Feline Leukemia Virus

Clinical Signs
FeLV virus EM(cc)                 cat eye(dd)                        EM FeLV(ee)

FeLV is a retrovirus, which is characterized by being an enveloped virus belonging to the viral family Retroviridae.  As a Retrovirus, FeLV contains RNA that requires an enzyme (reverse transcriptase) to allow the conversion of RNA to DNA which can then be incorporated into a host’s cell and initiate replication of viral units.   (14, 15)

Feline leukemia virus (FeLV) affects a small percentage (<5%) of the cat population in North America, but can be found throughout the world. Differences in the prevalence of FeLV show a divergence depending on the age, health, environment, and lifestyle of the domestic cat.  Significant increase in prevalence (>13%) can be seen in cats that are ill, very young, or are not protected by vaccination. (3, 14)

                                                                    FeLV infected cat(ff)

FeLV can be classified in to three types – A, – B, and – C, of which infected cats can be infected by one, two, or three in various combinations.  Type A is found in all cats infected with FeLV, and is responsible for a weakened immune system through T-cell immunosuppression.  Type B is found in half of the cats infected with FeLV, and this type is responsible for an increased occurrence of tumors and abnormal growths than is found in cats only infected with Type A.  Type C is found in a very small number of cats (1%) which present with severe anemia. (16)
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The spread of the virus from an infected cat to a susceptible cat can occur in different ways.  Cats that are infected shed the virus from bodily fluids and secretions.  Saliva and nasal secretions are the most prominent method of spread, but blood, urine, feces, and milk have also been recognized as sources of infection.

Transmission of virus is more commonly due to direct exposure between two cats by means of bite wounds from fighting, or membranes associated with the eye, mouth or nose is contaminated by grooming. Indirect exposure between an infected cat and a susceptible cat is less likely to result in the spread of disease, because the virus is labile and does not survive in the environment very long (<2hrs).  As a result, it is rare for transmission to occur due to events such as sharing a litter box or feeding dish at separate times. FeLV can also be spread from an infected queen to her fetuses by the virus crossing the placenta, or to her kittens during nursing. Kittens infected prior to birth are considered persistently infected and are a major source of infection to other cats because they do not show signs of being infected themselves.   (14, 17)

  nursing(gg)              shared litter(hh)
Cats that are more likely to be infected are those that have an increased exposure due to their lifestyle or the environment that they inhabit. Cats that live outdoors or have unsupervised time outside and are involved in fights are more likely to contract the virus. Cats that are living with, or having prolonged exposure to, other cats that are infected or have an unknown history, have an increased risk of contracting the virus. An infected mother, as mentioned before, will pass on the virus to her kittens, and is the most certain method of contracting the virus. (14)

Risk factors which increase the chance of infection include:    male cat(ii)

Age – young are more susceptible (<6yrs) as well as old cats (>14yrs)

Sickness – any disease which lowers the immune response in a cat increases the chance of infection.

Sex – males are more likely to become infected due to their lifestyle than females

Lifestyle – outdoor or stray cats have increased exposure as compared to indoor or non free-roaming cats.

Numbers – multicat households have an increased likelihood of infection especially when factors involving lifestyle are considered. (14, 16)

                                                       cats(jj)                                                                                                                                                                                                                                                                                                       Return to top

Clinical Signs

Signs of infection are directly related to the Type of FeLV infection the cat has.  As mentioned before, cats can present with immunosuppression, neoplasia, or with anemia.  (3)

These three conditions related to FeLV types can have a multiple of effects on an infected cat. Anemia can result in an assortment of blood disorders, while neoplasia is detected as cancer.  Immunosuppression has the most varied effects because the cat is susceptible to a variety of infections without an adequate defense system. Opportunistic pathogens made up of viruses, bacteria, fungi, protozoa, and parasites that do not cause harm to healthy cats and are found in their normal environment, will seriously affect a cat infected with FeLV.  Known as secondary infections, these pathogens result in the diseases and signs that make the FeLV infected cat ill and can potentially lead to its death.  (14)                             

                                                                         leukemia blood(kk)

Signs can fall into a wide range from nothing clinically detectable in the early stages of an infection, to a progressively deterioration and recurrent illness interspersed with periods of relative health that becomes more apparent with time. General signs of a cat that has an infection that has resulted in an immune response and can include an FeLV primary or secondary infection are:

  • Loss of appetite
  • Slow and progressive weight loss, which changes to a sudden and severe wasting late in the disease process
  • Poor coat condition
  • Enlarged lymph nodes (mild to severe)
  • Inflammation of the upper respiratory tract
  • Persistent fever
  • Pale gums and other mucus membranes
  • Inflammation of the gums (gingivitis) and mouth (stomatitis), and periodontium (periodontitis)
  • Infections of the skin, urinary bladder, and upper respiratory tract
  • Chronic external ear infections
  • Persistent diarrhea
  • Seizures, behavior changes, and other neurological disorders
  • A variety of eye conditions (inflammation of the conjunctiva or cornea)
  • In unspayed female cats, abortion of kittens or other reproductive failures
Infected kittens have slightly differing signs which are generally described as the  “fading kitten syndrome”.   These kittens have an increased risk of infection, are lethargic, and have slow growth.  (14, 35)

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Complete blood count will show abnormally low numbers of RBC’s and WBC’s

Depending on the body system that is affected by FeLV or a secondary infection the biochem/UA will show outside the normal range.

Specific laboratory tests for the blood to identify the presence of FeLV include the whole blood indirect immunofluorescent antibody assay which identifies a specific antigen (p27) belonging to the virus, and can be detected by the 4th week of infection. To detect FeLV in a variety of bodily fluids and secretions, an ELISA is used to detect the same antigen. A positive on either of these tests requires retesting in 12 weeks to confirm and determine if infection is persistent.

Kittens can be subjected to imaging that will show the absence of the thymus in infected individuals.

 blood smear(mm)

Bone marrow aspirates is a method used to evaluate the condition and ability of the bone marrow to produce Red Blood Cell’s and White Blood Cell’s, and to help determine the nature of the anemia.   (14, 15, 17, 18, 35)
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Unfortunately at this time, there is no treatment or cure for cats infected with FeLV, but that does not mean that a diagnosis of infection is an immediate death sentence. A change in lifestyle to indoors-only both protects other cats as well as lowers exposure to secondary infections, helping to prolong the life of infected cats.  Furthermore, owners must be educated to be aware of increased risks to the health of their FeLV cat and encouraged to maintain routine healthcare, vaccination, and dental care.  Treatment in most cases is at best supportive, and can include blood transfusions, and prednisone.   (15, 17)


Type B FeLV, the form that causes cancer, has been shown to have a better outcome with treatment, which includes either chemotherapy or radiation depending on the tumor present. Other treatments may involve glucocorticoids, interferon, and Protein A as well as supportive care.   (15, 17)

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Most cats with FeLV can survive up to 3 years from a diagnosis with supportive care.   (15, 17)

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Vaccination is one of the best methods of protecting cats that are free of the disease, but considered at risk of exposure to FeLV.  Kittens, due to their increased risk, are highly recommended to receive vaccination. Vaccinations can be given after 8 weeks of age, and the initial vaccination should be boostered 3 to 4 weeks later with a second booster 1 year after the initial vaccinations.


In addition to vaccination, owners should consider the risk factors mentioned above and try to the best of their ability minimize them in order to reduce their cat’s exposure. If there is any doubt about a cats exposure, testing should be done.   (3, 15, 17)

Vaccines are available in preparations with or without an adjuvant, and the virus can be inactivated or recombinant, but all types are shown to be effective in eliciting a protective immunity for 12 months.  Prior to vaccination cats should be tested to ensure no exposure to the virus has occurred.  Vaccination is only recommended for cats that are at an increased level of risk due to lifestyle (outdoor cats, multicat-home, and living with a known infected cat). Kittens are strongly recommended to receive the vaccine due to the high risk of contracting the infection and becoming persistently viremic. The vaccine is more commonly administered to the left hind leg as far down as possible.  This is done to aid in identification of which vaccine may have potentiated a reaction which could results in a tumor.

                                                          FeLV vaccination(oo)

Kittens 8wks  - first dose

          12wks  - second dose

Booster  - annually (especially for cats at risk of exposure)  (3)
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