Efficacy of an immunocontraceptive in cats
Published: March 13, 2018
Winn Feline Health Foundation

Fischer A, Benka VA, Briggs J, Driancourt MA, Maki J, et al. Effectiveness of GonaCon as an immunocontraceptive in colony-housed cats. J Feline Med Surg. 2018 Feb 1;:1098612X18758549. PubMed PMID: 29463201.

Control of free-roaming cat populations is a major topic of discussion in veterinary, public health, shelter, and municipal government communities. Traditional methods of population control (such as culling or relocating) have largely fallen out of favor as inhumane or ineffective. In modern times, Trap-Neuter-Return (TNR) programs have become the gold standard for population control. These provide the advantage of gradually reducing a population in a humane fashion while also allowing for vaccination, parasite control, and other interventions. A major downside to TNR programs is the effort and expense required to transport animals to a facility where veterinary teams can perform surgery. To this end, non-surgical induction of sterility has been a recent area of exploration.

GonaCon is a commercial vaccine against GnRH (the hormone which controls ovulation) initially formulated for use in deer and equids but recently demonstrated to be safe and effective in cats. While previous studies have shown GonaCon to be effective in inducing sterility, the purpose of this study was to determine the length of time sterility was induced for. The study was designed as a prospective double-blind placebo-controlled trial.

33 female and 7 male cats at high risk for euthanasia were enrolled from local animal control agencies and housed in a 1400 square foot indoor controlled environment with daytime access to a 1/3 acre outdoor area. Light indoors was set to mimic the natural photoperiod. Food and water were offered ad libitum. Female cats were nonpregnant, free of obvious disease (on exam, bloodwork, abdominal ultrasound, Wood’s lamp), retrovirus negative, and vaccinated and dewormed. Male cats were assessed as above with the inclusion of semen analysis to confirm fertility.

Female cats were randomized by age to receive 0.5mL of either sterile saline (10 cats) or GonaCon (20 cats) intramuscularly in a double-blinded manner. Bloodwork was repeated on all cats at day 34 and injection sites were palpated regularly. Male cats were introduced 115 days after treatment, coinciding with the start of the usual estrus period in cats (early February). Males were cycled through one at a time to maximize breeding success and minimize intra-cat aggression.

Female cats were ultrasound once monthly after introduction of the males and were ovariohysterectomized within 8 days when pregnancy was confirmed (prior to 45 days gestation). Time to conception was determined by crown-rump length at day of diagnosis.

6 control cats (60%) and 8 GonaCon cats (40%) became pregnant within the first month. By the fourth month all control cats and 60% of GonaCon cats were pregnant. At 1 year, 70% of cats were pregnant. The GonaCon group had a median time to conception of 212 days, compared to 127.5 days for control cats.  Litter size was also smaller in the GonaCon group (3.9 vs 5.6 fetuses).

Of the 6 cats that did not become pregnant, 4 did not exhibit estrus behaviour over the course of the study.

9/20 treated cats developed significant injection site granulomas histologically associated with acid-fast bacteria, likely as a result of the mycobacterial vaccine adjuvant. After the end of the study, all cats were sterilized and adopted to private homes.

The rate of pregnancy, litter size, and time to conception were all significantly different in the treatment group, suggesting some degree of vaccine efficacy. However, results were much less significant than demonstrated in previous studies, and were not considered sufficient for population control.

The major concern with this study is how different the results were from previous investigations (which showed good efficacy). The authors propose several possible reasons for this, including differences in batch efficacy, individual differences in cats, or environmental effects.

The authors conclude that there is little evidence for efficacy of GonaCon for fertility control in cats, and terminated the study due to failure of efficacy. Further work is needed into alternative methods of non-surgical sterilization in cats. (MRK)

See also:

Vansandt LM, Kutzler MA, Fischer AE, et al. Safety and effectiveness of a single and repeat intramuscular injection of a GnRH vaccine (GonaCon) in adult female domestic cats. Reprod Domest Anim 2016; 51: 1–6.