat household. Most veterinarians counsel against introducing a new cat into a household with a FeLV-positive cat, because he or she may be at risk for contracting the infection – even with vaccination. In addition, the stress of a newcomer may adversely affect the FeLV-positive cat.

You will need to monitor your cat for symptoms of infection and keep in touch with the veterinarian regarding follow-up treatment and testing. More than 50 percent of cats that persistently have cat leukemia in their blood (known as FeLV viremic cats) succumb to related diseases within two to three years after infection.


feline leukemia

In 2006, the United States Department of Agriculture issued a conditional license for a new treatment aid termed Lymphocyte T-Cell Immunomodulator (LTCI). Lymphocyte T-Cell Immunomodulator is manufactured and distributed exclusively by T-Cyte Therapeutics, Inc.

Lymphocyte T-Cell Immunomodulator is intended as an aid in the treatment of cats infected with feline leukemia virus (FeLV) and/or feline immunodeficiency virus (FIV), and the associated symptoms of lymphocytopenia , opportunistic infection, anemia, granulocytopenia, or thrombocytopenia. The absence of any observed adverse events in several animal species suggests that the product has a very low toxicity profile.

Lymphocyte T-Cell Immunomodulator is a potent regulator of CD-4 lymphocyte production and function. It has been shown to increase lymphocyte numbers and Interleukin 2 production in animals.

Lymphocyte T-Cell Immunomodulator is a single chain polypeptide. It is a strongly cationic glycoprotein, and is purified with cation exchange resin. Purification of protein from bovine-derived stromal cell supernatants produces a substantially homogeneous factor, free of extraneous materials. The bovine protein is homologous with other mammalian species and is a homogeneous 50 kDa glycoprotein with an isoelectric point of 6.5. The protein is prepared in a lyophilized 1 microgram dose. Reconstitution in sterile diluent produces a solution for subcutaneous injection.


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