Feline Leukemia
Gammaretrovirus
Definition:
This is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal’s immune system, the virus can cause diseases which can be lethal. FeLV is categorized into four subgroups, A, B, C and T. An infected cat has a combination of FeLV-A and one or more of the other subgroups. Symptoms, prognosis and treatment are all affected by subgroup.
Signs & Symptoms:
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.
Progression:
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.
Transmission:
Once a cat has been infected with FeLV-A, additional mutated forms of the original FeLV-A virus may arise, as may FeLV subgroups B, C and T.
In addition to domestic cats, some other members of Felidae are now threatened by FeLV (e.g. lynx and Florida panther). Overwhelming epidemiologic evidence suggests FeLV is not transmissible to either humans or dogs.
Approximately 0.5% of pet cats are persistently infected with FeLV, but many more pet cats (>35%) have specific IgG antibodies which indicate prior exposure and subsequent development of immunity instead of infection. FeLV is highly infectious.
Kittens can be born with it, having contracted it from their mother while in utero.
Infection is far higher in city cats, stray or owned, than in rural cats: this is entirely due to the amount of contact the cats have with each other.
Diagnosis and Prognosis:
Testing for FeLV is possible with ELISA tests that look for viral antigens, free particles found in the bloodstream. These ELISA tests use blood samples most often, but can also use saliva or eye secretions. The sample is added to a container or dish that contains the viral antigen. If antibodies to the antigen are present in the sample, they will bind together and an indicator on the test will change color. These give a definitive diagnosis, but it cannot differentiate between acute or persistent infections. Therefore, it is recommended that the cat be retested in three to four months after the positive result to determine if the virus has been cleared from the body. Diagnosis can also be made by reference lab testing, using an immunofluorescence (IFA) test. The IFA test uses a blood sample and will detect the virus once it is in the bone marrow by detecting the virus’s presence in white-blood cells. IFA testing will not give positive results for transient, primary infections – the infection must be persistent to get a positive result on this test. Other than ELISA and IFA testing, routine laboratory blood work may show changes that indicate infection but cannot be used as a definitive diagnosis. There may be blood cell count changes like leukopenia, decreased Packed Cell Volume (PCV) and Total Protein (TP) levels due to anemia, hemoconcentration and hypoglycemia due to vomiting and diarrhea, electrolyte imbalance caused by dehydration and anorexia, and recurrent urinary tract infections.
Cats diagnosed as persistently infected by ELISA testing may die within a few months or may remain asymptomatic for longer; median survival time after diagnosis is 2.5 years.
FeLV is categorized into four subgroups.
FeLV-A: Is responsible for the immunosuppression characteristic of the disease. All cats with FeLV have FeLV-A.
FeLV-B: Causes an additional increase in the incidence of tumors and other abnormal tissue growths. About half of FeLV infected cats have FeLV-B.
FeLV-C: Causes severe anemia. Approximately 1% of FeLV infected cats have FeLV-C.
FeLV-T: Leads to lymphoid depletion and immunodeficiency.
Prevention:
Two types of vaccines for FeLV are available: an adjuvanted killed virus noninfectious vaccine and a nonadjuvanted canarypox virus-vectored recombinant infectious vaccine (ATCvet code QI066AA01 and various combination vaccines), though no currently available vaccine offers 100% protection from the virus. Vaccination is recommended for high-risk cats: those that have access to the outdoors, feral cats, cats that do not have the virus but live with an infected cat, multicat households, and cats with an unknown status, such as cats in catteries and shelters.
Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received the adjuvanted killed virus vaccine 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.
Merial produces a recombinant vaccine consisting of canarypox virus carrying FeLV gag and env genes (sold as PUREVAX FeLV in the US 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.
Viral structure:
Feline leukemia virus (FeLV) is an RNA virus in the subfamily Oncovirinae belonging to the Retroviridae family. The virus comprises 5′ and 3′ LTRs and three genes: Gag (structural), Pol (enzymes) and Env (envelope and transmembrane); the total genome is about 9,600 base pairs.
See the entry on retroviruses for more details on the life cycle of FeLV.
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