Affects: Cats, Dogs
Feline leukemia virus (FeLV) is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved. If the animal's immune system fails to neutralize the virus, it may become severely weakened by it, potentially leading to lethal disease. Because FeLV is cat-to-cat contagious, FeLV+ cats should only live with other FeLV+ cats.
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.
FeLV+ cats often have a shortened lifespan, but can still live relatively normal, healthy lives.
Signs And 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 are still immune suppressed, with potential delayed and decreased antibody responses.
Transmission: Cats infected with FeLV can serve as sources of infection of FeLV-A. Cats can pass the virus between themselves through saliva and close contact, by biting another cat, and (rarely) through a litter box or food dish used by an infected cat.
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.
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 antibodies to the viral antigens. If the antigens are present in the sample, the antibodies will bind to them 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.
Prevention: Three types of vaccines for FeLV are available: an adjuvanted killed virus noninfectious vaccine, an adjuvanted subunit 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.