Dear Editor,
Thank you for the opportunity to respond to the letter from Drs. Doornenbal, Ingwersen, and Cloutier regarding our recent review article on feline retroviruses “Feline leukemia and feline immunodeficiency virus in Canada: Recommendations for testing and management” ( Can Vet J 2011;52:849–855). We appreciate their interest in the topic.
As is apparent, we consider many of the same issues important in feline health care: the impact of FIV infection-associated morbidity, the need for better FIV prevention and therapy, and the need for better tests. However, we differ in our interpretation of available information on FIV vaccine efficacy. As cited by Doornenbal et al. and us, there are several studies from researchers collaborating with the vaccine manufacturer showing good efficacy of the licensed FIV vaccine against different viral clades, and there is only one study from researchers unaffiliated with the vaccine manufacturer showing lack of efficacy. These highly discrepant findings are difficult to interpret in the context of a virus that is constantly evolving and recombining, and for which laboratory strains can differ markedly from field strains. Furthermore, the amount of virus and the route of inoculation that represent a “natural challenge” versus a challenge in a laboratory setting are not clear.
Of paramount importance when considering whether to vaccinate cats against FIV remains the inability to distinguish vaccinated from infected cats by antibody ELISA. The antibody ELISA is the mainstay of diagnosis, and currently an alternate test of similar sensitivity and specificity is not available. Under conditions encountered by rescue groups and humane societies, a cat testing positive for FIV antibody is assumed to be infected and may be euthanized, even though the test result may be a result of vaccination and not natural infection.
Finally, the currently available FIV vaccine is an adjuvanted product. Adjuvanted vaccines are more likely to induce inflammation, which in turn may lead to development of tumors. Hence, we cannot recommend an adjuvanted vaccine with an extremely variable reported efficacy and which interferes with the mainstay of testing.
Prevention of FIV infection with a consistently effective vaccine remains a highly desirable goal, and we welcome additional evidence from independent researchers showing efficacy under field conditions.