Comparative Efficacy of Feline Leukemia Virus (FeLV) Inactivated Whole-Virus Vaccine and Canarypox Virus-Vectored Vaccine during Virulent FeLV Challenge and Immunosuppression

Abstract

Four vaccines for feline leukemia virus (FeLV) are available in the United States. This study's purpose was to compare the efficacy of Nobivac feline 2-FeLV (an inactivated, adjuvanted whole-virus vaccine) and PureVax recombinant FeLV (a live, canarypox virus-vectored vaccine) following FeLV challenge. Cats were vaccinated at 9 and 12 weeks with Nobivac feline 2-FeLV (group A, n = 11) or PureVax recombinant FeLV (group B, n = 10). Group C (n = 11) comprised unvaccinated controls. At 3 months postvaccination, cats were immunosuppressed and challenged with FeLV-A/61E. The outcomes measured were persistent antigenemia at 12 weeks postchallenge (PC) and proviral DNA and viral RNA at 3 to 9 weeks PC. Persistent antigenemia was observed in 0 of 11 cats in group A, 5 of 10 cats in group B, and 10 of 11 cats in group C. Group A was significantly protected compared to those in groups B (P < 0.013) and C (P < 0.0001). No difference was found between groups B and C (P > 0.063). The preventable fraction was 100% for group A and 45% for group B. At 9 weeks PC, proviral DNA and viral RNA were detected 1 of 11 cats in group A, 6 of 10 cats in group B, and 9 of 11 cats in group C. Nucleic acid loads were significantly lower in group A than in group C (P < 0.01). Group A had significantly lower proviral DNA loads than group B at weeks 6 to 9 (P < 0.02). The viral RNA loads were significantly lower in group A than in group B at weeks 7 to 9 (P < 0.01). The results demonstrate that Nobivac feline 2-FeLV-vaccinated cats were fully protected against persistent antigenemia and had significantly smaller amounts of proviral DNA and plasma viral RNA loads than PureVax recombinant FeLV-vaccinated cats and unvaccinated controls.

FIG 1

Serum IgG antibodies to FeLV vaccination phase. All cats were negative for FeLV antibody prior to vaccination on study day 0. Ten of 11 cats in the Nobivac feline 2-FeLV group were positive for FeLV antibody on study days 20 (average S/P ratio, 3.0544 ± 1.007) and 111 (average S/P ratio, 1.002 ± 0.48). The PureVax recombinant FeLV-vaccinated cats and control-group cats remained antibody negative during the vaccination phase of the study. Statistically significant (P = 0.00) differences were seen between the Nobivac feline 2-FeLV group and the Purevax recombinant FeLV group (*) and the control group (**).

FIG 2

p27 antigen, proviral DNA, and viral RNA results by weeks postchallenge. All cats were tested for p27 antigen, FeLV plasma viral RNA, and FeLV proviral DNA. Cats in the Nobivac feline 2-FeLV group were negative for p27 antigen throughout the postchallenge period; 50% of the PureVax-vaccinated cats and 91% of the control-group cats became persistently antigenemic postchallenge. At week 9 postchallenge, plasma viral RNA was detected in 1 of 11 (9%) of the Nobivac feline 2-FeLV-vaccinated cats, in 6 of 10 (60%) of the PureVax recombinant FeLV-vaccinated cats, and in 9 of 11 (82%) of the unvaccinated control cats. From weeks 7 to 9, viral RNA loads were significantly lower in the Nobivac group than in the PureVax recombinant FeLV group (P < 0.01). Proviral DNA was detected in 1 of 11 (9%) of the Nobivac feline 2-FeLV-vaccinated cats, in 6 of 10 (60%) of the PureVax recombinant FeLV-vaccinated cats, and in 9 of 11 (82%) of the unvaccinated control cats. From weeks 6 to 9, cats in the Nobivac feline 2-FeLV group had significantly lower proviral DNA loads than those in the PureVax recombinant FeLV group (P < 0.02).

FIG 3

FeLV persistent antigenemia. FeLV p27 antigen ELISA to detect persistent antigenemia in the three groups (group A, Nobivac feline 2-FeLV; group B, PureVax recombinant FeLV; group C, control). Optical densities (OD) ≥0.200 were considered positive for the FeLV p27 antigen. Testing was performed during both the vaccination and challenge phases of the study. Percentages of cats persistently antigenemic were calculated at 0% for the Nobivac feline 2-FeLV group, 50% for the PureVax recombinant FeLV group, and 91% for the control group.

FIG 4

FeLV p27 antigenemia ELISA data. FeLV p27 antigen ELISA to detect persistent antigenemia in the three groups (group A, Nobivac feline 2-FeLV; group B, PureVax recombinant FeLV; group C, control group). Optical densities (OD) ≥ 0.200 were considered positive for the FeLV p27 antigen. Statistically significant differences were seen between the Nobivac feline 2-FeLV and control groups at all time points (P < 0.03). Statistically significant differences were seen between the Nobivac feline 2-FeLV and PureVax recombinant FeLV groups at weeks 3, 4, 6 to 9, and 11 postchallenge (*, P < 0.01). No statistically significant differences were seen between the PureVax recombinant FeLV and control groups at any time points (P > 0.08).

FIG 5

FeLV plasma viral RNA and proviral DNA results by PCR. Quantitative detection of FeLV virus in proviral DNA by real-time PCR and in plasma viral RNA by reverse transcription real-time PCR. The limit of detection for DNA was approximately 400 copies of DNA/1 ml. The limit of detection for RNA was approximately 1,000 copies of RNA/1 ml. At the end of the challenge, proviral DNA and plasma viral RNA were detected in 1 of 11 (9%) of the Nobivac feline 2-FeLV-vaccinated cats, in 6 of 10 (60%) of the PureVax recombinant FeLV-vaccinated cats, and in 9 of 11 (82%) of the unvaccinated control cats. Statistically significant differences were seen between the Nobivac feline 2-FeLV and control groups at all time points for plasma viral RNA and proviral DNA (both P < 0.01). Statistically significant differences were seen between the Nobivac feline 2-FeLV and PureVax recombinant FeLV groups between weeks 7 and 9 for plasma viral RNA (P < 0.01) and between weeks 6 and 9 for proviral DNA (P < 0.02).