Feline Leukemia Virus

FeLV causes immunosuppression in pet cats and in their larger wild cat relatives. Currently, much evidence suggests FeLV is not transmissible to either humans or dogs.  Not one verifiable report exists in the medical/veterinary literature for FeLV infection in either a puppy or adult dog, and there is no report of an FeLV infection in a human child or of an adult pet owner.

A cat with FeLV
http://www.cvm.tamu.edu/oncology/faq/questions/dog01.html

There are many possible outcomes as to how successfully the cat’s immune system will be in defending the body from the virus. About forty percent of cats fight the virus off without complications.  Many others are able to put the virus into a latent stage, in which the virus will remain until the cat becomes stressed causing the FeLV to re-emerge. The remainder proceed through the following six stages of infection at varying rates: Phase one is when the virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsillar immune cells. These white blood cells then filter down to the lymph nodes and begin to replicate producing new virus infected cells. In phase two, the virus enters the blood stream and begins to distribute throughout the body. Phase three starts when the lymphoid tissues  become infected, leading to  further viral distribution throughout the body. Phase four is the main point in the infection, where the virus can take over the body's immune system causing viremia. During this phase the circulatory system and intestines become infected and virus particles spread to the spleen, lymph nodes, epithelial cells of the intestine and bladder, salivary glands, and bone marrow.

 If the cat's immune system does not fight off the virus, then the infection enters phase five where the 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 white blood cells. At 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, pharynx, kidney tubules, bladder, pancreas, and glandular ducts from the muzzle.

Despite the widespread use of vaccines, FeLV remains one of the most important causes of death and illness in cats. It causes a variety of cancers including lymphoma and  leukemia.  Persistent infection can also cause severe immunosuppression similar to that seen in cases of FIV infection and profound anemia.  Immune complexes formed in the presence of persistent viral presence can cause systemic inflamation involving the viens, joints and kidney, and a variety of other immune disorders.  Feline Leukemia Virus has also been linked to infertility in queens as well as increased rates of abortion.  The virus is also implicated as a cause of gastro intestinal inflamation and neurologic disorders.

The virus does also cause clinical signs in the accute, or early, stage of infection which occurs 2-6 wk after infection.  These signs include fever and lethargy and are rarely detected.