Vaccine 24 (2006) 1087–1094 Reassessment of feline leukaemia virus (FeLV) vaccines with novel sensitive molecular assays Regina Hofmann-Lehmann a,∗ , Ravi Tandon a,1 , Felicitas S. Boretti b,2 , Marina L. Meli a,3 , Barbara Willi a,4 , Valentino Cattori a,5 , Maria A. Gomes-Keller a,6 , Pete Ossent c,7 , Matthew C. Golder d,8 , J. Norman Flynn d,9 , Hans Lutz a,10 a Clinical Laboratory, Vetsuisse Faculty, University of Zurich, Winterthurerstr. 260, CH-8057 Zurich, Switzerland b Clinic of Small Animal Internal Medicine, Vetsuisse Faculty, University of Zurich, Winterthurerstr. 260, CH-8057 Zurich, Switzerland c Institute of Veterinary Pathology, Vetsuisse Faculty, University of Zurich, Winterthurerstr. 260, CH-8057 Zurich, Switzerland d Institute for Comparative Medicine, University of Glasgow, Bearsden, Glasgow G61 1QH, UK Received 19 May 2005; received in revised form 29 August 2005; accepted 5 September 2005 Available online 19 September 2005 Abstract We previously described antigen negative, provirus positive cats. Subsequently, we hypothesized that efficacious FeLV vaccines cannot prevent minimal viral replication. Thus, we vaccinated cats with either a canarypox-vectored live or a killed virus vaccine and analyzed the challenge outcome with quantitative PCR and a newly established real-time RT-PCR. When judged by conventional parameters (antigenaemia, virus isolation), most of the vaccinated cats were, as expected, protected from persistent viraemia. However, all cats were found to be plasma viral RNA positive. The loads were significantly associated with the infection outcome. Thus, commonly used FeLV vaccines understood to be successful model antiretroviral vaccines protecting against FeLV-related diseases do not confer sterilizing immunity. © 2005 Elsevier Ltd. All rights reserved. Keywords: FeLV vaccines; Plasma viral RNA load; TaqMan real-time RT-PCR ∗ Corresponding author. Tel.: +41 44 635 8322; fax: +41 44 635 8923. E-mail addresses: rhofmann@vetclinics.unizh.ch (R. Hofmann-Lehmann), rtandon@vetclinics.unizh.ch (R. Tandon), fboretti@vetclinics.unizh.ch (F.S. Boretti), mmeli@vetclinics.unizh.ch (M.L. Meli), bwilli@vetclinics.unizh.ch (B. Willi), vcattori@vetclinics.unizh.ch (V. Cattori), mgomes@vetclinics.unizh.ch (M.A. Gomes-Keller), ossent@vetpath.unizh.ch (P. Ossent), M.Golder@udcf.gla.ac.uk (M.C. Golder), Norman.Flynn5@homeoffice.gsi.gov.uk (J.N. Flynn), hlutz@vetclinics.unizh.ch (H. Lutz). 1 Tel: +41 44 635 8695; fax: +41 44 635 8923. 2 Tel: +41 44 635 8360; fax: +41 44 635 8930. 3 Tel: +41 44 635 8385; fax: +41 44 635 8906. 4 Tel: +41 44 635 8694; fax: +41 44 635 8923. 5 Tel: +41 44 635 8749; fax: +41 44 635 8923. 6 Tel: +41 44 635 8386; fax: +41 44 635 8906. 7 Tel: +41 44 635 8555; fax: +41 44 635 8934. 8 Tel: +44 141 330 6938; fax: +44 141 330 5748. 9 Present address: P.O. Box 6779, Dundee DD1 9WN, Scotland. Tel: +44 138 222 3189; fax: +44 138 222 1571. 10 Tel: +41 44 635 8312; fax: +41 44 635 8906. 1. Introduction Feline leukaemia virus (FeLV) [1] is a gamma-retrovirus and a well-known feline pathogen occurring worldwide in domestic cats and some related felids [2,3]. FeLV infec- tion can lead to fatal neoplasia, degenerative diseases of the haematopoietic system, and immunodeficiency. It is not only of great veterinary but also of human medical interest as it is an important animal model for tumor and AIDS research [4,5]. In contrast to infections with lentiviruses, such as the feline or human immunodeficiency viruses, the susceptibil- ity of cats to FeLV varies remarkably and different infec- tion outcomes are known [2,6–11]. A proportion of cats develop progressive infection with persistent viraemia; they lack FeLV specific humoral and cellular immunity [2,12,13], and ultimately develop FeLV-associated diseases [14]. The majority of FeLV exposed cats develop contained infec- tion characterized by transient or undetectable viraemia and 0264-410X/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2005.09.010