If it invades the bone marrow, the cat is infected for life. It then spreads through the blood through the circulation. Tissues such as the tear glands, salivary glands, and urinary bladder become infected. The cat is now shedding the virus, and becomes infectious to other cats. To prevent FeLV infection, recommendations are to keep cats indoors and away from strays who might be carrying the virus.
Several vaccines are available which are produced by different methods but give the same protection.
Some vaccinated cats may become temporarily FeLV-positive after exposure to the virus, but they will not develop the disease. Although most cats experience no reaction to a vaccine, some may display malaise for a few hours to two days after the shot. The relatively rare allergic reaction can be easily treated. Annual booster vaccinations are also recommended. Antibodies against FeLV that are produced as a result of vaccination are not detected by the tests.