Feline leukemia virus was initially isolated from a household of cats in Scotland by Dr. Jarrett in 1964. He had been trying
to determine the cause of a high incidence of lymphosarcoma in specific catteries. The AIDS epidemic has made available funding
for research in both the feline leukemia virus (FeLV) and the feline immunodeficiency virus (FIV). These retrovirus studies
have broadened our understanding of this group of viruses. We will discuss the pathophysiology of the disease, the spectrum
of clinical presentation and the guidelines for vaccination.
Feline leukemia virus is endemic wherever there are free-roaming cats. Currently about 2 to 3% of cats that go outdoors are
FeLV positive. The infection is easily spread indoors when infected cats share food and water dishes with uninfected cats.
Saliva can have over 1 million viral particles per ml. Sharing litter pans can also spread the disease. The test and eliminate
strategy in multi-cat households and in breeding colonies has largely eliminate the disease in strictly indoor cats. Intact
male cats are especially capable distributors of the virus through fighting.
The virus produces neoplastic, immunosuppressive and bone marrow suppressive diseases. The most easily transmissible form
of the virus is the FeLV-A type. The FeLV-A has little pathogenic effect but recombination with endogenous viral elements
in the cat's DNA results in the production of FeLV-B which will produce tumors. A mutated form, FeLV-C, specifically targets
red blood cell precursors resulting in aplastic anemic. A recently described, FeLV-T, destroys T-lymphocytes resulting in
The most common neoplastic disease is lymphoma though any of the marrow cell lines may become neoplastic. The lymphoma may
occur as leukemia or may be localized to the gut, thymus, eyes, nose, central nervous system, skin or a combination of these
sites. The nature of the disease seems to be changing with intestinal lymphoma becoming more common than the multicentric
or thymic forms. The thymic form is more likely to occur in young cats that are FeLV antigen positive while the gastrointestinal
forms are in older cats that are FeLV antigen negative. There appears to be a relation between Helicobacter and gastric lymphoma in cats.
The non-neoplastic manifestations of FeLV infection are more common than the neoplastic diseases. The virus produces immunosuppression
by gradual depletion of normal lymphocytes and depression of immune response. Feline infectious peritonitis (FIP) is much
more common in households infected with FeLV. Fortunately, most breeders and multiple cat households screen for FeLV infection
and remove infected cats thereby reducing their incidence of FIP. Chronic gingivitis, non-healing abscesses, mycoplasma infections
(haemobartonellosis), reproductive failures and abortion and persistent upper respiratory virus infections have all been associated
with FeLV infection. Suppression of bone marrow cell lines is common producing anemia and/or neutropenia. Neutropenia also
contributes to the development of diseases associated with immunosuppression. Many of these syndromes also occur with FIV
infection so testing should include both viruses.