Safety and effectiveness of a single and repeat intramuscular injection of a GnRH vaccine (GonaCon™) in adult female domestic cats

Sterilization is a key strategy to reduce the number of domestic cats entering and killed in shelters each year. However, surgical sterilization is expensive and labour‐intensive and cannot fully address the 70 million free‐roaming cats estimated to exist in the United States. GonaCon™ is a gonadotropin‐releasing hormone vaccine originally developed for use as a wildlife immunocontraceptive. An earlier formulation was tested in domestic cats and found to be safe and effective for long‐term contraception. However, the current Environmental Protection Agency (EPA)‐registered formulation consists of a different antigen‐carrier protein and increased antigen concentration and has never been tested in cats. A pilot study was undertaken to evaluate the short‐term safety of a single GonaCon immunization, assess the consequences of vaccinated cats receiving an accidental second GonaCon injection and determine the humoral immune response to immunization. During Phase 1, cats in Group A (n = 3) received a single intramuscular injection of GonaCon and Group B (n = 3) received a single intramuscular injection of saline. During Phase 2, Group A received a second GonaCon injection and Group B received their initial GonaCon injection. All cats developed GnRH antibodies within 30 days of vaccine administration. The endpoint titre (1:1,024,000) was similar among all cats, and levels remained high throughout the duration of the study. Four cats developed a sterile, painless, self‐limiting mass at the site of injection. The mean number of days to mass development was 110.3 (range, 18–249 days). In conclusion, this preliminary study suggests that the EPA‐registered GonaCon formulation is safe for continued testing in domestic cats, an accidental revaccination should not increase the risk of a vaccine reaction and the EPA‐registered formulation effectively elicits a strong humoral immune response.