**3.5. Immunocontraception**

**3.4. GnRH antagonists**

148 Canine Medicine - Recent Topics and Advanced Research

or work dogs [19].

The GnRH antagonists limit its action by a competitive block of the hormone receptors achieving the annulation of the effects of circulating GnRH. Consequently, the function of the hypothalamus-pituitary-gonadal axis is impaired. Long-term effects of GnRH antagonists also include the down-regulation of the GnRH receptors [4, 42]. Either way, follicular waves are suppressed and ovulation is compromised [19]. Several generations of peptides with GnRH antagonist activity have been tested in dogs, but their use is still limited all over the world, mainly because they present a rather low efficiency as contraceptives. Also, the first generation compounds showed several important side effects, derived from the need for higher doses of these peptides to reach the desired effect. Peptide GnRH antagonists act only for short-term estrus suppression [19, 43], which make them a poor agent when longer periods of contraception are foreseen. Therefore, its use is mostly restricted to the short-term contraception in show

**Figure 4.** SWOT analysis to the use of GnRH antagonists as a contraceptive in dogs.

expected to enter a new cycle within 3 weeks of treatment [4].

available on these molecules.

In dogs, the third generation GnRH antagonist, Acylin®, is the most used antagonist. It should be administered within the first 3 days of proestrus, subcutaneously, at a dose of 100 µg/kg. Suspension of the follicular stage is obtained, and ovulation inhibited; however, the bitch is

Companies are now exploiting some nonpeptide molecules as GnRH antagonists, alike those tested for humans, aiming to obtain the long-term release formulations that may be applied for the long-term contraception. However, to our best knowledge, no information is yet Immunocontraception is yearned for long estrus control, either for owned and free-roaming dogs. Classical targets in immunocontraception include the GnRH and LH along with their corresponding receptors, as well as the sperm or *Zona pellucida* (ZP) proteins [4, 44]. Vaccines are usually conjugated with various antigens to enhance the immune response against the target compound(s). Contraception would be maintained through regular boosting [45]. Either approach shows unsatisfactory results till present [4]. Particularly, the resumption of estrous cycles or fertility after withdrawal of the treatment is still a concern.

The use of vaccines targeting GnRH will induce suppression of the estrous cycle, while vaccines against LH will interfere with ovulation because the preovulatory LH-surge is suppressed, and progesterone secretion is also compromised. Depending on the vaccine used, the cycle may be suspended for periods ranging from 5 months to 5 years with a single administration. On the other hand, vaccines targeting sperm or *Zona pellucida* proteins will not disturb the normal estrous cycle of the female dog, but will inhibit egg-perm binding in the female genital tract and fertilization [15]. *Zona pellucida* vaccines do not succeed to induce infertility in dogs [4, 15].

GonaCon™ is a vaccine against GnRH. It was developed to control the reproduction in the wildlife population, in which infertility was achieved for a period of 1–4 years with a single vaccination [46]. However, its use in dogs is still controversial, as it seems that dogs present intense reactions at the site of injection, with formation of long-lasting abscesses and granulomas, due to a greater sensitivity to the adjuvant used in the vaccine. Also, there is no available data on the duration of estrus suppression [15]. Recently, in Mexico, a new formulation of GonaCon (with the adjuvant AdjuVac™) was tested in shelter dogs during a campaign for rabies and control of canine reproduction [47]. According to the data reported, the proportion of animals presenting abscesses at day 60 was lower than the expected from previous studies using different vaccine adjuvants, but it was still not devoid of other local side effects (like muscular atrophy) [47].

Another study, using an unidentified commercial vaccine against GnRH, administered twice at a 4-week interval, however, reported that none of the four animals used showed adverse reactions to vaccination, remaining clinically healthy for the length of the study (20 weeks) [48]. That study also showed that the reduction in LH and testosterone as well as in the size of the gonads started by week 4 and it was maximal by week 12. However, at week 20 the parameters were similar to those recorded at week 4, suggesting that the vaccine effects were reverting. However, the length of the study [48] does not confirm this hypothesis or establish the schedule for revaccination.

**Figure 5** summarizes the analysis to the application of antiGnRH vaccines as contraceptives in dogs.

**Figure 5.** SWOT analysis to the use of antiGnRH vaccines as a contraceptive in dogs.

Immunocontraception holds great promise for canine contraception; still, several drawbacks need to be overcome before being widely introduced into the veterinary practice. The need for regular revaccination may not be an issue in owned dogs but important questions needing answers respect the maintenance of fertility and the time to fertility restoration at the withdrawal of the treatment.
