It is pretty confusing to deal with feline leukemia in litters of young kittens. It is entirely possible for only part of the litter or even one kitten to be infected and the rest of the litter unaffected. Some kittens (or cats) who are infected with feline leukemia will develop immunity to the virus and will not ever become ill from it. Probably about 30% of the time this happens. Some kittens will be able to sequester the virus in the bone marrow or central nervous system where it will not cause harm until some future stress occurs and causes the immune system to fail in its suppression of the virus. This is usually considered to be a latent infection. Some cats can tolerate the virus but can not suppress it. These cats have virus in their bloodstream constantly. They are carriers of the disease because they are infectious to other cats. Finally, some cats die from the initial infection. This wouldn’t be too confusing except for one thing. There is no easy way that I know of to tell the difference between a cat who becomes immune and one who is latently infected. So it is very hard to be sure that future problems won’t crop up if a kitten tests positive for the virus and then later tests negative.
Cats with feline leukemia often have an increased susceptibility to bladder infections (actually to many infections). It would be best to be sure that this was not leading to the behavioral changes. Cats with feline leukemia also often have behavioral changes that do not always seem to have a specific physical cause. In some cases they may be too weak to make it to a litterpan in a location such as the second story of a house or even into a litterpan with high sides. If there is an suspicion this is the case it is definitely a problem you need to discuss with your vet. Lastly, cats with feline leukemia can have physical and behavioral problems totally unrelated to the feline leukemia. If this seems to be the case the standard advice for litterpan problems found in our cat information area would apply.
Feline leukemia virus is one of the more common causes of fluid accumulation in the chest of cats. This can be chylothorax (accumulation of white blood cells) or other effusions. In general the presence of fluid in the chest in combination with the history of feline leukemia is a poor prognostic sign. Many times a secondary tumor in the chest, associated with the feline leukemia, is causing the fluid accumulation. In this case radiation therapy or chemotherapy for the tumor (it is important to make sure there is a tumor) may provide short term relief but the prognosis for survival long term is still pretty grim probably about 3 or 4 months.
With aggressive care for illnesses and a good quality of life, many cats can live a long time with feline leukemia but almost all have a shortened life span.