Combined vaccination with anti-GnRH and rabies vaccines in owner-owned male domestic cats: One-year safety, immunogenicity, and reproductive effects

Domestic cat overpopulation is a global concern, affecting animal welfare, biodiversity, and public health. Surgical castration remains the standard method for controlling tom reproduction, but has notable drawbacks. Gonacon, an anti-GnRH vaccine, offers a potential non-surgical alternative; however, evidence in pet toms in their natural habitats, alongside rabies vaccination, is scarce. Our objective was to evaluate the one-year safety, immunogenicity, and reproductive effects of combined Gonacon and rabies vaccination in owner-owned toms, and to compare responses between indoor and outdoor cats. Forty-two intact toms (age ≥6 months) were allocated to either Gonacon and rabies vaccination (n = 38) or surgical castration and rabies vaccination (n = 4). Toms remained under owner care in their natural habitats and were monitored at 0,3,6, and 12 months by clinical examination, testicular measurements, penile spine evaluation, serum testosterone, and owner reports. Twenty-five toms completed follow-up (22 Gonacon; 3 castration). Injection-site nodules occurred only with Gonacon (18.2 % vs. 0 %; P = 0.0410). Anti-GnRH seropositivity was detected in 77.3 %, 36.4 %, and 13.6 % of Gonacon-vaccinated toms at 3,6, and 12 months, respectively. Among responders (n = 17), 52.9 % were positive only at 3 months, 29.4 % up to 6 months, and 17.6 % throughout. Anti-GnRH levels tended to be lower in outdoor than indoor toms (P = 0.0688). Higher anti-GnRH levels were associated with reduced testosterone and penile spine regression. Rabies antibody levels remained above the protective immunity threshold (≥0.5 IU/mL) in all toms, but were lower in outdoor toms (P = 0.0228). Combined Gonacon-Rabies vaccination was safe, partially effective, and housing-dependent, offering real-world insights into feline reproductive management and preventive health.