A comparative study of the efficacy of a canarypox based recombinant leukaemia vaccine against a natural contact FeLV challenge in cats

Objective: Feline leukaemia virus (FeLV) infection is a major infectious disease of cats, but its prevalence has decreased as a result of management of infected animals and vaccination. Efficacy of vaccines is tested in vaccination/challenge experiments which are unfortunately poorly representative of the natural conditions of infection and may in some instances lead to biased conclusions. To overcome this limitation of traditional efficacy studies, we tested the efficacy of a non-adjuvanted recombinant canarypox-FeLV vaccine in a natural contact challenge model, and compared it to an adjuvanted commercially available vaccine.

Methods: Vaccinated and unvaccinated control cats were mixed with persistently viraemic cats and kept in close contact for more than 6 months. Cats were regularly tested for p27 antigenaemia and viraemia. Cats that were found positive for either p27 antigenaemia or viraemia for the last three blood samples were considered persistently viraemic.

Results: This contact challenge resulted in a high rate of persistent viraemia in control cats (78%). Vaccination efficiently protected cats against persistent viraemia with preventable fractions of 79% for the canarypox vectored vaccine and 68% for the adjuvanted commercial vaccine.

Conclusion: The natural contact challenge model proved to be a potent method to reproduce FeLV challenge, confirming that under appropriate conditions FeLV is efficiently transmitted. Despite the severity of the challenge, both vaccines provided a strong and sustained protection against persistent viraemia over a 6-month contact period. This study confirmed the efficacy of a non-adjuvanted canarypox-vectored FeLV vaccine against a contact challenge mimicking the natural conditions of infection.