Dear Susan: According to Dr. Pitcairn, in his book Natural Health for Dogs and Cats, there are six stages of infection for FeLV. He indicates that cats are not actively shedding the virus and infectious to other cats until they have reached stage five or six. Stage five is the point at which the virus infects the bone marrow and at this point a cat will remain viremic for the rest of its life. Stage six is the point at which a cat becomes significantly and seriously symptomatic and eventually "crashes."
So if you have a cat who has tested ELISA+, but is IFA negatiave, that would most likely indicate a cat in the earlier stages of infection and one not contagious, yet, to other cats. An IFA+ correlates well with stage four to six of infection, and most likely infectious. Cats that are seriously symptomatic should be considered infectious. The biggest unknown would be a stage-five, latent carrier, who would not be symptomatic but contagious. That is my understanding. I would love to find some recent research that could either corroborate or clarify that interpretation. Sally in San Jose
