Detection of transient and persistent feline leukaemia virus infections

A study was made of cats persistently or transiently viraemic with feline leukaemia virus (FeLV) following experimental oronasal infection. Cats of two ages were exposed to the virus. One group was infected when eight weeks old in the expectation that most of the cats would become persistently viraemic, and the second group when 16 weeks old, so that some would show signs of a transient infection and then recover. The periods following infection when virus was detectable in the blood and in the oropharynx were determined for each group. Three methods for detecting viraemia were compared: virus isolation, immunofluorescence on blood smears and an enzyme-linked immunosorbent assay (ELISA). There was good overall agreement among the three tests in detecting virus-positive cats. Virus was found sooner after infection by virus isolation than by the other methods, and virus appeared in the blood slightly sooner in cats which developed persistent viraemia than in transiently viraemic cats. Infectious FeLV was isolated from the oropharynx of all of the persistently viraemic cats, in most cases simultaneously with virus in the plasma. Virus was also isolated from the mouth of most transiently viraemic cats. Under field conditions such transient excretion of virus lasting only a few days would rarely be detected in a single sampling. This might explain how FeLV is maintained in free range urban cats in the absence of a large number of cats with persistent active FeLV infection. For routine diagnosis, immunofluorescence would appear to offer the best chance of differentiating transient and persistent infections by FeLV.