Westman ME, Parr Y, Martin C, Wuestner E, Pan S, Norris JM, McDonald M, Dunbar D, Weir W, Hall E, Nakamura M, Atkin N, Hajjar R, Meggiolaro MN, Hughes D, Malik R, Hosie MJ. Antibody responses to feline leukemia vaccination: exploring the effect of sex, boosting schedule, site of administration and vaccine type. J Feline Med Surg. 2025 Sep; 27(9) doi: 10.1177/1098612X251353080.
This study investigated the feline immune response to vaccination for Feline Leukemia Virus, or FeLV. The main purpose was to address whether alternative injection sites, adopted primarily due to safety concerns over Feline Injection-Site Sarcomas (FISSs), provide an equivalent level of immunological protection compared to traditional injection sites. The paper analyzed the effects of the cat’s sex, the vaccination schedule, the specific anatomical location of the injection, and the type of vaccine used on resulting antibody concentrations.
FeLV remains a pathogen of significant global concern that can be associated with a poor long-term prognosis due to conditions like immunosuppression, anemia, and lymphoma. Vaccination is a common preventative measure recommended for kittens and adult cats with potential outdoor exposure. Historically, feline vaccines were routinely administered in the skin on the back of the neck (the scruff) due to ease and convenience. However, aggressive tumors can form as a result of certain vaccines, and when they occur in this area they are extremely difficult to remove surgically. This led veterinary organizations to recommend alternative injection sites, specifically the distal hindlimbs or the distal tail. The rationale for these sites is that if a sarcoma were to develop, amputation of the affected area might be possible. However, these recommendations were made with limited evidence confirming that immune responses generated at these alternative sites were comparable to the traditional scruff injection site.
This study recruited 125 healthy, mixed-breed kittens that were negative for FeLV. These kittens were distributed into nine groups, which included combinations of three commercially available FeLV vaccines (Fel-O-Vax 5, Fel-O-Vax Lv-K, and Leucogen FeLV) and three injection locations (scruff, left distal hindlimb, and tail). Blood samples were collected on Day 0 (initial vaccination), Day 28 (after the first dose), Day 56 (after the second/booster dose), and Day 365 (one year later). The primary immunological measurement was an ELISA (enzyme-linked immunosorbent assay) to quantify the antibodies against FeLV. Additional tests, including PCR and neutralizing antibody assays, were used to confirm that the cats remained uninfected by FeLV throughout the study. All cats remained free of FeLV infection through the study. A difference in antibody response based on the cat’s sex was noted: female kittens exhibited approximately 1.6-fold greater antibody concentrations than those found in males. This suggests a disparity in the humoral immune response between sexes.
Antibody levels at Day 56, following the second dose, were higher than the levels observed at Day 28, after only one dose. This finding supports the established vaccination protocol that a booster dose is required to elicit a substantially elevated and likely more protective antibody concentration. The analysis found no significant differences in the overall mean FeLV antibody concentrations when comparing the three different injection sites—scruff, left hindlimb, and tail. This result indicates that, in terms of stimulating a measurable antibody response, the distal injection sites are immunologically comparable to the traditional scruff site. Similarly, there were no significant differences in the overall antibody response between the three vaccine formulations tested.
Overall, this research supports that FeLV vaccines can be administered in the scruff, the left hindlimb, or the tail with comparable overall antibody responses. This evidence suggests that veterinarians can adhere to recommendations for using alternative, distal injection sites to allow easier tumor removal in cases of FISS without negatively affecting the cat’s systemic immune response to the vaccine. The study also reinforces the importance of the two-dose primary schedule and identifies sex as a possible variable influencing the immune response. ~MK
See Also:
Lutz H, Addie D, Belak S, et al. .Feline leukaemia: ABCD guidelines on prevention and management. J Feline Med Surg 2009; 11: 565–574
Hartmann K. Clinical aspects of feline retroviruses: a review. Viruses 2012; 4: 2684–2710. doi: 10.3390/v4112684
Little S, Levy J, Hartmann K, et al. AAFP feline retrovirus testing and management guidelines. J Feline Med Surg 2020; 22: 5–30. doi.org/10.1177/1098612X19895940