As of the time of this posting (February 2015) non-lethal alternatives to bow-hunting or sharpshooting are practically limited to sterilization, as presented in this Program, and immuno-contraception. There are two chemical contraception options available: GonaCon™ and Porcine Zona Pellucida (PZP).
GonaCon™ (including a formulation known as “GonaCon-Blue”) is a gonadotropin-releasing hormone (GnRH) immunocontraceptive vaccine that stimulates the production of antibodies that bind to GnRH, a hormone that signals the production of sex hormones (such as estrogen and progesterone). By binding to GnRH, the antibodies reduce GnRH’s ability to stimulate the release of these sex hormones. As a result, mating activity is reduced and a deer receiving GonaCon™ will be infertile for as long as a sufficient level of antibody activity is present. GonaCon™ has been registered with the EPA for use in white tailed deer 1 year of age or older.
Currently, a single shot of GonaCon™ is highly effective in the first year, and in some deer antibody levels remain high enough to prevent pregnancy for up to 5 years. However, antibody levels fall off more quickly in other deer, such that 20% to 50% of treated deer regain their fertility between treatments. The efficacy rates tend to be higher in controlled studies (e.g., with captive deer) than in typical field conditions. GonaCon™ currently is only EPA registered for hand injection each time it is given (rather than being registered for remote darting). This means that a deer who is first treated with GonaCon™ at age 1 will likely need to be captured, sedated, and vaccinated at least twice in her lifetime, and may still give birth to multiple fawns.
Scientists are working to develop improvements to GonaCon™, including possible oral or remote darting delivery methods. These improvements are not yet available and their timing is uncertain.
Porcine Zona Pellucid (PZP)
PZP is a naturally occurring protein found in pig eggs. When the PZP vaccine is injected into the muscle of a female deer, it stimulates her immune system to produce antibodies against the vaccine. These antibodies block fertilization, but unlike GonaCon™, do not suppress the stimulation of sex hormones, so deer receiving PZP continue to go into heat. In fact, PZP appears to extend by approximately 2 months the estrus cycles of treated deer who do not become pregnant. There is no evidence this harms the treated deer, though it does extend the mating season and may increase undesirable mating behavior such as chasing.
PZP has been registered with the EPA for use in wild horses and burros. It has not yet been registered for use in white tailed deer but is approved routinely for research purposes.
The most researched formulations of PZP have been found to be highly effective when given twice in the first year (the vaccine and a booster several months later) and annually thereafter. Newer formulations currently being tested in field studies do not require a first year booster and can last up to 2 or 3 years, and treatments after the first one can be delivered remotely by dart. The efficacy of the multi-year formulations of PZP start at 90% to 95% and may decline further in years 2 and 3 (meaning that at least 5% to 10% of the treated deer will give birth to fawns between treatments). A deer who is first treated with PZP at age 1 will likely need to be treated 3 to 4 times in her lifetime.
As with GonaCon™, scientists are working to develop improvements to PZP (to increase the length of time needed between treatments, as well as its efficacy) and current delivery technology. These improvements could make contraception using PZP a much more desirable option, but they are not yet available and their timing is uncertain.
Advantages of Sterilization over Immunocontraceptives Today
We believe sterilization is the best choice at this time because:
- Sterilization is 100% effective over the lifetime of the deer. The effectiveness of multi-year immunocontraceptives declines over time. Any non-lethal program will only be successful in reducing the deer population if natural attrition through death and emigration exceeds natural increases through births and immigration. Even a 10% “break through rate” (deer who become pregnant between treatments) can be enough to offset natural attrition.
- Sterilization requires only one treatment over the lifetime of the deer. Immunocontraceptives require multiple treatments.
- It is less expensive to sterilize each deer once than to vaccinate every deer multiple times.
- Deer subject to repeated darting tend to become “educated”, making them less approachable and less easily lured to bait traps. Locating and successfully revaccinating previously treated deer becomes more challenging and labor intensive over time. This does not happen with sterilization.
- A deer who is sterilized and emigrates out of the study area will remain infertile and will not contribute to fawning in her new area. A vaccinated deer who leaves the study area (or who can’t be located or darted when it is time for her revaccination) will become fertile again.
- If the ODNR does not allow the Program to continue at the end of the 5 year study period, sterilized deer will remain sterilized. The investment will not be lost. Deer who have been vaccinated will become fertile again, and the investment in contraceptives will be lost.
- Deer who have received ovariectomies do not go into heat, so bucks are not attracted to the area during mating season (adding to the deer population) and undesirable “chasing” and other breeding activity do not occur. With PZP, the breeding season is actually extended by up to 2 months.
- It is our understanding that the ODNR has granted permits for two PZP studies (a very small one in Columbus, and a larger one in Cleveland) in the past, but after several years declined to issue renewal permits. And as recently as last year, the ODNR reportedly placed conditions on a proposed PZP study in Avon Lake, Ohio that effectively scuttled the proposal. To our knowledge, the ODNR has never been asked to approve a sterilization proposal and has no negative institutional experience with it.
- An experienced sterilization program provider is available to work in Cincinnati now. Immunocontraceptive studies typically require more deer for research purposes than are present in the Clifton Parks, so finding an experienced contraception program provider (particularly within the Park Board’s timeframe) may be challenging.
- If contraception becomes a better choice in the future, there will be no loss or impediments to switching methods on immigrant deer.