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Symptoms of Feline Leukemia Virus (FeLV)

Source: PetWave, Updated on July 16, 2015
Leukemia Virus


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:

  • Fever
  • Loss of appetite (inappetence; anorexia)
  • Weakness
  • Lethargy
  • 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
  • Diarrhea
  • Nasal discharge
  • Ocular discharge (from the eye)
  • Muscle wasting (atrophy)
  • Spontaneous abortion
  • Infertility
  • 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.

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