Difficulties in demonstrating long term immunity in FeLV vaccinated cats due to increasing age-related resistance to infection

In all studies, all of the younger aged control kittens showed persistent FeLV p27 antigenaemia confirming that the challenge virus was severe and efficacious. In contrast, the control cats did not show the required level of persistent antigenaemia, with a maximum of 45% cats affected in the middle duration study and only 10% in the longer study. However, apart from one animal in the short duration study, all of the cats vaccinated with Versifel FeLV were negative for persistent antigenaemia and can be considered treatment successes.In conclusion, we have shown that although age-related resistance to infection with a virulent FeLV challenge is evident from as early as 10 months of age, vaccination with Versifel FeLV may aid in the protection of cats from FeLV related disease up to three years after primary vaccination as kittens.Feline leukaemia virus (FeLV) is a gamma-retrovirus [1] and a significant pathogen of domestic cats throughout the world, and also other small felids such as the wildcat and lynxes [2]. Contact and saliva are the main modes of transmission although faeces have also been shown to contain virus shed from infected cats [3]. Cats exposed to FeLV may become either persistently viraemic or recover from the infection, with young cats being more rapidly infected following contact exposure [4]. A third category of cats infected with FeLV are those with a latent infection; they are not viraemic, but have a covert infection and do not develop FeLV disease [5]. Viraemic animals do not respond to FeLV antigens and show limited response to the virus, and are as a result at higher risk of developing fatal disease. Those cats which do recover from the infection produce virus neutralising antibodies which are considered important in preventing re-infection and can be passed through colostrum to protect neonatal kittens [6]. Antibodies to the feline oncornavirus-associated cell membrane antigen (FOCMA) can also be induced, which protect against the developmen