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After a cat is orally exposed to FeLV, the lymphoid tissue of the oropharynx is infected followed by a viremia. The initial infection may be seen clinically as malaise and lymphadenopathy. After an initial phase of viremia, FeLV replicates within rapidly dividing lymphoid, myeloid, and epithelial cells, such as those lining the intestinal crypts. When cellular destruction exceeds the ability of the host's immune system to suppress viral replication, persistent viremia and progressive FeLV-related disease results. Feline Leukemia positive cats can be characterized by severely depleted CD4+ immune cells as well as mildly decreased CD 8+ cells as these are the immune cells targeted by the virus. However, Hoffman-Lehmann et al. (1997) found no evidence of persistent leukopenias or neutropenias in cats with the disease. The decrease in CD4+ and CD8+ cells leads to an immune compromised individual. It is often common for the patients to die of secondary infections or complications due to immune complexes collecting in the nephrons of the kidney leading to glomerular nephritis. Myeloproliferative disorders, several types of anemia, the panleukopenia-like and thymic atrophy syndromes are also commonly seen. Perhaps the most common side effect of contracting persistent feline leukemia is the development of lymphosarcomas which is characterized by uncontrolled proliferation of lymphocytes.

Once the virus has entered the cat, there are six phases to a FeLV infection:

Phase One: The virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsorial B-lymphocytes and macrophages. These white blood cells then travel to the lymph nodes and begin to replicate.
Phase Two: The virus enters the blood stream and begins to distribute throughout the body.
Phase Three: The lymphoid system (which produces antibodies to attack infected cells) becomes infected, with further distribution throughout the body.
Phase Four: The virus can take over the body's immune system and cause viremia. During this phase the hemolymphatic system and intestines become infected.

If the cat's immune system does not fight off the virus, then it goes onto:

Phase 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 4-7 days later in infected neutrophils, and sometimes lymphocytes, monocytes, and eosinophils.
Phase Six: The cat's body is overwhelmed by infection and mucosal and glandular epithelial cells 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.

Hoffman-Lehmann et al., 1997; Kipar et al., 2007; Pedersen et al., 1977;
Rojko et al., 1979; Scott et al., 2007; Sykes, 2010; Trainin et al., 1983, Wernicke et al., 1986; Wikipedia, 2010