Gn-RH vaccines
The principle of Gn-RH vaccines is that antibodies to Gn-RH prevent its binding to Gn-RH receptors, shutting down the release
of LH and FSH. Gn-RH must be coupled with a large carrier in order to be immunogenic.13 Some disadvantages of Gn-RH vaccines are variations in individual responses, the need for repeated immunization to keep
titers high, and the need for an adjuvant.13 The Gn-RH vaccines have the advantages of inhibiting hormonal cycling and associated behaviors, and being effective in both
females and males.27
GonaCon, a vaccine containing Gn-RH coupled to keyhole limpet hemocyanin and the mycobacterial adjuvant AdjuVac, was originally
developed for the control of white-tailed deer populations by scientists at the USDA Wildlife Service's National Wildlife
Research Center.21 GonaCon has been tested in male and female cats.27,28 There was a wide variation in response and duration of action. It was about 75% effective in female cats, and some cats
had estrus prevention for up to 3.5 years after a single dose.28 However, some of the females developed a granuloma at the injection site after 24 months, likely caused by the mycobacterial
adjuvant.28
GonaCon is not effective for inducing long-term infertility in dogs, and it resulted in severe injection site reactions that
persisted for a year until surgical removal.29 New formulations are being developed to prevent this problem in dogs.
Another Gn-RH vaccine consisting of the antigen IPS-21, a commercially available adjuvant, and the immuno-stimulant dimethyl
dioctadecyl ammonium bromide was tested in four male and 10 female cats.30 IPS-21 is a recombinant protein made up of eight tandem repeats of Gn-RH fused to a fragment of leukotoxin A from Mannheimia haemolytica. It was shown to be effective as evidenced by immunoneutralizing titers to GnRH for almost two years, but required several
immunizations. None of the females exhibited estrous behavior or became pregnant. All the cats, including the cat injected
with a placebo, showed nonpainful, palpable tissue reactions that resolved by day 28. There were no serum chemistry profile
abnormalities.30
LH receptor vaccines
Female dogs and cats have been vaccinated with bovine LH receptor (LH-R) encapsulated in a silastic subdermal implant, followed
by four booster injections of LH-R intramuscularly.31,32 This resulted in the formation of LH-R antibodies with subsequent suppression of serum progesterone concentrations and lack
of estrus for about one year. None of the females in these studies was mated to determine whether pregnancy was prevented.
Further studies are needed to determine the sufficient LH-R antibody titer to cause infertility.
CONCLUSION
With the worldwide overpopulation of dogs and cats, a variety of options are needed for population control. Nonsurgical methods
of contraception are one such option. Gonazon, Suprelorin, Neutersol, Esterilsol, and Infertile are the only approved products
for dogs or cats, but they are limited to certain countries. Extensive research and development are under way on Gn-RH agonists
and antagonists, VCD, immunocontraception, and chemical castration. None of the products meets all of the criteria set forth
by the Alliance for Contraception in Cats and Dogs. However, it is likely that new products will be available in the future
that offer hope for controlling the free-roaming dog and cat populations.
Megan Cathey, DVM
Mushtaq A. Memon, BVSc, PhD, DACT
Department of Veterinary Clinical Sciences
College of Veterinary Medicine
Washington State University
Pullman, WA 99164
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