They are both caused by a virus and therefore there is no difference there. Feline leukaemia virus disease complex (FeLV) is transmitted by infected saliva. Bites, sharing water or food bowls and cat-to-cat grooming can spread the disease. Feline AIDS (Feline Immunodeficiency Virus – FIV) is spread in a similar fashion because that too is transmitted in saliva but it is believed that the major source of transmission is during fights among tomcats when they bite each other.
Feline leukaemia is responsible for more cat diseases than any other infectious agent. The feline leukaemia virus suppresses the cat’s immunity allowing other diseases to develop. It is the most important cause of cancer in cats. In this way it is similar to feline AIDS. Feline AIDS does not produce symptoms on its own because it does not produce a disease in cats. It suppresses the immune system i.e. is a major cause of chronic immunodeficiency in cats. It is related to AIDS in humans.
As for how common they are, is believed that about 1 to 2% of healthy, freeroaming cats are infected with feline leukaemia. In multi-cat households and in some catteries the incidence could be higher. In some cases 20 to 30% of cats show the presence of feline leukaemia virus disease complex. About 50% have antibodies to this disease indicating a previous infection. As for feline AIDS it has been reportedly found in cats throughout the United States and believed to have infected 2 to 4% of cat in the general population. Outdoor cats are more likely to be infected especially males in the age range of 3 to 5 years.
Feline leukaemia is a more complex disease in my opinion then feline aids. There is no effective treatment as far as I know for either feline leukaemia or feline aids. Feline AIDS is treated by treating the symptoms of secondary infections and reducing the symptoms such as anti-inflammatory drugs, parasite control, immune-enhancing drugs, electrolyte replacement therapy and so on. As for feline leukaemia many cats who test positive to this disease are otherwise healthy and can live long, full lives. They need high quality caretaking including parasite control, minimal stress, regular grooming, high quality nutrition and an indoor life. Once they become ill (and about 30% of cats do not develop an infection at all) drugs such as ImmunoRegulin, Interferon and Acemannan appear to be prescribed by some veterinarians as they have shown some benefits. Cancers caused by the presence of feline leukaemia cannot be cured and cats with cancers so produced have an average survival time of about six months with extensive treatment.
I’m not going to discuss treatment any more because that is the role of a veterinarian. As can be seen there are similarities between these diseases.
Note: AIDS stands for Acquired Immune Deficiency Syndrome.