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Feline Leukemia Virus (FeLV)


The feline leukemia virus is a highly contagious retrovirus that can cause immunosuppression, secondary opportunistic infections and a number of neoplastic (cancerous) and hematologic (blood) abnormalities in cats. In fact, FeLV infection is one of the leading causes of death among companion felines and is responsible for more feline disease than any other identified infectious agent. It also contributes to cancer and a number of other diseases, due to its immunosuppressive effects.

Causes & Prevention

Causes of FeLV

Most FeLV-positive cats become infected by direct contact with saliva or blood from the oral or nasal secretions of infected cats. This commonly occurs either through mutual cat-to-cat grooming, playing, shared water or food dishes, weeping wounds or cat bites. The virus is shed to a lesser extent in urine, feces and tears. FeLV can be transmitted through blood transfusions, and also in utero from an infected queen to her unborn kittens. Young newborns tend to be more susceptible than older cats to clinical disease and often contract the infection from the saliva of their infected mothers. Because this virus is not very hardy and is highly susceptible to environmental conditions such as heat and disinfectants, environmental contamination is an uncommon cause of FeLV infection.
However, not all cats exposed to FeLV become clinically ill. Some mount an effective immune response and eliminate the virus entirely. It appears that prolonged or repeated exposure to the virus is necessary for infection, especially in otherwise healthy adult cats. Stress, sickness, poor hygiene, overcrowding and poor nutrition can all weaken a cat's immune system and make it more susceptible to infection.
Some cats mount an ineffective or incomplete immune response and become viremic, meaning that the virus replicates in their lymphoid tissue and bone marrow and enters circulation. If an effective immune response eventually kicks in, some of these cats will become latent carriers, while others will become persistently infected and will shed the virus continually in their saliva and other body secretions. It is this latter group of cats that is most likely to develop blood abnormalities, anemia, cancer, chronic opportunistic infections or other diseases due to the progressive weakening of their immune systems caused by FeLV.
There are three subgroups of FeLV. Subgroup A is the most common and is present in all cats with observable disease and viremia (presence of the virus in circulation). It is responsible for the immunosuppression that makes FeLV-positive cats susceptive to so many other infections and illnesses. Subgroup B occurs in combination with subgroup A in about one-half of cats and appears to be responsible for FeLV-associated cancers. Subgroup C is uncommon and occurs together with subgroup A in cats that develop nonregenerative anemias and bone marrow complications.

Prevention of FeLV Infection

There is no fool-proof way to prevent FeLV infection, although keeping cats indoors and away from free-roaming strays is perhaps the best prevention. Uninfected (naïve) cats should be kept away from FeLV-positive cats and should not share their food and water bowls or litter boxes. They should not live in the same household due to the high risk of contagion. There is an FeLV vaccine that may be useful in high-risk cats, such as those kept primarily or exclusively outdoors. However, it has been associated with vaccine-induced injection site sarcomas and currently is not recommended for use in indoor-only animals. New cats should not be introduced into multi-cat households or catteries without first being tested twice (3 months apart) with an IFA test (see, PetWave article on FeLV – Diagnosis and Test). They should be isolated from other cats during this process. In addition, cats should be tested for FeLV before being bred, and should not be used in a breeding program if they are FeLV-positive.

Special Notes

Feline leukemia virus has not been shown to be transmittable from infected cats to people. However, it reportedly can replicate in human cells in a laboratory setting. People with weakened immune systems (such as those who are HIV-positive), and women who are pregnant or considering becoming pregnant, probably should avoid contact with FeLV-positive cats as a precautionary measure. Unfortunately, there is no current cure for FeLV infection.

Symptoms & Signs

FeLV is a contagious virus that can weaken a cat's immune system and contribute to nonregenerative anemias, secondary bacterial infections and several forms of cancer. Some cats never become infected, either because they mount a strong immune response or because they were not exposed to the virus for a long or constant enough period of time. Other cats develop a transient infection, where the virus is present in circulating blood, saliva and other bodily secretions for a few months (called viremia) and then is effectively neutralized by the cat's immune system. These animals will go on to live a life unaffected by FeLV. A third group of cats develop what is called persistent viremia, where the virus remains in their circulatory system and saliva for longer than 3 or 4 months. This weakens their immune system and suppresses their ability to respond normally to other infections or diseases. Persistently infected cats continue to shed the virus and are infective to other cats; they usually will die from associated illnesses within roughly 3 years, if not sooner. That last group of cats develops a latent infection, whereby they eventually produce antibodies that are able to eliminate the virus from their blood and saliva so that they are not continuously infective to other animals. However, the virus continues to survive in their bone marrow and certain cells of the immune system. Most of latently infected cats eventually are able to neutralize the virus sufficiently to avoid clinical illness. Periods of stress, however, can cause a recurrence of viremia and viral shedding.

Symptoms of Feline Leukemia Virus

Acute infection with the feline leukemia virus typically lasts for up to 16 weeks post-exposure and causes one or more of the following non-specific symptoms in those cats that develop clinical disease:

Loss of appetite (inappetence; anorexia)
Enlarged lymph nodes (peripheral lymphadenopathy)
Anemia (abnormally low numbers or concentration of circulating red blood cells)
Pale mucous membranes (pallor)

The signs may be so mild as to go unnoticed by even the most observant owners. Newborns infected with FeLV may die within their first few weeks of life ("fading kitten syndrome"), although this is not common. Persistent viremia, which is the worst form of this illness, may cause additional symptoms, including:

Weight loss
Poor body condition
Poor hair coat
Nasal discharge
Ocular discharge (from the eye)
Muscle wasting (atrophy)
Spontaneous abortion
Hematologic (blood) abnormalities

Persistently infected cats are predisposed to developing virus-related cancer(s) months to years following exposure, including malignant lymphosarcoma/lymphoma and leukemia, among others. Cats with FeLV-associated lymphoma often develop difficulty breathing (dyspnea) due to cancerous masses in their chest (mediastinal tumors), although the cancer can spread elsewhere. Cats with persistent FeLV infection are also prone to developing other diseases due to their progressively weakened immune systems, such as feline infectious anemia, feline infectious peritonitis, toxoplasmosis, chronic urinary tract infections, periodontal disease, viral respiratory disease, bone marrow suppression and other opportunistic illnesses that their bodies are simply unable to fend off.

Cats at Increased Risk

Young cats under 4 months of age are predisposed FeLV infection. There does not seem to be a gender or breed predisposition.

Diagnosis & Tests

Feline leukemia viral infection is a significant disease among domestic cats. A number of routine and confirmatory tests can be used in combination to confirm a diagnosis of FeLV infection.

How FeLV Infection is Diagnosed

It typically takes at least four weeks of fairly constant exposure to the feline leukemia virus for it to appear in blood samples of affected animals. After 20 weeks of prolonged exposure, roughly 80% of exposed cats probably will become infected.
Most veterinarians presented with a cat showing non-specific signs of systemic illness will conduct an initial database of blood work (complete blood count and serum chemistry panel), a urinalysis and thoracic radiographs (chest X-rays). The results of these tests may be unremarkable even in infected cats. But, they may identify anemia (low circulating red blood cell levels), secondary urinary tract infections, chest masses and/or abnormal fluid accumulation in the thoracic cavity. More advanced confirmatory tests include an enzyme-linked immunosorbent assay (ELISA) test of blood, saliva or tears (blood is the preferred sample), and an immunofluorescent antibody (IFA) test of blood and/or bone marrow. These tests can identify the actual viral antigen in the sample.
Most veterinary practitioners recommend using an in-clinic ELISA test for initial screening, as it is more likely to detect early and transient FeLV infections. If the ELISA is positive, the IFA test is recommended, because it is more likely to identify persistent viremia (presence of the virus in circulation) and viral shedding, with bone marrow involvement. If the results of both tests are positive, the cat is considered to be viremic and will remain infected with FeLV for life. Tests on saliva or tears are not considered to be reliably diagnostic. A polymerase chain reaction (PCR) test of blood or fixed tumor tissues is also available at some referral laboratories.

It is suggested that both the ELISA and IFA tests be repeated in 2 to 3 months unless both tests are positive, to determine whether the virus has been eliminated by the cat's immune response or whether it has progressed from transient to persistent, permanent infection.

Special Notes

Cats with latent FeLV infection will typically test negative on both the ELISA and IFA tests, because they do not have viral particles in circulating blood. Cats previously vaccinated against FeLV do not test positive on these blood tests as a result of the vaccination.

Treatment Options

Feline leukemia virus is a contagious retrovirus that causes immunodeficiency, anemia and cancerous diseases in domestic cats. Unfortunately, infection by FeLV cannot be cured under current medical protocols. The goals of therapy are to manage the symptoms caused by secondary opportunistic infections and to control hematologic (blood), neoplastic (cancerous) and other conditions associated with the immunosuppressive effects of this disease.

Treatment Options

Many FeLV-positive cats can live long, full lives if they are otherwise healthy. Once an infected cat becomes ill, however, the treatment options are rather limited. Secondary opportunistic bacterial infections are common and should be treated aggressively with appropriate antibiotics, to allow the cats' immune systems to mount as strong a response to the associated FeLV infection as possible. Vitamin and mineral supplements may be recommended. Anemia (low levels of circulating red blood cells) may be managed with blood transfusions and close monitoring of red blood cell levels. Chemotherapeutic drugs may be helpful to treat FeLV-associated lymphoma, leukemia or other cancers, with varying degrees of success. However, FeLV-associated cancers are not curable, and most cats diagnosed with these cancers have an average survival time of less than 12 months.
Some anti-viral medications have been used to address FeLV infection in cats and appear to be most effective when given within a very short time (3 to 4 days) of exposure to the virus. While these drugs may reduce the symptoms of the disease for some period of time, they will not clear the virus from the cat's body and can have significant adverse side effects. Sometimes, immunosuppressive doses of steroids, such as prednisone, can be helpful in increasing the levels of circulating white blood cells to boost the cat's immune response to infection.


Cats that are FeLV-positive but otherwise healthy may live for years after diagnosis, with an excellent quality of life. They will need attentive supportive care, including high-quality nutrition, effective parasite control, a strictly indoor living environment and minimal stress. Unfortunately, cats that develop cancer, blood abnormalities, secondary infections or other diseases associated with FeLV have a guarded to poor prognosis, and they most often do not respond well to chemotherapy or other forms of medical management.

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